Tasks

All the tasks of the course in one place. First 50 standalone tasks from "Hello, World" to the shortest path in a grid, each with an example and a solution in JavaScript checked with a real compiler. Then the tasks from the lessons, by module: each one leads to the lesson that explains what you need.

50 tasks to practise

1. Hello World
Print the text Hello, World! to the console; the program takes no input.
No input.
Output
```
Hello, World!
```
What you need is in the lesson: Your first program
Solution
```
console.log("Hello, World!");
```

2. Greet the user

Read a name from one line and greet the person by name as "Hello, <name>!".

Maria

Hello, Maria!

What you need is in the lesson: Input and output

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const name = await ask("");
console.log("Hello, " + name + "!");
rl.close();

3. Print the age

Read the user's age (a whole number), store it in a variable and print it in a sentence.

You are 17 years old.

What you need is in the lesson: Variables and types

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const age = Number(await ask(""));
console.log("You are " + age + " years old.");
rl.close();

4. Unit converter

Read a length in inches (it may be a decimal) and convert it to centimetres: 1 inch = 2.54 cm. Print the result with 2 decimal places.

12.5

31.75 cm

What you need is in the lesson: Operators and arithmetic

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const inches = Number(await ask(""));
console.log((inches * 2.54).toFixed(2) + " cm");
rl.close();

5. Even or odd

Read a whole number and print whether it is even or odd.

7 is odd.

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const number = Number(await ask(""));
if (number % 2 === 0) {
  console.log(number + " is even.");
} else {
  console.log(number + " is odd.");
}
rl.close();

6. Sum of two numbers

Read two whole numbers, each on its own line, and print their sum.

12
30

The sum is 42.

What you need is in the lesson: Operators and arithmetic

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const first = Number(await ask(""));
const second = Number(await ask(""));
console.log("The sum is " + (first + second) + ".");
rl.close();

7. Area of a square

Read the side of a square (a whole number) and print its area.

The area is 25.

What you need is in the lesson: Operators and arithmetic

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const side = Number(await ask(""));
console.log("The area is " + side * side + ".");
rl.close();

8. Volume of a sphere

Read the radius r (it may be a decimal) and print the volume of the sphere V = 4/3 · π · r³ with 2 decimal places.

The volume is 113.10.

What you need is in the lesson: Operators and arithmetic

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const r = Number(await ask(""));
const volume = (4 / 3) * Math.PI * r ** 3;
console.log("The volume is " + volume.toFixed(2) + ".");
rl.close();

9. Compare two numbers

Read two whole numbers, each on its own line, and print the larger one (if they are equal, either will do).

8
15

The larger one is 15.

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const a = Number(await ask(""));
const b = Number(await ask(""));
if (a > b) {
  console.log("The larger one is " + a + ".");
} else {
  console.log("The larger one is " + b + ".");
}
rl.close();

10. Triangle check

Read three whole numbers, each on its own line, and check whether they can be the sides of a triangle: any two of them must add up to more than the third.

3
4
5

They can form a triangle.

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const a = Number(await ask(""));
const b = Number(await ask(""));
const c = Number(await ask(""));
if (a + b > c && a + c > b && b + c > a) {
  console.log("They can form a triangle.");
} else {
  console.log("They cannot form a triangle.");
}
rl.close();

11. Grade to words

Read a mark on the Bulgarian scale from 2 to 6 (it may be a decimal) and print it in words: 5.50 and up is Excellent, 4.50 and up Very good, 3.50 and up Good, 3.00 and up Average, below 3.00 Poor.

4.75

Very good

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const grade = Number(await ask(""));
if (grade >= 5.5) {
  console.log("Excellent");
} else if (grade >= 4.5) {
  console.log("Very good");
} else if (grade >= 3.5) {
  console.log("Good");
} else if (grade >= 3) {
  console.log("Average");
} else {
  console.log("Poor");
}
rl.close();

12. Hours to seconds

Read a number of hours (a whole number) and print how many seconds that is.

7200 seconds

What you need is in the lesson: Operators and arithmetic

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const hours = Number(await ask(""));
console.log(hours * 3600 + " seconds");
rl.close();

13. Count to N

Read a whole number N and print the numbers from 1 to N, each on a new line.

