Use Python for loops with range and while loops with a condition, including accumulators and avoiding infinite loops

What this dot point is asking

SEAB wants you to use both kinds of Python loop, the for loop with range, and the while loop with a condition, including building accumulators and avoiding loops that never end. The central idea is that a for loop repeats a known number of times while a while loop repeats while a condition holds, and choosing the right one makes a program both correct and clear.

The answer

The for loop with range

A for loop repeats once for each value in a sequence. With range, it repeats a known number of times:

for i in range(5):
    print(i)        # prints 0, 1, 2, 3, 4

range(5) gives $0$ to $4$ (five values, starting at $0$). range(1, 6) gives $1$ to $5$: the start is included, the stop is excluded. range(0, 10, 2) counts in steps of $2$.

The while loop

A while loop repeats while a condition is true. Use it when you do not know in advance how many repetitions are needed:

total = 0
while total < 100:
    total = total + 25

The condition is tested before each pass. Something inside the loop must move it toward false, or the loop never stops.

Choosing for or while

• Use for when the number of repetitions is known (loop over a range or a list).
• Use while when it depends on a condition (loop until the user enters valid data).

Accumulators and counters

An accumulator is a variable that builds up a result across a loop. Set it before the loop, then update it each pass:

total = 0                 # accumulator, set first
for n in range(1, 6):
    total = total + n     # grows each pass
print(total)              # 15

A counter works the same way to count occurrences.

Avoiding infinite loops

A while loop runs forever if its condition never becomes false. Make sure the body changes a value used in the condition (read new input, increase a counter, or shrink a value) so the loop can end.

Examples in context

Example 1. Summing a shopping basket. A program loops with for item in prices over a list of prices, adding each to a total accumulator. Because the number of items is known from the list, a for loop is the natural, clear choice.

Example 2. A login attempt limit. A while loop keeps asking for a password while it is wrong and attempts remain, increasing an attempts counter each pass. The counter ensures the loop ends even if the user never gets it right, avoiding an infinite loop.

Try this

Q1. State what range(0, 10) produces. [2 marks]

• Cue. The numbers $0$ to $9$ (start included, stop excluded).

Q2. Write a for loop that prints the numbers $1$ to $5$. [2 marks]

• Cue. for i in range(1, 6): print(i).

Q3. Explain one way to stop a while loop running forever. [2 marks]

• Cue. Make the loop body change a value used in the condition (such as reading new input or increasing a counter) so the condition can become false.