Computer Science · Class 11 · Practice Programs

PracticeOne outcome⏱️ 14 min read

if Statement Programs

Six programs, each built round a single if. Read the problem, plan it, write it — and only then look at the program printed here. Every output on this page is a real run, so if yours differs, yours is telling you something.

The lesson these programs practiseThe if Statement →

Note

What a plain if is for. One outcome. The block runs when the condition is True, and when it is False nothing happens — no error, no message, no stopping. The program simply carries on with the next line at the margin. Half the marks lost on this shape are lost to that half.

1Program 1 — is the number positive?

📋 The problem

Ask the user for a number. If it is positive, say so. The program should end politely either way.

Input

what we ask the user for

one number, num

Process

what we work out

test whether num > 0

Output

what we show

a message, but only when the test is true
a closing line, always

positive_check.py

# print a message only when the number is positive

num = int(input('Enter a number: '))

if num > 0:
    print('That number is positive')

print('Checked')

Output

Enter a number: 7
That number is positive
Checked

Now the run that matters more — the one where the condition is false:

positive_check.py — the same program, run again

Output

Enter a number: -4
Checked

num = int(input('Enter a number: '))

input() always hands back text, so int() turns '7' into the number 7. Without it, num > 0 would be comparing text with a number and Python would refuse.

if num > 0:

The condition. Python works out num > 0 first — with 7 in num that is True, with -4 it is False — and the colon says the indented block below belongs to this line.

print('That number is positive')

Indented by four spaces, so it is the if's block. It runs only when the condition was True.

print('Checked')

Back at the margin, so it belongs to nobody. It runs on both runs — which is how you can tell the program did not stop when the condition was false.

Watch Out

“It printed nothing, so it crashed.” No — a false condition is not an error. Look at the second run: the block was skipped and Checked still printed. If your program seems to do nothing, print something after the if and you will see it is alive.

2Program 2 — a discount on bills over ₹500

📋 The problem

A shop gives 10% off, but only on bills above ₹500. Ask for the bill amount and print what the customer has to pay.

Input

what we ask the user for

the bill amount, bill

Process

what we work out

if bill > 500, work out 10% and take it off

Output

what we show

the discount, when there was one
the amount to pay, always

bill_discount.py

# a 10% discount, but only on bills above 500

bill = float(input('Enter the bill amount: '))

if bill > 500:
    discount = bill * 10 / 100
    bill = bill - discount
    print('Discount given:', discount)

print('Amount to pay:', bill)

Output

Enter the bill amount: 800
Discount given: 80.0
Amount to pay: 720.0

bill_discount.py — a bill that misses the offer

Output

Enter the bill amount: 400
Amount to pay: 400.0

Two things are worth noticing. First, the block has three statements in it — a block is not one line, it is every line indented under the header. Second, bill = bill - discount puts the new amount back into the same box, so the last line prints the reduced amount without knowing whether a discount happened.

Tip

Why float() and not int()? Because a bill can be 499.50, and int('499.50') raises a ValueError. Whenever the value could have a decimal point, read it with float().

3Program 3 — turn a negative number positive

📋 The problem

Ask for a number and print its size, ignoring the minus sign. (−25 and 25 are both a size of 25.)

make_positive.py

# turn a negative number into a positive one

num = int(input('Enter a number: '))

if num < 0:
    num = -num

print('The size of the number is', num)

Output

Enter a number: -25
The size of the number is 25

This is the neatest shape a plain if has: a value is corrected when it needs correcting and left alone otherwise, and the printing happens once, afterwards. A positive number never enters the block, so it reaches the last line untouched.

make_positive.py

num = int(input('Enter a number: '))

if num < 0:
    num = -num

print('The size of the number is', num)

num = int(input('Enter a number: '))

if num < 0:
    num = -num

print('The size of the number is', num)

4Program 4 — does this subject have a practical?

📋 The problem

Four subjects have a practical exam. Ask the user for a subject and warn them if theirs is one of them.

practical_warning.py

# a reminder for the subjects that have a practical exam as well

practical_subjects = ['Computer Science', 'Physics', 'Chemistry', 'Biology']

subject = input('Enter the subject: ')

if subject in practical_subjects:
    print('Remember: this subject has a practical exam too')

print('Timetable printed')

Output

Enter the subject: Computer Science
Remember: this subject has a practical exam too
Timetable printed

practical_warning.py — a subject that is not on the list

Output

Enter the subject: History
Timetable printed

A condition does not have to be about numbers. in asks is this value somewhere in that list? and answers True or False, which is all an if ever needs. Writing this with four separate comparisons joined by or would work too — and would have to be edited every time the school adds a subject.