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
for (let i = 1; i <= n; i++) {
  console.log(i);
}
rl.close();

14. ASCII value

Read one character (the first character of the line) and print its ASCII code, the number the computer stores it as.

The code of A is 65.

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const symbol = (await ask(""))[0];
console.log("The code of " + symbol + " is " + symbol.charCodeAt(0) + ".");
rl.close();

15. Quadratic equation

Read a, b and c, each on its own line, and solve ax² + bx + c = 0. With two roots print "x1 = …, x2 = …" (x1 uses +√D), with one root "x = …", with 2 decimal places; if there are no real roots or a is 0, say so.

1
-3
2

x1 = 2.00, x2 = 1.00

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const a = Number(await ask(""));
const b = Number(await ask(""));
const c = Number(await ask(""));
if (a === 0) {
  console.log("This is not a quadratic equation");
} else {
  const d = b * b - 4 * a * c;
  if (d > 0) {
    const x1 = (-b + Math.sqrt(d)) / (2 * a);
    const x2 = (-b - Math.sqrt(d)) / (2 * a);
    console.log("x1 = " + x1.toFixed(2) + ", x2 = " + x2.toFixed(2));
  } else if (d === 0) {
    console.log("x = " + (-b / (2 * a)).toFixed(2));
  } else {
    console.log("No real roots");
  }
}
rl.close();

16. Positive or negative

Read a whole number and print whether it is positive, negative or zero.

-4

Negative

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const number = Number(await ask(""));
if (number > 0) {
  console.log("Positive");
} else if (number < 0) {
  console.log("Negative");
} else {
  console.log("Zero");
}
rl.close();

17. Seasons

Read a month number (1–12) and print its season: 12, 1, 2 are Winter, 3–5 Spring, 6–8 Summer, 9–11 Autumn. For any other number print "Invalid month".

Spring

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const month = Number(await ask(""));
if (month < 1 || month > 12) {
  console.log("Invalid month");
} else if (month === 12 || month <= 2) {
  console.log("Winter");
} else if (month <= 5) {
  console.log("Spring");
} else if (month <= 8) {
  console.log("Summer");
} else {
  console.log("Autumn");
}
rl.close();

18. Reverse a number

Read a positive whole number and print the number with its digits in reverse order: 123 becomes 321. Zeros that end up in front disappear (1200 becomes 21). Take the last digit with % 10 and drop it by dividing by 10.

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

let n = Number(await ask(""));
let reversed = 0;
while (n > 0) {
  reversed = reversed * 10 + (n % 10);
  n = Math.floor(n / 10);
}
console.log(reversed);
rl.close();

19. Temperature converter

Read a temperature in degrees Celsius (it may have a decimal point) and print it in Fahrenheit with two decimal places. The formula is F = C * 9 / 5 + 32.

36.6

97.88 °F

What you need is in the lesson: Operators and arithmetic

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const celsius = Number(await ask(""));
const fahrenheit = (celsius * 9) / 5 + 32;
console.log(`${fahrenheit.toFixed(2)} °F`);
rl.close();

20. Longest name

The first line holds the number of names n, and each of the next n lines holds one name. Print the name with the most letters and their count in brackets; if two names are equally long, print the first of them.

4
Mia
Alexandra
John
Charlotte

Alexandra (9 letters)

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
let longest = "";
for (let i = 0; i < n; i++) {
  const name = await ask("");
  if (name.length > longest.length) {
    longest = name;
  }
}
console.log(`${longest} (${longest.length} letters)`);
rl.close();

21. Word counter

Read one line of text and print how many words it has. Words are separated by one or more spaces, so the empty pieces between two spaces are not words.

I write  code every day and   get better

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const line = await ask("");
const words = line.split(" ").filter((word) => word !== "");
console.log(words.length);
rl.close();

22. Prime number

Read a whole number n and check whether it is prime: greater than 1 and divisible only by 1 and itself. It is enough to try the divisors i for which i * i <= n.

