Lesson 26

An if inside an if

After this lesson: You'll ask a second question only when the answer to the first one is True.

Video

The video for this lesson is coming soon.

Explanation

Any line can be one of an if's indented lines - even... another if! It's like a castle with chambers: first the gate, and only inside it the doors to the next rooms. 🏰

A castle with chambers 🎬

Click Next - the second question is asked only behind the first gate.

Bajtek

🎟️➡️🏰

Code

if ticket:

print("You enter the castle")

else:

print("Buy a ticket first")

🖥️ Screen

The castle gate

You enter the castle 🏰 with a ticket. Without one the guard says: "Buy a ticket first" - and nothing else happens.

Step 1 / 5

The castle: Ola, age 10

A fork inside a fork: the robot goes through the gate, and inside it picks another road.

👧1ticket = True

2age = 10

if ticket

4print("You enter the castle 🏰")

if age >= 12

6print("You may go up the tower")

🗼

elsein every other case

8print("Welcome to the garden 🌷")

🌷

🏰

elsein every other case

10print("Buy a ticket first")

🎟️

Click the arrow - I'll walk the program's path, line by line.

Code

1ticket = True
2age = 10
3if ticket:
4    print("You enter the castle 🏰")
5    if age >= 12:
6        print("You may go up the tower")
7    else:
8        print("Welcome to the garden 🌷")
9else:
10    print("Buy a ticket first")

Screen

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The castle: no ticket

The first gate is closed - the whole inside of the castle is grey, because the robot doesn't go in.

🧒1ticket = False

2age = 14

if ticket

4print("You enter the castle 🏰")

if age >= 12

6print("You may go up the tower")

🗼

elsein every other case

8print("Welcome to the garden 🌷")

🌷

🏰

elsein every other case

10print("Buy a ticket first")

🎟️

Click the arrow - I'll walk the program's path, line by line.

Code

1ticket = False
2age = 14
3if ticket:
4    print("You enter the castle 🏰")
5    if age >= 12:
6        print("You may go up the tower")
7    else:
8        print("Welcome to the garden 🌷")
9else:
10    print("Buy a ticket first")

Screen

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Levels of indentation

Line	Spaces	Belongs to
`if ticket:`	0	the main program
`print("You enter...")`	4	the indented lines of `if ticket`
`if age >= 12:`	4	the indented lines of `if ticket`
`print("You may go...")`	8	the indented lines of `if age >= 12`
`else:` (under `if age`)	4	the else of the inner if
`else:` (under `if ticket`)	0	the else of the outer if

Which else belongs to which if? The one standing in line with it - the same number of spaces.

The pool 🏊

People from 140 cm 📏 may use the big pool. Type the height, then 1 if you can swim, or 0 if you can't - Enter ⏎ after each number.

🏊1height_str = input()

2height = int(height_str)

3swims_str = input()

4swims = int(swims_str)

if height >= 140

if swims == 1

7print("The big pool 🏊")

🛟

elsein every other case

9print("A swimming lesson first")

🦆

🏊

elsein every other case

11print("The paddling pool 🦆")

Click the arrow - I'll walk the program's path, line by line.

Code

1height_str = input()
2height = int(height_str)
3swims_str = input()
4swims = int(swims_str)
5if height >= 140:
6    if swims == 1:
7        print("The big pool 🏊")
8    else:
9        print("A swimming lesson first")
10else:
11    print("The paddling pool 🦆")

Screen

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The biggest of three numbers 🎬

Click Next - a tournament where every match is one if.

Bajtek

a = 4b = 9c = 7

A tournament of three numbers

Three numbers a, b and c play a tournament: the biggest wins. First the match a against b.

Step 1 / 3

The tournament on the podium

Watch which bars are compared - and where the crown ends up.

🧒

👧

🧑

📦 a📦 b📦 c

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

Code

1a = 4
2b = 9
3c = 7
4if a > b:
5    if a > c:
6        biggest = a
7    else:
8        biggest = c
9else:
10    if b > c:
11        biggest = b
12    else:
13        biggest = c
14print(biggest)

Screen

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The tournament as a road

The same program - two levels of forks.

🏆1a = 4

2b = 9

3c = 7

if a > b

if a > c

6biggest = a

elsein every other case

8biggest = c

elsein every other case

if b > c

11biggest = b

elsein every other case

13biggest = c

14print(biggest)

Click the arrow - I'll walk the program's path, line by line.

Code

1a = 4
2b = 9
3c = 7
4if a > b:
5    if a > c:
6        biggest = a
7    else:
8        biggest = c
9else:
10    if b > c:
11        biggest = b
12    else:
13        biggest = c
14print(biggest)

Screen

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An if inside an if, or and?

Sometimes an if inside an if can be replaced with and - when there's no else inside:

if height >= 140:
    if swims == 1:
        print("The big pool")

is the same as:

if height >= 140 and swims == 1:
    print("The big pool")

But when every situation has a different answer (the big pool, a swimming lesson, the paddling pool), an if inside an if is clearer - you can see every road separately.

A leap year 🎬

Click Next - a real puzzle from the calendar, solved with an if inside an if.

Bajtek

📅29 February

An extra day

A leap year has 29 February - 366 days. The rule: a year is a leap year when it divides by 4... but with exceptions!

Step 1 / 3

Is it a leap year? 📅

Type a year 📅 (e.g. 2024, 2026, 1900 or 2000) and press Enter ⏎ - go through the three gates.

📅1year_str = input()

2year = int(year_str)

if year % 4 == 0

if year % 100 == 0

if year % 400 == 0

6print("TAK")

elsein every other case

8print("NIE")

elsein every other case

10print("TAK")

elsein every other case

12print("NIE")

Click the arrow - I'll walk the program's path, line by line.

Code

1year_str = input()
2year = int(year_str)
3if year % 4 == 0:
4    if year % 100 == 0:
5        if year % 400 == 0:
6            print("TAK")
7        else:
8            print("NIE")
9    else:
10        print("TAK")
11else:
12    print("NIE")

Screen

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What will this program print?

x = 5
if x > 0:
    if x > 10:
        print("A")
    else:
        print("B")
else:
    print("C")

How many spaces does an indented line of an if have, when that if is itself indented inside another if?

Was 1900 a leap year?

Remember

An if's indented lines can include another if - a question behind a gate.
The inner if is checked only when the outer condition was True.
Every level is 4 more spaces: 4, 8, 12...
An else belongs to the if it stands in line with.
With no else inside, an if inside an if can be replaced with and.

On to the tasks! Draw the gates and doors in your head - or on paper.

Tasks from the video

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

1The castle 2The biggest of three 3A leap year 4Weather and clothes 5The quarter 6The cinema 7A parcel 8A big even number

Practice tasks

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

9The smallest of three 10Who is positive?11The roller coaster 12A number from both sides