Chapter 5: Loops

How Does It Work?

For Loops: Iterating Over a Sequence

A for loop goes through each item in a sequence — one at a time. A sequence can be a range(), a list, or even a string (where each character is an item).

The basic structure is:

# Structure of a for loop
for item in sequence:
    # do something with item
    print(item)

# Example: looping through each letter in a word
for letter in "Python":
    print(letter)

Each time around the loop is called an iteration. The loop variable (like item or letter) automatically takes on the next value in the sequence with each iteration. You do not need to update it yourself — Python handles that for you.

The range() Function

The range() function generates a sequence of numbers. It is one of the most useful tools for for loops. There are three ways to use it:

Notice that range() always stops before the end number. This catches many beginners out — if you want to count from 1 to 10, you need range(1, 11), not range(1, 10).

While Loops: Repeat While a Condition is True

A while loop keeps repeating as long as its condition remains True. You must make sure the condition eventually becomes False, or the loop will never stop.

The basic structure is:

# Structure of a while loop
count = 1

while count <= 5:
    print("Count is:", count)
    count = count + 1

print("Loop finished!")

Notice the three essential parts of a while loop:

1. Initialise the variable before the loop (count = 1)
2. Test the condition at the start of each iteration (while count <= 5)
3. Update the variable inside the loop (count = count + 1)

If you forget step 3, you will create an infinite loop!

Count-Controlled vs Condition-Controlled Loops

This is essential exam terminology that you absolutely must know for your GCSE. Exam boards use these terms frequently, so make sure you understand the difference.

Here is an easy way to decide which to use:

• "Do this 10 times" → You know the number → for loop (count-controlled)
• "Keep going until the user says stop" → You do not know when → while loop (condition-controlled)
• "Process every item in this list" → You know the items → for loop (count-controlled)
• "Keep guessing until correct" → Could be 1 guess or 100 → while loop (condition-controlled)

break and continue: Controlling Loop Flow

Sometimes you need to exit a loop early or skip certain steps. Python gives you two keywords for this:

• break — immediately exits the loop entirely
• continue — skips the rest of the current iteration and moves to the next one

# Using break to exit a loop early
print("--- break example ---")
for i in range(1, 11):
    if i == 6:
        print("Stopping at 6!")
        break
    print(i)

# Using continue to skip an iteration
print("\n--- continue example ---")
for i in range(1, 6):
    if i == 3:
        continue  # skip number 3
    print(i)

Think of break as an emergency exit — you leave the entire loop immediately. Think of continue as skipping a song on a playlist — you move to the next one without finishing the current one.

Trace Tables: Following a Loop Step by Step

A trace table is a technique for tracking the values of variables as a program runs, line by line. They are essential for your GCSE exam — you will almost certainly be asked to complete one. Trace tables are particularly useful for understanding loops because they show you exactly what happens at each iteration.

Consider this simple while loop:

# Simple doubling program
x = 1
count = 0

while x < 20:
    x = x * 2
    count = count + 1
    print("Iteration", count, "- x =", x)

Here is the trace table for this program:

On iteration 5, x becomes 32. When the loop checks the condition again, 32 < 20 is False, so the loop stops. The final value of x is 32 and count is 5.

Worked Example: Finding the Average

Let us work through a complete example step by step. We want to calculate the average of a list of test scores using a for loop and the accumulator pattern.

The problem: A teacher has five test scores: 72, 85, 90, 68, and 95. Calculate the average score.

Step 1: Store the scores in a list and set up an accumulator variable.

Step 2: Loop through each score, adding it to the total.

Step 3: Divide the total by the number of scores.

# Worked Example: Finding the average test score
scores = [72, 85, 90, 68, 95]
total = 0

for score in scores:
    total = total + score
    print("Added " + str(score) + " - running total: " + str(total))

average = total / len(scores)
print("\nTotal: " + str(total))
print("Number of scores: " + str(len(scores)))
print("Average: " + str(average))

Trace table for this program:

After the loop, total is 410. Dividing by 5 (the number of scores) gives us an average of 82.0.

Common Mistakes

Here is a summary of the most common loop mistakes and how to avoid them:

Key Vocabulary

Make sure you understand all of these terms. They come up frequently in exams and in discussions about programming.

Chapter Summary

Here is what you have learned in this chapter:

• A loop repeats a block of code multiple times, saving you from writing the same instructions over and over.
• A for loop (count-controlled) is used when you know how many times you want to repeat — it iterates through a sequence like a range(), list, or string.
• A while loop (condition-controlled) is used when you do not know how many times to repeat — it keeps going as long as a condition is True.
• The range() function generates sequences of numbers and is memory-efficient thanks to lazy evaluation.
• The accumulator pattern lets you build up a total or result across iterations of a loop.
• Nested loops place one loop inside another — the inner loop runs completely for each iteration of the outer loop.
• break exits a loop immediately; continue skips to the next iteration.
• A trace table helps you track variable values through each iteration of a loop — an essential exam skill.
• The most common mistakes are off-by-one errors with range() and infinite loops from forgetting to update a counter.