91 is not prime

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
let prime = n > 1;
for (let i = 2; i * i <= n; i++) {
  if (n % i === 0) {
    prime = false;
    break;
  }
}
if (prime) {
  console.log(`${n} is prime`);
} else {
  console.log(`${n} is not prime`);
}
rl.close();

23. Day of the week

Read a number from 1 to 7 and print the name of that day of the week, where 1 is Monday. Keep the names in an array; for any other number print "No such day".

Friday

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const day = Number(await ask(""));
const days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"];
if (day >= 1 && day <= 7) {
  console.log(days[day - 1]);
} else {
  console.log("No such day");
}
rl.close();

24. Factorial

Read a whole number n from 0 to 15 and print n! = 1 * 2 * ... * n (0! is 1). Already 13! does not fit in an int, so in Java and C# keep the product in a long.

15! = 1307674368000

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
let factorial = 1;
for (let i = 2; i <= n; i++) {
  factorial *= i;
}
console.log(`${n}! = ${factorial}`);
rl.close();

25. Valid date

Read a day, a month and a year, each on its own line, and print "Valid date" or "Invalid date". The year is at least 1, the month is from 1 to 12 and the day from 1 to the number of days in that month; February has 29 days in a leap year (divisible by 4 but not by 100, unless it is divisible by 400).

29
2
2100

Invalid date

What you need is in the lesson: Conditions

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const day = Number(await ask(""));
const month = Number(await ask(""));
const year = Number(await ask(""));
const leap = (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
let days;
if (month === 2) {
  days = leap ? 29 : 28;
} else if (month === 4 || month === 6 || month === 9 || month === 11) {
  days = 30;
} else {
  days = 31;
}
if (year >= 1 && month >= 1 && month <= 12 && day >= 1 && day <= days) {
  console.log("Valid date");
} else {
  console.log("Invalid date");
}
rl.close();

26. Letter frequency

Read one line of text and print how many times each letter appears in it, ignoring upper and lower case. Print the letters in alphabetical order, one per line, as "a: 5"; spaces and punctuation are not counted.

Hello, World!

d: 1
e: 1
h: 1
l: 3
o: 2
r: 1
w: 1

What you need is in the lesson: Map and Set

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const text = (await ask("")).toLowerCase();
const counts = new Map();
for (const c of text) {
  if (/\p{L}/u.test(c)) {
    counts.set(c, (counts.get(c) ?? 0) + 1);
  }
}
for (const c of [...counts.keys()].sort()) {
  console.log(`${c}: ${counts.get(c)}`);
}
rl.close();

27. Minimum and maximum in an array

The first line holds the count n, and each of the next n lines holds one whole number. Read them into an array and print the smallest and the largest, starting from the first element and comparing it with the rest.

Smallest: -3
Largest: 12

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
const numbers = [];
for (let i = 0; i < n; i++) {
  numbers.push(Number(await ask("")));
}
let min = numbers[0];
let max = numbers[0];
for (let i = 1; i < n; i++) {
  if (numbers[i] < min) {
    min = numbers[i];
  }
  if (numbers[i] > max) {
    max = numbers[i];
  }
}
console.log(`Smallest: ${min}`);
console.log(`Largest: ${max}`);
rl.close();

28. Sort an array

The first line holds the count n, and each of the next n lines holds one whole number. Sort them yourself in ascending order with bubble sort (n - 1 passes, each swapping neighbours that are in the wrong order) and print them on one line, separated by spaces.

1 2 5 5 6 9

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
const numbers = [];
for (let i = 0; i < n; i++) {
  numbers.push(Number(await ask("")));
}
for (let pass = 0; pass < n - 1; pass++) {
  for (let i = 0; i < n - 1 - pass; i++) {
    if (numbers[i] > numbers[i + 1]) {
      const temp = numbers[i];
      numbers[i] = numbers[i + 1];
      numbers[i + 1] = temp;
    }
  }
}
console.log(numbers.join(" "));
rl.close();

29. Replace the vowels

Read one line of text and print it with every vowel, upper or lower case, replaced by the character *. The vowels are a, e, i, o, u.