Watch Out

in on a list is exact. 'computer science' is not 'Computer Science', and 'Physics ' with a stray space is not 'Physics'. Real programs guard against that with subject.strip().title() before the test.

5Program 5 — the scholarship, which needs two things

📋 The problem

A scholarship needs at least 75 marks and at least 80% attendance. Ask for both and say whether the student is eligible.

scholarship.py

# a scholarship needs good marks AND good attendance

marks = int(input('Enter the marks: '))
attendance = int(input('Enter the attendance percentage: '))

if marks >= 75 and attendance >= 80:
    print('Eligible for the scholarship')

print('Application checked')

Output

Enter the marks: 82
Enter the attendance percentage: 91
Eligible for the scholarship
Application checked

scholarship.py — good marks, but the attendance is short

Output

Enter the marks: 82
Enter the attendance percentage: 60
Application checked

and needs both sides to be true, so one short attendance is enough to keep the block shut. Had the rule been “marks or attendance”, or would be the word — and 82 marks alone would have won the scholarship.

Watch Out

Write the second comparison out in full. marks >= 75 and attendance >= 80 is right; marks >= 75 and >= 80 is a SyntaxError. Each side of an and has to be a complete question, with its own value on the left.

6Program 6 — the larger of two numbers, with two `if`s

📋 The problem

Ask for two numbers and say which is larger. Use only plain if statements.

larger_two_ifs.py

# the larger of two numbers, using two separate if statements

a = int(input('Enter the first number: '))
b = int(input('Enter the second number: '))

if a > b:
    print(a, 'is larger')

if b > a:
    print(b, 'is larger')

print('Done')

Output

Enter the first number: 15
Enter the second number: 9
15 is larger
Done

It works. Two independent ifs, and exactly one of them opens… until the two numbers are the same:

larger_two_ifs.py — the same program, with two equal numbers

Output

Enter the first number: 12
Enter the second number: 12
Done

Key Takeaway

Separate ifs leave gaps. Neither 12 > 12 nor 12 > 12 is true, so both blocks are skipped and the user is told nothing. Nothing is broken — the program did exactly what it was asked. This gap is the reason else exists, and it is the next page.

7Recap

One outcome

The block runs, or the program carries on. There is no third possibility and no error either way.

The condition is a question

Anything that comes out True or False will do: a comparison, an and/or of two comparisons, or an in test on a list.

Indentation joins the block to the if

Every line indented under the header belongs to it — one line or five. The first line back at the margin does not.

Separate ifs leave gaps

Two ifs can both be false at once. If every case must be covered, you want else, not a second if.

✍️ Now write these yourself

1
Ask for a number and print Even only when it is even.
Hint · The test is num % 2 == 0. Nothing is printed for an odd number, and that is correct.
2
Ask for the temperature and warn the user if it is above 40.
Hint · Read it with float() — temperatures have decimal points.
3
Ask for a password and print a warning if it is shorter than 8 characters.
Hint · len(password) < 8. No int() here — the password is meant to stay text.
4
Ask for three marks and print Distinction when all three are above 90.
Hint · Two ands in one condition is perfectly normal: a > 90 and b > 90 and c > 90.
5
Ask for a year and add 1 to a leap_count variable if the year divides by 4.
Hint · Start the counter before the if, or the name will not exist when you print it.

Quick Check

A program reads a number and has if num > 100: print('Big'). The user types 40. What happens?

Quick Check

In the discount program, why is print('Amount to pay:', bill) at the margin rather than indented?

Quick Check

Two separate ifs test a > b and b > a. For a = 12 and b = 12 the program prints only 'Done'. Why?

← Previous

🏁 else with a Loop

if...else Programs ⚖️

1Program 1 — is the number positive?

2Program 2 — a discount on bills over ₹500

3Program 3 — turn a negative number positive

4Program 4 — does this subject have a practical?

5Program 5 — the scholarship, which needs two things

6Program 6 — the larger of two numbers, with two ifs

7Recap

6Program 6 — the larger of two numbers, with two `if`s