Computer Science · Class 11 · Sample Programs

Sample programBrackets matter⏱️ 6 min read

Average of Three Numbers

The problem: ask the user for the marks of three subjects and display the average. The maths is from primary school. The trap is one pair of brackets — and if you get it wrong, Python will not tell you.

1Plan it first

Input

what we ask the user for

three marks: a, b, c

Process

what we work out

total = a + b + c
avg = total / 3

Output

what we show

the average, with a message

Notice the process is written as two steps. You could do it in one, but splitting it is easier to read and much easier to check — and it makes the brackets look after themselves.

2The program

average.py

# program to find the average of three numbers

a = float(input('Enter the first number: '))
b = float(input('Enter the second number: '))
c = float(input('Enter the third number: '))

total = a + b + c
avg = total / 3

print('Total =', total)
print('Average =', avg)

Output

Enter the first number: 40
Enter the second number: 50
Enter the third number: 60
Total = 150.0
Average = 50.0

Why float() and not int()? Not because the answer has a decimal point — it has one either way, since / always produces a float, even for 150 / 3. The reason is the user. You do not know whether they will type 50 or 50.5, and the two functions are not equally forgiving:

Python prompt — interactive mode

>>> float('50') # a whole number? fine

50.0

>>> float('50.5') # a decimal? also fine

50.5

>>> int('50') # a whole number? fine

>>> int('50.5') # a decimal? refused

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ValueError: invalid literal for int() with base 10: '50.5'

float() accepts everything int() accepts, and more. int() is the fussy one — it rejects any text with a decimal point in it, even '50.0'. So float() is the safer choice whenever the value could sensibly be either, which makes the program work for whoever runs it rather than only for the person who wrote it.

So when would you use int()?

When a decimal would be wrong, not merely unlikely — a number of shirts, a count of students, a year. There int() refusing 2.5 is the program protecting itself. For a measurement, a mark or an amount of money, reach for float().

Try it with three marks of your own:

average.py

a = float(input('Enter the first number: '))
b = float(input('Enter the second number: '))
c = float(input('Enter the third number: '))

total = a + b + c
avg = total / 3

print('Total =', total)
print('Average =', avg)

a = float(input('Enter the first number: '))
b = float(input('Enter the second number: '))
c = float(input('Enter the third number: '))

total = a + b + c
avg = total / 3

print('Total =', total)
print('Average =', avg)

3Doing it in one line — carefully

You can write the calculation as a single line, and this is where the marks are lost. These two lines look almost identical and give completely different answers:

two_ways.py

a = 40
b = 50
c = 60

wrong = a + b + c / 3
right = (a + b + c) / 3

print('wrong gives', wrong)
print('right gives', right)

Output

wrong gives 110.0
right gives 50.0

Division runs before addition, so the first line divides only c by 3 and then adds it to the other two. The brackets in the second line force the addition to happen first — which is what “the average of three numbers” actually means.

No error message, ever

a + b + c / 3 is perfectly legal Python. It runs, it prints, and it is wrong. That makes it a logical error — the kind only you can catch. Test with three numbers whose average you already know (40, 50, 60 → 50) and a wrong answer stands out at once.

4Now you try

The program below has the bug in it on purpose. Run it with 40, 50 and 60 — it will say 110.0. Fix the brackets so it says 50.0, then extend it to print the percentage as well (the total out of 300).

fix_average.py

a = float(input('Enter the first number: '))
b = float(input('Enter the second number: '))
c = float(input('Enter the third number: '))

avg = a + b + c / 3

print('Average =', avg)

a = float(input('Enter the first number: '))
b = float(input('Enter the second number: '))
c = float(input('Enter the third number: '))

avg = a + b + c / 3

print('Average =', avg)

5Recap

Key Takeaway

Average = total ÷ how many. In code that is (a + b + c) / 3 — the brackets are not decoration, because / runs before +. Splitting the work into total and then avg avoids the trap altogether and reads better.

Quick Check

With a = 40, b = 50, c = 60, what does a + b + c / 3 print?

Quick Check

Why is float() the safer choice for reading these three marks?

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