Python for kids · Module 2

Lesson 22

if and else - one of two

After this lesson: You'll write a program that always picks one of two roads: if or else.

Film to watch

Video

Two roads: if and else 🎬

Play the film ▶ - you'll see that with if and else exactly one road always runs.

Loading the film...

A YouTube video for this lesson will appear here.

Explanation

An if on its own does something or nothing. But often we want to do one of two things: take an umbrella or sunglasses, TAK or NIE, Ola won or Kuba did. That's what the word else is for - otherwise. ☂️😎

How an if-else is built

if temperature < 15:
    print("Put on a jacket")
else:
    print("A T-shirt is enough")

Piece	What it means
`if condition:`	the road when the condition is True
`else:`	the road in every other case - no condition, with a colon
indentation	each road has its own indented lines
`else` in line with `if`	the else belongs to that if

The road with a fork now has two lanes - and the robot always picks exactly one.

Jacket or T-shirt?

8 degrees - cold. The robot takes the if lane.

❓ question⚖️ answer🛣️ choosing a road🤝 together again

🤖1temperature = 8

🚦

2if temperature < 15:

True➜ the if's indented lines

4 spaces = these lines belong to the if

3print("Put on a jacket 🧥")

🧥

False➜ the lines under else

4else:

5print("A T-shirt is enough 👕")

👕

🤝 the roads join

6print("You can go out!")

Click the arrow - 🤖 will walk the program's road, line by line.

Code

1temperature = 8
2if temperature < 15:
3    print("Put on a jacket 🧥")
4else:
5    print("A T-shirt is enough 👕")
6print("You can go out!")

Screen

▍

1/5 · line 1 runs next

A hot day

The same program for 27 degrees - this time the else lane.

❓ question⚖️ answer🛣️ choosing a road🤝 together again

🤖1temperature = 27

🚦

2if temperature < 15:

True➜ the if's indented lines

4 spaces = these lines belong to the if

3print("Put on a jacket 🧥")

🧥

False➜ the lines under else

4else:

5print("A T-shirt is enough 👕")

👕

🤝 the roads join

6print("You can go out!")

Click the arrow - 🤖 will walk the program's road, line by line.

Code

1temperature = 27
2if temperature < 15:
3    print("Put on a jacket 🧥")
4else:
5    print("A T-shirt is enough 👕")
6print("You can go out!")

Screen

▍

1/5 · line 1 runs next

TAK or NIE - like at an olympiad 🎬

Click Next - this is how you answer in contest tasks.

Bajtek

📜TAK or NIE?

1

A question in a task

In olympiad tasks (and in many of our tasks) you often see: "print TAK if the number is positive, otherwise NIE" (TAK is Polish for yes, NIE for no).

Step 1 / 3

TAK or NIE

For -3 the robot takes the else lane.

❓ question⚖️ answer🛣️ choosing a road🤝 together again

🧑‍⚖️1n = -3

🚦

2if n > 0:

True➜ the if's indented lines

4 spaces = these lines belong to the if

3print("TAK")

✅

False➜ the lines under else

4else:

5print("NIE")

❌

🤝 the roads join

Click the arrow - 🧑‍⚖️ will walk the program's road, line by line.

Code

1n = -3
2if n > 0:
3    print("TAK")
4else:
5    print("NIE")

Screen

▍

1/4 · line 1 runs next

Even or odd? 🔢

Type a number 🔢 and press Enter ⏎. An even number divides by 2 with no remainder - more about it in lesson 24.

❓ question⚖️ answer🛣️ choosing a road🤝 together again

🤖1n_str = input()

2n = int(n_str)

🚦

3if n % 2 == 0:

True➜ the if's indented lines

4 spaces = these lines belong to the if

4print("Even 👯")

👯

False➜ the lines under else

5else:

6print("Odd 🧍")

🧍

🤝 the roads join

Click the arrow - 🤖 will walk the program's road, line by line.

Code

1n_str = input()
2n = int(n_str)
3if n % 2 == 0:
4    print("Even 👯")
5else:
6    print("Odd 🧍")

Screen

▍

1/2 · line 1 runs next

Each road can calculate

The if and else roads can hold any lines - for example putting a value into a box. That's how we find the bigger of two numbers:

if a > b:
    bigger = a
else:
    bigger = b
print(bigger)

Each road puts something into the box bigger - so after the if-else the box is surely full.

The bigger of two on the podium

The bars are the numbers a and b - the crown goes on the bigger one.

?

🧒

?

👧

📦 a📦 b

Click the arrow - the numbers will stand on the podium as bars.

Code

1a = 4
2b = 9
3if a > b:
4    bigger = a
5else:
6    bigger = b
7print(bigger)

Screen

▍

1/6 · line 1 runs next

Who has more apples? 🍎

Type the number of apples Ola 👧 has, then Kuba 🧒 - two different numbers, Enter ⏎ after each.

❓ question⚖️ answer🛣️ choosing a road🤝 together again

🍎1ola_str = input()

2ola = int(ola_str)

3kuba_str = input()

4kuba = int(kuba_str)

🚦

5if ola > kuba:

True➜ the if's indented lines

4 spaces = these lines belong to the if

6print("Ola has more")

👧

False➜ the lines under else

7else:

8print("Kuba has more")

🧒

🤝 the roads join

Click the arrow - 🍎 will walk the program's road, line by line.

Code

1ola_str = input()
2ola = int(ola_str)
3kuba_str = input()
4kuba = int(kuba_str)
5if ola > kuba:
6    print("Ola has more")
7else:
8    print("Kuba has more")

Screen

▍

1/2 · line 1 runs next

And when they both have the same?