I love programming

* l*v* pr*gr*mm*ng

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const text = await ask("");
const vowels = "aeiou";
let result = "";
for (const c of text) {
  if (vowels.includes(c.toLowerCase())) {
    result += "*";
  } else {
    result += c;
  }
}
console.log(result);
rl.close();

30. Sum of the digits

Read a whole non-negative number and print the sum of its digits. The last digit is n % 10, and n / 10 drops it (n // 10 in Python).

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

let n = Number(await ask(""));
let sum = 0;
while (n > 0) {
  sum += n % 10;
  n = Math.floor(n / 10);
}
console.log(sum);
rl.close();

31. Palindrome

Read one line of text and check whether it is a palindrome, that is, it reads the same backwards. Upper and lower case count as the same and spaces are skipped. Print "Palindrome" or "Not a palindrome".

Never odd or even

Palindrome

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const text = (await ask("")).toLowerCase().replaceAll(" ", "");
let palindrome = true;
for (let i = 0; i < text.length / 2; i++) {
  if (text[i] !== text[text.length - 1 - i]) {
    palindrome = false;
    break;
  }
}
if (palindrome) {
  console.log("Palindrome");
} else {
  console.log("Not a palindrome");
}
rl.close();

32. Strong password

Read passwords, one per line, until a strong one arrives: at least 8 characters, at least one digit, one uppercase and one lowercase letter. For each weak one print the first rule it breaks, in this order ("Too short", "No digit", "No uppercase letter", "No lowercase letter"); for the strong one print "Password accepted" and stop.

kotka
kotkamiau
kotkamiau7
Kotkamiau7

Too short
No digit
No uppercase letter
Password accepted

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

while (true) {
  const password = await ask("");
  let digit = false;
  let upper = false;
  let lower = false;
  for (const c of password) {
    if (c >= "0" && c <= "9") {
      digit = true;
    } else if (c !== c.toLowerCase()) {
      upper = true;
    } else if (c !== c.toUpperCase()) {
      lower = true;
    }
  }
  if (password.length < 8) {
    console.log("Too short");
  } else if (!digit) {
    console.log("No digit");
  } else if (!upper) {
    console.log("No uppercase letter");
  } else if (!lower) {
    console.log("No lowercase letter");
  } else {
    console.log("Password accepted");
    break;
  }
}
rl.close();

33. Reverse an array

The first line holds the count n, and each of the next n lines holds one whole number. Read them into an array and print them in reverse order on one line, separated by spaces: walk the array from the last index down to zero.

3 7 -2 10

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
const numbers = [];
for (let i = 0; i < n; i++) {
  numbers.push(Number(await ask("")));
}
let line = "";
for (let i = n - 1; i >= 0; i--) {
  line += numbers[i];
  if (i > 0) {
    line += " ";
  }
}
console.log(line);
rl.close();

34. Count the elements

Read one line of words separated by single spaces and print how many times each word appears, as "word: count". The words come out in the order in which they first appeared.

apple pear apple plum pear apple

apple: 3
pear: 2
plum: 1

What you need is in the lesson: Map and Set

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const line = await ask("");
const counts = new Map();
for (const word of line.split(" ")) {
  counts.set(word, (counts.get(word) ?? 0) + 1);
}
for (const [word, count] of counts) {
  console.log(`${word}: ${count}`);
}
rl.close();

35. Prime numbers

Read a number N and print all prime numbers from 2 to N on one line, separated by spaces.

2 3 5 7 11 13 17 19 23 29

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
const primes = [];
for (let i = 2; i <= n; i++) {
  let prime = true;
  for (let d = 2; d * d <= i; d++) {
    if (i % d === 0) {
      prime = false;
      break;
    }
  }
  if (prime) {
    primes.push(i);
  }
}
console.log(primes.join(" "));
rl.close();

36. Average

One line holds whole numbers separated by spaces. Print their average with two digits after the decimal point.

