Conditions

Problem

The shipping that should not be there

An online shop promises free shipping on orders of €50 and up. Under €50, shipping is €4.99. The order is €62, and the program still adds shipping:

total = 62.0
shipping = 4.99
print(f"To pay: €{total + shipping}")

Output

To pay: €66.99

The program runs every line, one after another. Here it has to choose: one line in one case, another line in another.

The idea

A comparison answers yes or no

First the program has to ask: is the total €50 or more? For this there are comparison operators. Their answer is not a number, but True or False: a value of type bool.

total = 62.0
print(total >= 50)
print(total == 50)
print(total != 50)

Output

True
False
True

You write	It asks
a > b, a < b	greater, less
a >= b, a <= b	greater or equal, less or equal
a == b	are they equal
a != b	are they different

Be careful: == compares, and = stores a value. total == 50 asks whether the total is 50, and total = 50 makes it 50.

Text in Python is compared with the same ==:

answer = "yes"
yes = answer == "yes"
print(yes)

Output

True

More precisely, if you are curious

Decimal numbers are rarely exactly equal. The computer stores 0.1 and 0.2 approximately, so their sum is not exactly 0.3:

print(0.1 + 0.2 == 0.3)
print(0.1 + 0.2)

Output

False
0.30000000000000004

For decimal numbers, ask whether they are close enough: abs(a - b) < 0.000001. Comparisons with > and < do not have this problem.

The idea

if and else: one of two paths

Now the program can choose. if checks the condition after it. If it is true, it runs the block after the colon. If it is not, it runs the block after else:. The block is the lines indented by 4 spaces: there are no braces, the indentation says what is inside.

total = 62.0
if total >= 50:
    shipping = 0
else:
    shipping = 4.99
print(f"Shipping: €{shipping}")

Output

Shipping: €0

For an order of €18.50 the same program prints Shipping: €4.99. Only one of the blocks runs, never both.

else is not required. Without it, if the condition is not true, the block is simply skipped and the program continues after it.

More precisely, if you are curious

• In Python indentation is not for looks, it is part of the code. A line indented as much as the if itself is already outside the block and always runs.
• The condition can be any value, not only True or False. The number 0 and the empty text "" count as false, everything else as true:

total = 0
if total:
    print("There is a total")
else:
    print("0 counts as false")

Output

0 counts as false

• if total = 50: stops even before the program starts, with SyntaxError: Python does not allow = in a condition.

Try it

An order bigger than the stock

There are 3 items in the warehouse, and the customer orders 5.

What will the program print?

stock = 3
ordered = 5
if ordered > stock:
    print("Not enough in stock")
    ordered = stock
print(f"Sending {ordered} items")

Answer

Not enough in stock
Sending 3 items

Sending 5 items

Not enough in stock
Sending 5 items

Check

The idea

elif: more than two paths

A teacher turns test points into a grade. There are four possibilities. Between if and else you put elif (short for else if):

points = 76
if points >= 90:
    print("Excellent")
elif points >= 75:
    print("Very good")
elif points >= 50:
    print("Good")
else:
    print("Fail")

Output

Very good

The checks go from top to bottom. The first branch with a true condition runs, and the rest are skipped. 76 is not >= 90, but it is >= 75, so it is "Very good". The program never gets to >= 50, even though it is true too.

That is why the order matters: the strictest condition comes first. If points >= 50 were at the top, everyone with 50 points or more would get "Good".

points = 76
if points >= 90:
    print("Excellent")
elif points >= 75:
    print("Very good")
elif points >= 50:
    print("Good")
else:
    print("Fail")

Step by step, in words

1. points is 76.
2. 76 >= 90? No. The "Excellent" branch is skipped.
3. 76 >= 75? Yes. We go into the "Very good" branch.
4. It prints "Very good".
5. 76 >= 50 is true too, but it is not checked: the chain stops at the first true branch.

Try it

Find the bug

Everyone with 90 points or more gets "Excellent". A student with exactly 90 points gets "Very good". Which line is the bug on?

Pick the line with the bug.