Try typing two equal numbers in the previous example. The program says "Kuba has more" - and that's not true! When ola > kuba is False, else always runs - also in a draw. How do we add a third road "A draw"? That's in lesson 23!

Traps with else

Code	What's wrong
`else`	no colon
`else n < 0:`	else has no condition
`else:` indented differently from `if`	else must stand in line with the if
something between the if's indented lines and `else`	else must come right after the if's indented lines

What will this program print?

x = 5
if x > 10:
    print("A")
else:
    print("B")
print("C")

Which else line is correct?

How many roads of an if-else can run at once?

Remember

else: is the road for every other case - when the if's condition is False.
else has no condition, it has a colon and stands in line with the if.
With if-else exactly one road always runs.
if raining: - the condition can be a variable holding True or False.
We print the answer TAK / NIE exactly like that - in capital letters, no full stop.

The tasks are waiting! In many of them the answer is TAK or NIE.

Tasks from the video

We solve them together in the video - pause it and try first on your own.

1Positive?2Even?3Passed?4The bigger of two 5Who has more?6A cinema ticket 7The distance 8Free delivery

Practice tasks

Try to solve them without the video. Stuck? Every task has hints and an editorial.

9More than a hundred?10A round number?11Hot or cold?12Half or double

Sign in to save your progress.

Watch the film

1. Title

Show the film's text

0:00Title

Narrator: Umbrella or sunglasses? Today you'll see how a program picks one of two roads.

Bajtek: Hi! Come with me!

0:10A rainy Monday

Narrator: It's Monday morning. Bajtek is getting ready for school.

Narrator: Before he goes out, he walks to the window. He has to check something.

Bajtek: Hmm... Is it raining?

Narrator: This question has only two answers: yes or no.

Narrator: Bajtek looks out of the window...

Narrator: It's raining! The answer is: yes.

Bajtek: It's raining, so I'll take my umbrella!

Narrator: Bajtek opens the door and goes out. Under the umbrella he stays dry!

0:50A sunny Tuesday

Narrator: The next day is Tuesday.

Bajtek: Is it raining today?

Narrator: Bajtek looks out of the window...

Narrator: The sun is shining! No rain. The answer is: no.

Guess! The sun is shining. What will Bajtek take? (☂️ An umbrella / 😎 Sunglasses / ☂️😎 Both)

Bajtek: No rain... I'll take my sunglasses!

Narrator: Bajtek goes to school. With his sunglasses on!

1:16The fork in the road

Narrator: Bajtek's choice is like a road that splits in two.

Narrator: Where it splits, there's a sign. The sign asks: is it raining?

Narrator: Raining? Then the answer is yes.

Narrator: So Bajtek takes the top road, the green one. To the umbrella!

Narrator: Not raining? Then the answer is no.

Narrator: So he takes the bottom road... the orange one. To the sunglasses!

Narrator: At a fork, you always take one road. Never both. Never none.

1:58if on its own

Narrator: Do you remember if?

Narrator: We met it in the last lesson.

Narrator: It's the one that asks: is it raining?

Narrator: Under the if, we write what should happen. Like: Take an umbrella.

Narrator: But this line doesn't start at the edge.

Narrator: We press Tab... and the line moves four spaces right.

Narrator: A line moved like that is indented. It belongs to the if.

Narrator: When it's raining, if runs its indented line.

Narrator: The screen shows: Take an umbrella.

Narrator: And when it's not raining? The indented line is skipped.

Narrator: The screen shows: nothing.

Bajtek: But then I want my sunglasses! I need a second road!

2:52The second road: else

Narrator: The second road has its own word: else.

Narrator: else means: otherwise.

Narrator: So: when it's not raining.

Guess! When will the line under else run? (☂️ When it's raining / 😎 When it's not raining / 🔁 Always)

Narrator: Look! else has no question. The if already asked it.

Narrator: It only has a colon.

Narrator: else lines up with if. No spaces in front.

Narrator: Under else comes an indented line: Take sunglasses.

Narrator: The indent shows which road a line belongs to.

Narrator: This one to if. And this one to else.

Narrator: Done! The program has two roads. Just like the fork!

3:40The program on a rainy day

Narrator: Let's run the program. First, on a rainy day.

Narrator: The first line puts True into the box raining. Because it's raining.

Narrator: Now the if. The sign asks: is it raining?

Guess! raining = True. What will the screen show? (☂️ Take an umbrella / 😎 Take sunglasses / 🤷 Nothing)

Narrator: The box holds True. The green if road opens.

Narrator: And the orange road is closed.

Narrator: Bajtek takes the if road. The program runs its line.

Narrator: The screen shows: Take an umbrella.

Narrator: The line under else is skipped. Today that road is closed.

Bajtek: I've got my umbrella! Off to school.

4:26The program on a sunny day

Narrator: And now, a sunny day.

Narrator: Now the box raining holds False. Because it's not raining.

Narrator: Again the if asks: is it raining?

Guess! raining = False. Tap the line that prints something.

Narrator: The box holds False. The green if road is closed.

Narrator: The program jumps over to else...

Narrator: ...and runs the line under it: Take sunglasses.

Bajtek: Sunglasses on!

5:01The rule

Narrator: Remember Bajtek's rule.

Narrator: When it's True, the green if road runs.

Narrator: When it's False, the orange else road runs.

Narrator: Exactly one road always runs. Never both. Never none.

Narrator: Your turn! Choose whether it's raining, and run Bajtek's program.

Your turn Choose whether it's raining, then press Run.

Narrator: Now build Bajtek's program yourself.

Guess! Build Bajtek's program: tap the lines in order.

Narrator: One more, last question.

Guess! What does this program print? (Take an umbrella / Take sunglasses / Take an umbrella + Take sunglasses)

Bajtek: Great! Now you know if and else!