5 8 12 6

Average: 7.75

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const parts = (await ask("")).split(" ");
let sum = 0;
for (let i = 0; i < parts.length; i++) {
  sum += Number(parts[i]);
}
const average = sum / parts.length;
console.log("Average: " + average.toFixed(2));
rl.close();

37. Sort by length

One line holds words separated by spaces. Sort them by length from shortest to longest (words of equal length keep their input order) and print them on one line.

cherry fig banana kiwi plum

fig kiwi plum cherry banana

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const words = (await ask("")).split(" ");
for (let i = 0; i < words.length - 1; i++) {
  for (let j = 0; j < words.length - 1 - i; j++) {
    if (words[j].length > words[j + 1].length) {
      const temp = words[j];
      words[j] = words[j + 1];
      words[j + 1] = temp;
    }
  }
}
console.log(words.join(" "));
rl.close();

38. Sentence counter

Read one line of text and print how many sentences it has. A sentence ends with a full stop, an exclamation mark or a question mark; a run of such marks (like "..." or "?!") is a single ending.

Hi! How are you? I am fine... And you?!

Sentences: 4

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const text = await ask("");
const ends = ".!?";
let count = 0;
for (let i = 0; i < text.length; i++) {
  const isEnd = ends.includes(text[i]);
  const nextIsEnd = i + 1 < text.length && ends.includes(text[i + 1]);
  if (isEnd && !nextIsEnd) {
    count++;
  }
}
console.log("Sentences: " + count);
rl.close();

39. Fibonacci numbers

The Fibonacci sequence starts with 1, 1, and every next number is the sum of the previous two. Read N (from 1 to 50) and print the N-th number of the sequence.

102334155

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const n = Number(await ask(""));
let previous = 0;
let current = 1;
for (let i = 1; i < n; i++) {
  const next = previous + current;
  previous = current;
  current = next;
}
console.log(current);
rl.close();

40. Finding the median

One line holds whole numbers separated by spaces. Print their median with one digit after the decimal point: the middle number once they are sorted, or the mean of the two middle ones when the count is even.

12 3 7 9 1 8

Median: 7.5

What you need is in the lesson: Arrays: push, splice and sort

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const numbers = [];
for (const part of (await ask("")).split(" ")) {
  numbers.push(Number(part));
}
numbers.sort((a, b) => a - b);
const n = numbers.length;
let median;
if (n % 2 === 1) {
  median = numbers[Math.floor(n / 2)];
} else {
  median = (numbers[n / 2 - 1] + numbers[n / 2]) / 2;
}
console.log("Median: " + median.toFixed(1));
rl.close();

41. Character chart

Read one line of text. For every character except the space, in order of first appearance, print a line "character | stars" with one star for each time it occurs in the text.

banana bread

b | **
a | ****
n | **
r | *
e | *
d | *

What you need is in the lesson: Strings: working with text

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const text = await ask("");
for (let i = 0; i < text.length; i++) {
  const c = text[i];
  if (c !== " " && text.indexOf(c) === i) {
    let count = 0;
    for (let j = 0; j < text.length; j++) {
      if (text[j] === c) {
        count++;
      }
    }
    console.log(c + " | " + "*".repeat(count));
  }
}
rl.close();

42. Transposing a matrix

The first line holds the number of rows R and columns C, followed by R lines of C numbers separated by spaces. Print the transposed matrix: C lines of R numbers, where rows become columns.

2 3
1 2 3
4 5 6

1 4
2 5
3 6

What you need is in the lesson: Two-dimensional arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const size = (await ask("")).split(" ");
const rows = Number(size[0]);
const cols = Number(size[1]);
const matrix = [];
for (let r = 0; r < rows; r++) {
  matrix.push((await ask("")).split(" ").map(Number));
}
for (let c = 0; c < cols; c++) {
  const line = [];
  for (let r = 0; r < rows; r++) {
    line.push(matrix[r][c]);
  }
  console.log(line.join(" "));
}
rl.close();

43. Paths in a maze

The first line holds R and C, followed by R lines of C characters: "." is a free cell and "#" is a wall. Moving only right or down and never stepping on a wall, print the number of all paths from the top-left to the bottom-right corner.

4 5
.....
.#.#.
...#.
#....

