CS6Iteration

Iteration is the repetition of a block of code. A loop saves writing the same lines again and again.

• Use definite iteration when you know how many times before the loop starts: ‘for each of the 7 days’.
• Use indefinite iteration when it depends on what happens: ‘keep asking until the answer is correct’.
• An indefinite loop’s condition can be at the start (WHILE) or at the end (REPEAT … UNTIL, DO … WHILE). You need to understand both, even though Python only has the first.
• A loop is shorter than repeated lines, and the number of passes is easy to change.

Iteration

The repetition of a block of code.

Definite iteration

The repetition of a block of code for a set number of times, e.g. a FOR loop.

Indefinite iteration

The repetition of a block of code controlled by a condition; the number of iterations is not known when entering the block of code, e.g. WHILE, REPEAT … UNTIL.

A FOR loop gives a counter variable each value in turn.

• Pseudo-code: FOR … TO … ENDFOR. The upper (or lower) limit is included, and the counter never goes past it. STEP is 1 unless given, and can be negative.
• The counter is declared in the FOR statement and doesn’t exist after the loop.
• Python: range(stop) starts at 0; range(start, stop); range(start, stop, step). The stop value is never reached.
• To match FOR i ← a TO b, use range(a, b + 1). Counting down, use range(a, b - 1, step).

Pseudo-code includes the limit; Python’s range stops before its stop value

for i in range(3, 16, 4):
    print(i)

Loop counter

The variable in a FOR loop that takes each value in turn, e.g. i.

range(start, stop, step)

Python’s list of counter values: from start, adding step, stopping before stop.

A WHILE loop repeats while its condition is True. The condition is checked before each pass.

• Output: 20, 13, 6. After 6, energy becomes −1, the condition is False, and the loop ends.
• If the condition is False the first time, the body never runs. Starting with energy ← 0, nothing is output.
• Something in the body must eventually make the condition False. If nothing does, the loop never ends (an infinite loop).
• Use WHILE when the loop might not need to run at all.

energy ← 20
WHILE energy > 0
   OUTPUT energy
   energy ← energy - 7
ENDWHILE

energy = 20
while energy > 0:
    print(energy)
    energy = energy - 7

WHILE loop

Indefinite iteration with the condition at the start; it may run zero times.

Infinite loop

A loop whose condition never becomes False (or True, for UNTIL), so it never stops.

With the condition at the end, the body runs first and is checked after, so it always runs at least once.

• AQA pseudo-code: REPEAT … UNTIL condition stops when the condition becomes True.
• Some languages use DO … WHILE condition: also checked at the end, but it carries on while the condition is True.
• Python has neither. AQA accepts two ways to get the same effect: write the body once before a while loop (AQA’s Python guide), or use while True: with break at the bottom (AQA’s marking guidance).

Where the condition is checked decides the smallest number of passes

energy ← 0
REPEAT
   OUTPUT energy
   energy ← energy - 7
UNTIL energy ≤ 0

energy = 0
while True:
    print(energy)
    energy = energy - 7
    if energy <= 0:
        break

REPEAT … UNTIL

Indefinite iteration with the condition at the end; runs at least once and stops when the condition is True.

DO … WHILE

Indefinite iteration with the condition at the end; runs at least once and continues while the condition is True.

A FOR … IN loop runs once for each character in a string, or each item in an array (a Python list).

• The loop variable (char) takes each character in turn, spaces included. Here it outputs 4 (H, W, U, K).
• It is definite iteration: the number of passes is the number of characters.
• Over an array: for score in scores: gives each item in turn. In AQA pseudo-code, you can’t change the array’s items through the loop variable.
• Arrays are covered in Arrays and records; picking out characters by position is in String handling.

message ← 'Hello World UK'
capitals ← 0
FOR char IN message
   IF char ≥ 'A' AND char ≤ 'Z' THEN
      capitals ← capitals + 1
   ENDIF
ENDFOR
OUTPUT capitals

message = "Hello World UK"
capitals = 0
for char in message:
    if char >= "A" and char <= "Z":
        capitals = capitals + 1
print(capitals)

FOR … IN

A loop that runs once for each character of a string or each item of an array.

When an iteration statement appears inside another iteration statement, it is nested.

• For each pass of the outer loop, the inner loop runs all the way through, starting again from its first value.
• The inner body runs outer × inner times: 3 × 4 = 12 here.
• The Python version prints each row on one line:
  1 2 3 4
2 4 6 8
3 6 9 12
• The spec’s example nests a FOR inside a WHILE: different loop types can be nested. Selection can be nested inside iteration too.

FOR row ← 1 TO 3
   FOR col ← 1 TO 4
      OUTPUT row * col
   ENDFOR
ENDFOR

for row in range(1, 4):
    for col in range(1, 5):
        print(row * col, end=" ")
    print()

Nested iteration

An iteration statement inside another iteration statement.

Most loop questions use the same few patterns.

• Set totals and counters to 0 before the loop. Inside the loop they would be reset on every pass.
• Output a total after the loop, unless the question asks for output on every pass.
• Only work out an average if the count is more than 0.

total = 0
items = 0
price = float(input("Price (0 to finish): "))
while price != 0:
    total = total + price
    items = items + 1
    price = float(input("Price (0 to finish): "))
print(f"{items} items, total {total}")

A loop that runs one time too many or too few is called an off-by-one error. Count the passes before you write.

• AQA’s FOR includes its limit, so FOR i ← 1 TO 5 is 5 passes. Python’s range excludes its stop value.
• Indexes start at 0, so the last index of an array (or string) is its length − 1. That is why loops over indexes run to LEN(x) - 1.
• To count the passes of FOR i ← a TO b STEP s: (b − a) DIV s + 1.

Read the brief, then pick the loop that matches it.

• Any FOR loop can be rewritten as a WHILE loop: set the counter before the loop, test it in the condition, and add the step at the end of the body.
• Changing a WHILE that counts into a FOR: find the first value, the last value and the step, then write the matching range.

i ← 1
WHILE i ≤ 3
   OUTPUT i
   i ← i + 1
ENDWHILE