6. Iteration

Iteration is a control structure that carries out repetition or looping using while or for.

6.1. Loops

There are two types of loops: * for loops, that keep count of the number of times a block of code is executed * while loops which perform an action until a specified condition is no longer true.

6.2. For loops

A for loop can be used to do an action a specific number of times.
The range function below, range (4), can be used to assign values of 0, 1, 2, and 3 to i as the for loop runs.
The last line in the pseudocode for a for loop can take two forms: ENDFOR or NEXT variable.

for i in range(4):
    print(i)

Pseudocode. The equivalent pseudocode is:

BEGIN
    FOR i ← 0 TO 3
        OUTPUT i
    NEXT i
END

Tasks

How many times does the loop run?

for i in range(4, 9):
    print(i)

Pseudocode. The equivalent pseudocode is:

BEGIN
    FOR i ← 4 TO 8
        OUTPUT i
    NEXT i
END

6.3. While loops: forever

while loops can do something forever.

while True: loops repeat forever, or until the program is stopped.

while True:
    print("Can't stop me.")

Pseudocode. The equivalent pseudocode is:

BEGIN
    WHILE TRUE
        OUTPUT "Can't stop me."
    ENDWHILE
END

6.4. While loops: pre-test

While loops test a condition and only run the indented code in the while block while that condition is True.
If the condition is False, the code in the while block doesn’t run.
The condition is tested before the rest of the while loop is run.

The general pattern is:

# pre-test loop
while <conditional>:
    <statements>

Example python code:

num = 1
while num < 10:
    print(num)
    num = num + 1

Pseudocode. The equivalent pseudocode is:

BEGIN
    num ← 1
    WHILE num < 10
        OUTPUT num
        num ← num + 1
    ENDWHILE
END

6.5. Pseudocode: post-test REPEAT UNTIL

The condition is tested after the rest of the while loop is run and so controls the exiting of the loop.
This guarantees that the while block is run at least once.
When the test condition is True, the loop is exited.

Example pseudocode:

BEGIN
    num ← 1
    REPEAT
        OUTPUT num
        num ← num + 2
    UNTIL num > 9
END

There is no direct post-test syntax in python. A while True loop is run with a break if the condition is met.

The general pattern is:

# post-test loop
while True:
    <statements>
    if <conditional>:
        break

The equivalent python code is:

num = 1
while True:
    print(num)
    num += 2
    if num > 9:
        break

6.6. Pseudocode: post-test DO WHILE

The condition is tested after the rest of the while loop is run and so controls the exiting of the loop.
This guarantees that the while block is run at least once.
When the test condition is False, the loop is exited.

Example pseudocode:

BEGIN
    num ← 1
    DO
        OUTPUT num
        num ← num + 2
    WHILE num < 10
END

There is no direct post-test syntax in python. A while True loop is run with a break if the condition is met.

The general pattern is:

# post-test loop
while True:
    <statements>
    if not <conditional>:
        break

The equivalent python code is:

num = 1
while True:
    print(num)
    num += 2
    if not(num > 9):
        break

6.7. Practice Questions

Tasks

What is the expected output from the code above?

How many times is the loop below executed?

BEGIN
    i = 0
    WHILE (i <= 10)
        print i
        i = i + 21
    ENDWHILE
END