Paths: 4

What you need is in the lesson: Recursion

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

function countPaths(maze, r, c) {
  if (r >= maze.length || c >= maze[0].length || maze[r][c] === "#") {
    return 0;
  }
  if (r === maze.length - 1 && c === maze[0].length - 1) {
    return 1;
  }
  return countPaths(maze, r + 1, c) + countPaths(maze, r, c + 1);
}

const size = (await ask("")).split(" ");
const rows = Number(size[0]);
const maze = [];
for (let r = 0; r < rows; r++) {
  maze.push(await ask(""));
}
console.log("Paths: " + countPaths(maze, 0, 0));
rl.close();

44. Grade statistics

One line holds grades from 2 to 6 (the Bulgarian school scale, 6 is best), separated by spaces. Print how many are excellent, very good, good, satisfactory and poor, each category on its own line.

6 4 5 6 3 5 2 6 5 4

Excellent (6): 3
Very good (5): 3
Good (4): 2
Satisfactory (3): 1
Poor (2): 1

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const names = ["", "", "Poor", "Satisfactory", "Good", "Very good", "Excellent"];
const counts = [0, 0, 0, 0, 0, 0, 0];
const parts = (await ask("")).split(" ");
for (let i = 0; i < parts.length; i++) {
  counts[Number(parts[i])]++;
}
for (let grade = 6; grade >= 2; grade--) {
  console.log(names[grade] + " (" + grade + "): " + counts[grade]);
}
rl.close();

45. Roman numerals

Read a whole number from 1 to 3999 and print it in Roman numerals (I, V, X, L, C, D, M, where 4 is IV, 9 is IX, 40 is XL and so on).

MCMXCIV

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

let number = Number(await ask(""));
const values = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1];
const symbols = ["M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"];
let roman = "";
for (let i = 0; i < values.length; i++) {
  while (number >= values[i]) {
    roman += symbols[i];
    number -= values[i];
  }
}
console.log(roman);
rl.close();

46. Coin toss simulation

Read a seed x (from 1 to 2147483646) and a number of tosses N, each on its own line. On every toss set x = x * 16807 % 2147483647; an even x is heads (H), an odd x is tails (T). Print the tosses on one line, then how many heads and how many tails came up.

42
10

HTTTHTTHHT
Heads: 4
Tails: 6

What you need is in the lesson: Loops

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

let x = Number(await ask(""));
const n = Number(await ask(""));
let tosses = "";
let heads = 0;
for (let i = 0; i < n; i++) {
  x = x * 16807 % 2147483647;
  if (x % 2 === 0) {
    tosses += "H";
    heads++;
  } else {
    tosses += "T";
  }
}
console.log(tosses);
console.log("Heads: " + heads);
console.log("Tails: " + (n - heads));
rl.close();

47. Symmetric array

One line holds whole numbers separated by spaces. Check whether the array is symmetric, that is, it reads the same from left to right and from right to left.

1 2 3 2 1

Symmetric

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const numbers = (await ask("")).split(" ").map(Number);
let symmetric = true;
for (let i = 0; i < Math.floor(numbers.length / 2); i++) {
  if (numbers[i] !== numbers[numbers.length - 1 - i]) {
    symmetric = false;
    break;
  }
}
console.log(symmetric ? "Symmetric" : "Not symmetric");
rl.close();

48. Balanced brackets

Read one line of text and check whether its brackets (), [] and {} are balanced: every opening bracket has a closing one of the same kind and the pairs do not cross. Other characters are ignored.

(a + [b) * c]

Not balanced

What you need is in the lesson: Stack, queue and linked list

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const text = await ask("");
const opening = "([{";
const closing = ")]}";
const stack = [];
let balanced = true;
for (const c of text) {
  if (opening.includes(c)) {
    stack.push(c);
  } else if (closing.includes(c)) {
    if (stack.length === 0 || opening.indexOf(stack.pop()) !== closing.indexOf(c)) {
      balanced = false;
      break;
    }
  }
}
if (balanced && stack.length === 0) {
  console.log("Balanced");
} else {
  console.log("Not balanced");
}
rl.close();

49. Run of equal elements

One line holds whole numbers separated by spaces. Find the longest run of equal neighbouring numbers and print it on one line; if several runs have the same length, print the first one.

