Iteration: For Loops with range()

Use range() to supply numbers to the same for loop.

range() is placed after in when a for loop needs a sequence of integers. The loop still follows exactly the same pattern: for variable in values:.

Not a second type of for loopfor name in names: and for number in range(5): are the same kind of Python loop. Only the source of the values is different.

Same loop, different source

same_pattern.py

names = ["Ava", "Ben", "Cara"]

for name in names:
    print(name)

for number in range(3):
    print(number)

Output

Ava
Ben
Cara
0
1
2

How it works

In the first loop, names supplies three strings.
In the second loop, range(3) supplies three integers: 0, 1 and 2.
Both loops take one value at a time and store it in the loop variable.
range() changes the values being supplied, not the type of loop.

Repeating a fixed number of times

repeat.py

for repeat in range(5):
    print("Hello")

Output

Hello
Hello
Hello
Hello
Hello

How it works

range(5) supplies five values: 0, 1, 2, 3 and 4.
The loop therefore runs five times.
The program does not use repeat inside the loop; the values simply control how many passes occur.

Using the number supplied by range()

numbers.py

for number in range(5):
    print("Number", number)

Output

Number 0
Number 1
Number 2
Number 3
Number 4

How it works

number receives each integer supplied by range(5).
range(5) starts at 0 and stops before 5.
Use the loop variable when the number itself is needed in the calculation or output.

Choosing the start and stop values

start_stop.py

for number in range(1, 6):
    print(number)

Output

How it works

The first value, 1, is the starting value and is included.
The second value, 6, is the stopping boundary and is not included.
range(1, 6) therefore supplies 1, 2, 3, 4 and 5.

The stop value is excludedrange() stops before its stop value. To produce the numbers 1 to 5, write range(1, 6).

Counting in steps

steps.py

for number in range(0, 11, 2):
    print(number)

for number in range(5, 0, -1):
    print(number)

Output

How it works

The third value is the step: how much the number changes after each pass.
range(0, 11, 2) counts upwards in twos.
range(5, 0, -1) counts downwards because the step is negative.
The stop boundary is still excluded in both examples.

Using range() for list indexes

Use indexes only when the position of each item is needed. If you only need the items, loop through the list directly.

indexes.py

names = ["Ava", "Ben", "Cara"]

for index in range(len(names)):
    print(index, names[index])

Output

0 Ava
1 Ben
2 Cara

How it works

len(names) is 3, so range(len(names)) supplies 0, 1 and 2.
Those are the valid indexes of the three list items.
names[index] accesses the item at the current position.
This is still the same for variable in values: loop pattern.

Do not use an index unless you need itWrite for name in names: when only the names are needed. Write for index in range(len(names)): when the position is also needed.

Choose the values needed after in

What the task needs	Values supplied after in	Code
Every item in a list	The list items	`for name in names:`
Exactly five repeats	Five integers	`for repeat in range(5):`
The numbers 1 to 5	Integers 1, 2, 3, 4, 5	`for number in range(1, 6):`
Every list position	Valid indexes	`for index in range(len(names)):`

What you have learned

range() supplies integer values to a normal for loop.
range(n) supplies 0 up to n - 1.
range(start, stop) includes the start but excludes the stop.
range(start, stop, step) changes by the given step.
Use range(len(list_name)) only when indexes are needed.
“For-each” and “count-controlled” describe purposes, not separate Python for statements.