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Computer Science · Class 11 · Practice Programs
PracticeKnown count⏱️ 16 min read

for Loop Programs

A for loop is for the jobs where you know how many times before you start — ten rows of a table, every letter of a word, every mark in a list. Nearly all of these programs are the same three lines in a different order: start a collector, add to it inside the loop, print it after.

The lesson these programs practiseThe for Loop →
Note
The collector, and where it goes. Three places, three different programs. Above the loop, a variable is created once and survives every round — that is a collector. Inside the loop it is wiped at the start of each round. After the loop it is too late to collect anything. Most wrong answers on this page are a right program with the collector one line out of place.

1Program 1 — the multiplication table of a number

📋 The problem

Ask for a number and print its table from 1 to 10, in the form 7 x 3 = 21.

Input
what we ask the user for
  • the number, num
Process
what we work out
  • multiply it by each of 1 to 10 in turn
Output
what we show
  • ten lines, one per row of the table
table.py
# the multiplication table of any number, from 1 to 10

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

for i in range(1, 11):
    print(num, 'x', i, '=', num * i)
Output
Enter a number: 7
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70
Watch Out
range(1, 11), not range(1, 10). The stop value is excluded, so range(1, 10) stops at 9 and the table is one row short. A table that ends at 9 is the single commonest range() mistake — and it looks fine until somebody counts the rows.

2Program 2 — the sum of the first n numbers

📋 The problem

Ask for a number n and print the total of all the whole numbers from 1 to n.

sum_n.py
# the sum of the first n natural numbers

n = int(input('Enter a number: '))
total = 0

for i in range(1, n + 1):
    total = total + i

print('The sum of the first', n, 'numbers is', total)
Output
Enter a number: 10
The sum of the first 10 numbers is 55
total = 0

Above the loop, so it is made once. Zero is the right starting value for a sum: adding it to anything changes nothing.

for i in range(1, n + 1):

n + 1 as the stop, because the stop is excluded and n itself has to be counted. With n = 10 the values handed out are 1 to 10.

total = total + i

The right-hand side is worked out first — the old total plus this round's number — and the answer goes back into the same box. Ten rounds, ten additions.

print('The sum of the first', n, 'numbers is', total)

At the margin, so it runs once, after the loop has finished. Indent it and you would get ten lines showing the running total.

Watch Out
Put total = 0 inside the loop and the answer becomes n. Each round would wipe the total and add just that round's number, so after the last round the total is whatever the last number was. No error — just an answer that happens to be right when n is 1.

3Program 3 — the factorial of a number

📋 The problem

The factorial of 5 is 1 × 2 × 3 × 4 × 5 = 120. Ask for a number and print its factorial.

factorial.py
# the factorial of a number: 5! = 1 x 2 x 3 x 4 x 5

n = int(input('Enter a number: '))
fact = 1

for i in range(1, n + 1):
    fact = fact * i

print('The factorial of', n, 'is', fact)
Output
Enter a number: 5
The factorial of 5 is 120

The same shape as the sum, with one change that is the whole lesson: fact starts at 1, not 0. The starting value has to be the one that does nothing to the operation — 0 for adding, 1 for multiplying.

factorial_wrong.py
# the same factorial, but the collector starts at 0

n = int(input('Enter a number: '))
fact = 0

for i in range(1, n + 1):
    fact = fact * i

print('The factorial of', n, 'is', fact)
Output
Enter a number: 5
The factorial of 5 is 0
Key Takeaway
Zero times anything is zero — for ever. The first round computes 0 * 1, and from then on there is no way back: every later round multiplies zero by something. The program runs perfectly and answers 0 for every number you give it.

4Program 4 — count the vowels in a word

📋 The problem

Ask for a word and count how many of its letters are vowels. Capitals count too.

count_vowels.py
# count the vowels in a word

word = input('Enter a word: ')
count = 0

for letter in word:
    if letter.lower() in 'aeiou':
        count = count + 1

print('The word has', count, 'vowels')
Output
Enter a word: banana
The word has 3 vowels
count_vowels.py — a word with none at all
Output
Enter a word: rhythm
The word has 0 vowels

A for loop does not need range(). Given a string it hands out one letter at a time, in order, and stops when the string runs out — so the loop counts the letters without anybody having to say how many there are.

