Lesson 28

Checking the data - before we calculate

After this lesson: You'll set up a guard: the program first checks the data and only then calculates - and doesn't crash.

Video

The video for this lesson is coming soon.

Explanation

Remember how in lesson 13 the program crashed when dividing by zero? Now that you know ifs, you can set up a guard 🛡️: first check whether the data makes sense, and only then calculate. Such a program won't crash, even when someone types something strange!

The guard at the division 🎬

Click Next - an if can save the program from an error.

Bajtek

🍬🍬🍬🍬🍬🍬÷0➡️💥

Code

sweets = 6

children = 0

print(sweets // children)

🖥️ Screen

ZeroDivisionError: integer division or modulo by zero

💥 the program crashed

Sweets for nobody

The program shares sweets 🍬 between children: sweets // children. But today nobody came - children = 0. Python tries to divide by zero and... ZeroDivisionError! The program crashes.

Step 1 / 4

Without a guard

Dividing by zero - the program crashes on the last line.

6 to share

🍬🍬🍬🍬🍬🍬

🪑 🪑 🪑0 children - nobody to give them to!

Click the arrow. In a moment Python tries to divide 6 by 0.

Code

1sweets = 6
2children = 0
3print(sweets // children)

Screen

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With a guard

The guard checks for zero before we get to the division - the program ends calmly.

🛡️1sweets = 6

2children = 0

if children == 0

4print("Nobody to give them to 🍬")

✋

elsein every other case

6print(sweets // children)

🍬

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

Code

1sweets = 6
2children = 0
3if children == 0:
4    print("Nobody to give them to 🍬")
5else:
6    print(sweets // children)

Screen

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Sharing sweets safely 🍬

Type the number of sweets 🍬, then the number of children 🧒, Enter ⏎ after each. Try 0 children too - the program won't crash!

🛡️1sweets_str = input()

2sweets = int(sweets_str)

3children_str = input()

4children = int(children_str)

if children == 0

6print("Nobody to give them to 🍬")

✋

elsein every other case

8print("Each gets:", sweets // children)

9print("Left over:", sweets % children)

🍬

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

Code

1sweets_str = input()
2sweets = int(sweets_str)
3children_str = input()
4children = int(children_str)
5if children == 0:
6    print("Nobody to give them to 🍬")
7else:
8    print("Each gets:", sweets // children)
9    print("Left over:", sweets % children)

Screen

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What can go wrong?

A guard is useful not only for zero. The data can also be nonsense - the program won't crash, but it will print silly things:

Data	Problem	Guard
children = 0	dividing by zero	`if children == 0:`
age = -5	an age can't be negative	`if age < 0:`
month = 13	there are only 12 months	`if month < 1 or month > 12:`
a withdrawal bigger than the money in the bank	it can't be paid out	`if withdrawal > balance:`

A clever trick: the guard checks whether the data is wrong (with or), and all the calculating sits in the else.

Bajtek's cash machine 🎬

Click Next - the guard makes sure no more leaves the account than there is in it.

Bajtek

🏦50 zloty-💸80 zloty=-30

50 zloty in the account

Ola has 50 zloty in the bank 🏦. Ola wants to withdraw 80 zloty for a bike. Without a guard the program would do 50 - 80 = -30 - and the bank would be surprised!

Step 1 / 3

The cash machine 🏧

The account holds 50 zloty. Type the amount to withdraw 💸 and press Enter ⏎ - try more and less than 50!

🏧1balance = 50

2withdrawal_str = input()

3withdrawal = int(withdrawal_str)

if withdrawal > balance

5print("Not enough money 🛑")

🛑

elsein every other case

7balance = balance - withdrawal

8print("Paid out! Left:", balance)

💰

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

Code

1balance = 50
2withdrawal_str = input()
3withdrawal = int(withdrawal_str)
4if withdrawal > balance:
5    print("Not enough money 🛑")
6else:
7    balance = balance - withdrawal
8    print("Paid out! Left:", balance)

Screen

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Which month is it? 📆

Type a month number 📆 and press Enter ⏎. Try 13 or 0 too - the guard will stop them!

📆1month_str = input()

2month = int(month_str)

if month < 1 or month > 12

4print("There is no such month!")

🛑

elif month <= 2 or month == 12

6print("Winter ❄️")

❄️

elif month <= 5

8print("Spring 🌷")

🌷

elif month <= 8

10print("Summer ☀️")

☀️

elsein every other case

12print("Autumn 🍂")

🍂

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

Code

1month_str = input()
2month = int(month_str)
3if month < 1 or month > 12:
4    print("There is no such month!")
5elif month <= 2 or month == 12:
6    print("Winter ❄️")
7elif month <= 5:
8    print("Spring 🌷")
9elif month <= 8:
10    print("Summer ☀️")
11else:
12    print("Autumn 🍂")

Screen

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Where should the guard (an if) checking for zero stand?

Which condition catches a wrong month number?

Remember

Check first, then calculate - the guard (an if) stands before the dangerous spot.
Dividing by zero: if children == 0:, and the division itself in the else.
Nonsense data (a negative age, the 13th month) is worth stopping too.
Wrong data is often caught with or: if m < 1 or m > 12:.

On to the tasks! In each one first think: what data could break the program?

Tasks from the video

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

1Safe division 2Sweets for the children 3A correct month 4The cash machine 5A school mark 6A time on the clock 7The average of marks 8A ticket with an age check

Practice tasks

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

9A percentage 10A day of the month 11The lift 12The area of a rectangle