Counting loops

Repeat with for

• A for loop repeats code a fixed number of times.
• The body of the loop is indented, like an if.
• A loop variable holds a new value on each pass.

Counting with range

• range(5) gives the numbers 0, 1, 2, 3, 4 (stops before 5).
• range(1, 6) gives 1, 2, 3, 4, 5 (start included, stop not).
• range(0, 10, 2) counts in steps of 2.

for i in range(5):
    print(i)

Build a total (accumulator)

• An accumulator is a variable that grows inside a loop.
• Start it at 0 before the loop.
• Add to it on each pass to build a running total.

total = 0
for n in range(1, 6):
    total = total + n
print(total)

In Cambridge pseudocode

• A Python for … range loop is the exam's count-controlled FOR … NEXT.

total ← 0
FOR i ← 1 TO 5
    total ← total + i
NEXT i
OUTPUT total

Common mistakes

• range(n) counts 0 to n - 1, not 1 to n.
• range(a, b) stops before b.
• The loop body must be indented under the for line.

Now you try

• Use a for loop with range(...) in each task.
• Press Check answer to test your code.