This is also the first program here with an if inside a loop. The indentation tells the whole story: the if is indented four spaces because it belongs to the loop, and count = count + 1 is indented eight because it belongs to the if. Pull that line back to four spaces and every letter is counted, vowel or not.

count_vowels.py

5Program 5 — the highest mark in a list

📋 The problem

A list holds five marks. Find the highest, without using max().

highest_mark.py
# the highest mark in a list, found the long way

marks = [56, 91, 43, 78, 65]
highest = marks[0]

for m in marks:
    if m > highest:
        highest = m

print('The marks are', marks)
print('The highest mark is', highest)
Output
The marks are [56, 91, 43, 78, 65]
The highest mark is 91

The collector here is not a total but a champion: the best value seen so far. Every round asks one question — is this one better? — and replaces the champion when the answer is yes.

Watch Out
Start it at marks[0], not at 0. Starting at 0 looks harmless and quietly breaks the moment every mark is negative — a list of temperatures like [-4, -9, -2] would report a highest of 0, a value that is not in the list at all. The first item of the list is always a safe champion, because it is a real member of it.

6Program 6 — reverse a word

📋 The problem

Ask for a word and print it backwards.

reverse_word.py
# reverse a word, one letter at a time

word = input('Enter a word: ')
backwards = ''

for letter in word:
    backwards = letter + backwards

print('Reversed:', backwards)
Output
Enter a word: python
Reversed: nohtyp

The collector is a string this time, and it starts empty — '' is to joining what 0 is to adding. The trick is the order: letter + backwards puts each new letter in front of everything collected so far. Write backwards + letter instead and you rebuild the word exactly as it was.

Tip
Trace it on paper with a three-letter word. Starting from '': 'c', then 'ac', then 'tac'. Three rounds, and the word is inside out. Tracing three rounds by hand is faster than staring at six lines.

7Program 7 — the total and average of a list

📋 The problem

A list holds five marks. Print their total and their average.

marks_average.py
# the total and the average of a list of marks

marks = [72, 65, 88, 91, 54]
total = 0

for m in marks:
    total = total + m

average = total / len(marks)

print('Total:', total)
print('Average:', average)
Output
Total: 370
Average: 74.0

len(marks) rather than 5, so adding a sixth mark to the list needs no other change. And the average is worked out after the loop — inside it, you would be dividing a part-finished total by the full count, which is not an average of anything.

Tip
Python already has sum() and len(). total = sum(marks) does the loop's job in one line, and in real code that is what you would write. Exam questions usually ask for the loop, because the loop is the part being examined — and because sum() cannot help you the moment the rule gets fussier, as it does on the continue page.

8Recap

Start the collector above the loop

0 for a sum, 1 for a product, '' for a string, marks[0] for a champion. Inside the loop it is wiped every round.

Print after the loop

At the margin, once. Indented, you get one line per round showing the running value — sometimes useful, rarely what was asked.

range(1, n + 1) to reach n

The stop is excluded. Any program whose answer is one short is nearly always this.

A for loop takes any sequence

range() for counting, a string for its letters, a list for its items. Same loop, no length needed.

✍️ Now write these yourself
  1. 1

    Print the table of a number from 1 to 20 instead of 1 to 10.

    Hint · Only the stop value changes — and remember it is excluded.

  2. 2

    Print all the even numbers from 1 to 50, using the loop's step rather than an if.

    Hint · range(2, 51, 2) — the third number is how far to jump.

  3. 3

    Ask for a number and print the sum of its table (7 + 14 + … + 70).

    Hint · The table program and the sum program, joined: one collector, one loop.

  4. 4

    Count how many marks in a list are 33 or above.

    Hint · A counter above the loop, an if inside it. The counter goes up by 1, not by the mark.

  5. 5

    Print the first ten numbers of the Fibonacci series: 0, 1, 1, 2, 3, 5…

    Hint · Two variables, a and b. Print a, then set both at once from the old pair — work the new value out before you overwrite anything.

Quick Check

A program totals 1 to n but has total = 0 written inside the loop instead of above it. For n = 10, what does it print?

Quick Check

Why does the factorial program start fact at 1?

Quick Check

In the reverse program, what does backwards = backwards + letter print for 'python'?

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