For loops and range

A count-controlled loop repeats a known number of times. In Python, for takes each value from a sequence. The range() function generates a sequence of integers for the loop to use.

This differs from the while loops you already know: while repeats while a condition is true; for is the clearer choice when the values or count are already known.

1. Read range(start, stop, step)

The start is included. The stop is excluded. The step is the change each time. Defaults are start 0 and step 1.

• range(4) generates 0, 1, 2, 3.
• range(2, 6) generates 2, 3, 4, 5.
• range(2, 12, 2) generates 2, 4, 6, 8, 10.

2. Predict before running

Read the loop variable values and output without executing this listing.

for number in range(3, 9, 2):
    print(number)

3. Construct a fixed count

Write a loop that prints the integers 1 to 5 inclusive, one per line. Store each current value in a loop variable named number.

To include 5, the excluded stop must be 6.

4. Repeat processing, not just output

A loop can update an accumulator, a variable that stores a running result. Start it before the loop, update it inside the loop, and use it after the loop.

total = 0
for value in range(1, 5):
    total = total + value
print(total)

The values 1, 2, 3 and 4 are added, so the final total is 10.

5. Independent application

Use for and range() to calculate the total of the even numbers from 2 to 10 inclusive. Store the running result in total and print only the final total.

The range must generate 2, 4, 6, 8 and 10. The program will be checked for the loop, the generated sequence's effect and the final value.

Route forward

You can control repetition with generated numbers. Next you will iterate directly over every item in a one-dimensional list and use a trace table to follow changing state.