Iteration

Repeat with iteration

• Iteration means doing the same steps again and again.
• Loops save you from writing the same line many times.
• AP CSP cares about loops because they let a little code do a lot of work.

Count with for and range

• for i in range(n): repeats the body n times.
• range(5) gives 0, 1, 2, 3, 4 (it stops before 5).
• The loop body is indented, like an if.

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

Build a total (accumulation)

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

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

Loop until a condition with while

• A while loop repeats as long as its condition is True.
• Something inside must move toward making it False.
• Use it when you do not know the count in advance.

count = 3
while count > 0:
    print(count)
    count = count - 1

Loop over a list (for-each)

• for item in mylist: visits each item in turn.
• The loop variable holds one item per pass.
• This is the clean way to process every value.

scores = [10, 20, 30]
for s in scores:
    print(s)

In AP CSP pseudocode

• CB has three loop shapes that match Python loops.
• REPEAT n TIMES = a fixed count; REPEAT UNTIL(cond) = loop until true.
• FOR EACH x IN list = a for-each loop. DISPLAY = print, ← = =, MOD = %.

REPEAT 5 TIMES
{
    DISPLAY "hi"
}

count ← 3
REPEAT UNTIL (count = 0)
{
    DISPLAY count
    count ← count - 1
}

FOR EACH s IN scores
{
    DISPLAY s
}

Common mistakes

• range(n) runs 0 to n - 1.
• Change the condition inside a while, or it never stops.

Now you try

• Use a loop in each task.
• Press Check answer to test your code.