1points = 902if points > 90:3 print("Excellent")4elif points >= 75:5 print("Very good")6elif points >= 50:7 print("Good")8else:9 print("Fail")

Check

The idea

And, or, not

The cinema gives a discount ticket to children and to students. There are two conditions, and one is enough. You connect them with logical operators:

age = 16
student = False
if age < 18 or student:
    print("Discount ticket: €6")
else:
    print("Ticket: €10")

Output

Discount ticket: €6

In Python the operators are words:

• a and b is true when both are true.
• a or b is true when at least one is true.
• not a is true when a is not true: not student.

Free shipping over €50 and only in the city is total >= 50 and in_city. A number is between 1 and 12 if month >= 1 and month <= 12. In Python the math notation works too, 1 <= month <= 12: it means the same thing.

month = 13
print(1 <= month <= 12)
print(month >= 1 and month <= 12)

Output

False
False

More precisely, if you are curious

• If the left operand already decides the answer, the right one is not checked: with False and … and with True or …. This is called short-circuit evaluation, and sometimes it protects you from an error: count != 0 and total / count > 10 never divides by 0 (in Python, division by 0 stops the program with ZeroDivisionError).
• and comes before or, like * before +. When you mix them, add parentheses so it reads clearly: (a and b) or c.
• A condition can also be inside another if. That is a nested if. This code is the same as if total >= 50 and in_city:

if total >= 50:
    if in_city:
        shipping = 0

Try it

By 5 and by 7

What goes in the blank so the condition is true only for numbers divisible by both 5 and 7?

number = 35
if number % 5 == 0  number % 7 == 0:
    print("Divisible by both 5 and 7")

Check

The idea

The conditional expression if … else

Sometimes the whole if is only there to pick one of two values. There is a short form for that on one line, the conditional expression:

points = 76
message = "You passed!" if points >= 50 else "You did not pass."
print(message)

Output

You passed!

It reads almost like a sentence: the value if it is true, if the condition, else the value if it is not. The same with if and else takes several lines, and here it is one.

In Java, C# and JavaScript the same thing is written with ? : and is called the ternary operator. Python has no ? :. Its equivalent is the conditional expression.

Use it to pick a value. If the program has to do different things in the two cases, write if.

More precisely, if you are curious

• The two values can be of different types: text in one case and a number in the other. The program runs, but afterward you do not know what the variable holds. Pick between values of one type.

points = 40
result = "Passed" if points >= 50 else 0
print(result)

Output

0

• Conditional expressions can be nested inside each other, but code like that is hard to read. For three or more possibilities, write elif.

The idea

match: choosing by value

The shop is open from 9:00 to 18:00 on weekdays, until 14:00 on Saturday, and it is closed on Sunday. If the day is a number (1 is Monday, 7 is Sunday), you compare it with several exact values. For this there is match:

day = 6
match day:
    case 6:
        print("10:00–14:00")
    case 7:
        print("Closed")
    case _:
        print("9:00–18:00")

Output

10:00–14:00

• match takes the value after it and compares it with each case from top to bottom. The first one that matches runs.
• After its case, match is done: the others do not run, and you do not need break.
• case _: is for all other values, like the last else.
• Several values with the same action are written with | in one case: case 6 | 7:.

match has been in Python since 3.10. In older code the same thing is written with if and elif. For checks like "from 50 and up", write if.

More precisely, if you are curious

• A case with a bare name, for example case sunday:, does not compare with the variable sunday. It matches any value and stores it in that name. Here the day is 3, and the program prints "Closed":

sunday = 7
day = 3
match day:
    case sunday:
        print("Closed")

Output

Closed

In a case, write the numbers and text themselves: case 7:, case "+":.

• In Java and C#, switch can also give a value. match cannot: store the value in a variable in each case. An example with |:

day = 7
match day:
    case 6 | 7:
        print("Weekend")
    case _:
        print("9:00–18:00")

Output

Weekend

Exercise

Programs that choose

Solve the tasks in your editor. Each number in the input is on a separate line. Try each solution with input that goes through the other branches too.

1. A cinema ticket. Read the age. Up to 6 years old you get in free, up to 17 the ticket is €6, up to 64 it is €10, from 65 up it is €5. A negative age is an error.