2 1 1 3 3 3 2 2 2 5

3 3 3

What you need is in the lesson: Arrays

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const numbers = (await ask("")).split(" ").map(Number);
let bestValue = numbers[0];
let bestLength = 1;
let length = 1;
for (let i = 1; i < numbers.length; i++) {
  if (numbers[i] === numbers[i - 1]) {
    length++;
  } else {
    length = 1;
  }
  if (length > bestLength) {
    bestLength = length;
    bestValue = numbers[i];
  }
}
const run = [];
for (let i = 0; i < bestLength; i++) {
  run.push(bestValue);
}
console.log(run.join(" "));
rl.close();

50. Shortest path in a grid

The first line holds R and C, followed by R lines of C characters: "." is a free cell, "#" is a wall, "S" is the start and "E" is the goal. Moving up, down, left and right, print the smallest number of moves from S to E, or "No path" if E cannot be reached.

5 6
S.#...
.##.#.
...#..
#.#..#
...E.#

What you need is in the lesson: Stack, queue and linked list

Solution

import { createInterface } from "node:readline";

const rl = createInterface({ input: process.stdin });
const lines = rl[Symbol.asyncIterator]();
async function ask(question) {
  process.stdout.write(question);
  return (await lines.next()).value;
}

const size = (await ask("")).split(" ");
const rows = Number(size[0]);
const cols = Number(size[1]);
const grid = [];
for (let r = 0; r < rows; r++) {
  grid.push(await ask(""));
}
const dist = [];
const queue = [];
for (let r = 0; r < rows; r++) {
  dist.push([]);
  for (let c = 0; c < cols; c++) {
    dist[r].push(-1);
    if (grid[r][c] === "S") {
      dist[r][c] = 0;
      queue.push([r, c]);
    }
  }
}
const dr = [-1, 1, 0, 0];
const dc = [0, 0, -1, 1];
let answer = "No path";
while (queue.length > 0) {
  const [r, c] = queue.shift();
  if (grid[r][c] === "E") {
    answer = String(dist[r][c]);
    break;
  }
  for (let d = 0; d < 4; d++) {
    const nr = r + dr[d];
    const nc = c + dc[d];
    if (nr >= 0 && nr < rows && nc >= 0 && nc < cols && grid[nr][nc] !== "#" && dist[nr][nc] === -1) {
      dist[nr][nc] = dist[r][c] + 1;
      queue.push([nr, nc]);
    }
  }
}
console.log(answer);
rl.close();

Tasks from the lessons

Language basics

Your first program: Hello World · Business card · Frame
Variables and types: Swap · Age · A hotel card
Input and output: Greeting by name · Sum of two numbers · Two texts in one · π with two digits
Operators and arithmetic: Average of three numbers · Seconds into hours · Volume of a sphere · Celsius to Fahrenheit
Conditions: A cinema ticket · A mini calculator · A quadratic equation · A valid date
Loops: Multiplication table · Prime number · The digits of a number · Guess the number
Arrays: Sum, even, odd · Reversed · Lowest, highest, average
Arrays: Sorting · Longest run · Grade statistics
Two-dimensional arrays: Diagonals and rows · Flip the table
Two-dimensional arrays: How many paths are there? · The shortest path
Strings: working with text: The name backward · Word count · Palindrome
Strings: working with text: Strong password

Functions

Functions: Receipt · Triangle
Parameters and return: Average · Prime number · Temperatures · Calculator · A triangle with a parameter
Parameters and return
Default values: The largest · Average grade · A receipt line · Free delivery
Recursion: Factorial two ways · Sum of digits · Fibonacci · Palindrome

Classes and objects

Classes and objects: Book · Car · The cart · Most expensive first
Constructors and this: Person · Animal · Dance move
Instance methods
Instance methods
Encapsulation: Person · An account with no minus · Animal
static: what all objects share: Orders · Calculator · Exchange

Inheritance and polymorphism

Inheritance: Family car · Student · Breakdance · Electric car
Polymorphism: Animal shelter · The student introduces themselves · Party
Abstract classes without abstract: Area and perimeter · Animals
Abstract classes without abstract: Vehicles · Performances
Interfaces without interface: Horn · Details · Dance · DJ · The pond
Type checks: Parking lot · Dances · Fleet · Zoo
OOP principles
OOP principles: A contractor · The terrace without if

Errors and exceptions

Exceptions: Text to number · No such product · Average order
try, catch and finally: Price per person · Calculator
Throwing and your own exceptions: Music shop · An account with rules · Invalid age

Text and files

Strings: Palindrome · A receipt with columns · Words in reverse
Regular expressions: Date · Mobile number · Email · Shopping list
Regular expressions: groups and replacing: Find and replace · Words with a prefix · Addresses in text · Dates to ISO
Files: A configuration file · A log file filter · Folder contents · An index file
Data in files: CSV analysis · Saving the hero · Duplicate text

Collections

Arrays: push, splice and sort: A bookshelf · An event log · A notebook · Mental math
Stack, queue and linked list: A toy train · A bus route · A stock ticker · Balanced brackets · Your own linked list
Map and Set: A user table · Word frequency · Documents by keyword · A contact network
Hash codes and hash functions: A key for a person · The robot in the warehouse · A hash of a file
Nested classes and data: Shopping cart · Web server monitor · Building manager · Ranking

Modern JavaScript

Arrow functions: Sorting by several fields · Filtering orders · Discounts one after another
Array methods: Sales by category · Top 3 products · Average grade · The words in a text
Data, switch and destructuring: Shapes · Delivery price · A point as a key
Design patterns: Discounts as strategies · "Notify me when it is in stock" · A notification factory · An old thermometer

Real-world programs

Tests with node:test: The discount calculator · The account · The bug in the average grade
Async, await and threads: Parallel sum · Downloading with a pool · A counter without losses
Network programming: Echo server · A server with commands · A chat for two
Databases: Catalog · Search by part of the name · A transfer that fails halfway
Reflection: All the fields and the chain · Mini JSON · Your own test runner

Let's make a game

How to think about a game: A race for two · Guess the number · Who won at tic-tac-toe?
RPG 1: the heroes: A sixth hero · The party roster · Who is the strongest?
RPG 2: the battle: A new monster · A healing potion · A battle report
RPG 3: the whole game: A menu · Progress · A boss
Epic Cosmos 1: the objects in space: The solar system · Distance · The whole tree
Epic Cosmos 2: resources, buildings, ships: 100 metal as fast as possible · A cargo ship · A 10-turn simulation
The locked lab 1: rooms and commands: Inventory · A new room · Synonyms
The locked lab 2: puzzles and the way out: The dark cellar · A timer · A hint for a move

Chronicles of Arcanum

Epic Cosmos: the empire

Project

Hotel reservations: Changing rooms

Made by Ivo Gergov as part of Evo Tech's campaign for accessible programming education ·

Tasks

50 tasks to practise

1. Hello World

2. Greet the user

3. Print the age

4. Unit converter

5. Even or odd

6. Sum of two numbers

7. Area of a square

8. Volume of a sphere

9. Compare two numbers

10. Triangle check

11. Grade to words

12. Hours to seconds

13. Count to N

14. ASCII value

15. Quadratic equation

16. Positive or negative

17. Seasons

18. Reverse a number

19. Temperature converter

20. Longest name

21. Word counter

22. Prime number

23. Day of the week

24. Factorial

25. Valid date

26. Letter frequency

27. Minimum and maximum in an array

28. Sort an array

29. Replace the vowels

30. Sum of the digits

31. Palindrome

32. Strong password

33. Reverse an array

34. Count the elements

35. Prime numbers

36. Average

37. Sort by length

38. Sentence counter

39. Fibonacci numbers

40. Finding the median

41. Character chart

42. Transposing a matrix

43. Paths in a maze

44. Grade statistics

45. Roman numerals

46. Coin toss simulation

47. Symmetric array

48. Balanced brackets

49. Run of equal elements

50. Shortest path in a grid

Tasks from the lessons

Language basics

Functions

Classes and objects

Inheritance and polymorphism

Errors and exceptions

Text and files

Collections

Modern JavaScript

Real-world programs

Let's make a game

Chronicles of Arcanum

Epic Cosmos: the empire

Project

Cookies