Input

15

Output

The ticket is €6

2. A mini calculator. Read a number, an operation (+, -, * or /) and a second number. Pick the action with match on the text of the operation. You cannot divide by 0.

Input

12
/
5

Output

Result: 2.4

3. A quadratic equation. Read a, b and c from ax² + bx + c = 0. The discriminant is D = b² − 4ac. If D is positive, there are two roots: (−b ± √D) / 2a. If D is 0, there is one root: −b / 2a. If D is negative, there are no real roots. If a = 0, the equation is not quadratic.

Input

1
-3
2

Output

x1 = 2.00, x2 = 1.00

4. A valid date. Read a day, a month and a year and say whether the date exists. Find the number of days in the month with match. February has 29 days in a leap year: divisible by 4 but not by 100, or divisible by 400.

Input

29
2
2023

Output

Invalid date

For 29, 2, 2024 the output is Valid date.

More tasks

• Even or odd. For 7: Odd.
• Positive, negative or zero. For −5: Negative.
• Greater than 10. For 15: Greater than 10.
• Are they equal. Read two numbers. For 5 and 5: Equal.
• The bigger of two. Read two numbers and print the bigger one. Do it once with if and once with … if … else ….
• Absolute value. Without abs: if the number is negative, change its sign.
• Yes or no. Read text. Print True for "yes", False for "no", and Answer not understood for anything else.
• By 5 and by 7. Say whether the number is divisible by both 5 and 7. For 35: yes, for 10: no.
• Three numbers in ascending order. Read three numbers and print them from the smallest to the biggest. For 7, 2, 5: 2 5 7.
• The smallest and the biggest of five. Read five numbers into five variables. Remember the first one as both the smallest and the biggest, then compare each next one with it.
• Day of the week. Read a number from 1 to 7 and print the day: 1 is Monday. For another number: No such day. Try it with match.
• Age group. Up to 12 years old is a child, up to 19 a teenager, otherwise an adult.
• Season by month. December, January and February are winter, March to May is spring, and so on. For 4: Spring. With match, put several values in one case: case 12 | 1 | 2:
• Is it a triangle. Three sides make a triangle if the sum of any two is bigger than the third. For 3, 4, 5: yes, for 1, 2, 3: no.
• A grade in words. Read a grade from 2 to 6 and print Fail, Average, Good, Very good or Excellent.
• Guess the animal. The program asks questions with a "yes" or "no" answer and guesses which animal you thought of. Start with a greeting and a short explanation of the game. You can ask all the questions and then decide with elif, or ask only what you need with nested if statements. The questions:

Question	If "yes"	If "no"
Does it fly?	a bird or an insect	go on
Does it have feathers?	a bird	if it flies, an insect
Does it swim?	if it does not fly: a fish or a small water mammal	go on
Does it meow?	a cat	not a cat
Does it have fur?	a dog, a cat or another mammal	a lizard or another reptile
Is it green?	if it does not fly and does not swim: a frog or a lizard	go on
Does it roar?	a cow or a lion	go on
Does it have a trunk?	an elephant	go on

An example (the input is the answer to each question, in order):

Input

no
no
no
yes
yes
no
no
no

Output

Think of an animal. I will ask you 8 questions. Answer yes or no.
Does it fly?
Does it have feathers?
Does it swim?
Does it meow?
Does it have fur?
Is it green?
Does it roar?
Does it have a trunk?
I think it is a cat.

Test

Check yourself

1. What does 62.0 >= 50 give?

62.0FalseTrue

2. What is the difference between = and ==?

= stores a value, == compares= compares numbers, == compares textThere is no difference

3. The chain if / elif checks points >= 50 first, then points >= 75. What happens with 95 points?

Both runThe second branch runsThe first branch runs

4. There is a discount if age < 18 or age >= 65. Does a 70-year-old get a discount?

It depends on the order of the conditionsYesNo

5. The day is 6. The match has case 6: and case 7: under it. What runs?

The blocks of case 6: and of case 7:Nothing, match does not work without breakOnly the block of case 6:

Check

Start the lesson over