Iterations with for

This lesson introduces the iterative instruction for in range, which we can consider a kind of special, quite useful case of the iterative instruction while.

Writing many numbers

Let's consider again a program that reads a number n, and writes all the numbers between 1 and n, one per line. Recall that this is one possible solution:

n = read(int)
i = 1
while i <= n:
    print(i)
    i = i + 1

If we look at this code from the third line, we find a very common pattern when building loops: make a variable (called control variable) start at a certain value, and advance to a certain value with a certain increment each iteration. In this case, the control variable is i, starting from 1, ending at n incrementing by 1 each time.

The iterative instruction for in range allows us to write this pattern more concisely. This is its most complete version:

for <variable> in range(<start>, <end>, <step>):
    <instructions>

which is equivalent to:

<variable> = <start>
while <variable> < <end>:
    <instructions>
    <variable> = <variable> + <step>

(If the <step> is negative, the loop stops when <variable> is greater than or equal to <end>).

Therefore, the loop from the previous program could be written like this:

for i in range(1, n + 1, 1):
    print(i)

Note that the <end> is not part of the loop.

In fact, the <step> of range is optional, defaulting to 1. Therefore, the loop from the previous program could be simplified a bit like this:

for i in range(1, n + 1):
    print(i)

Also, the <start> of range is optional, defaulting to 0: Therefore, the loop from the previous program could also be written like this:

for i in range(n):
    print(i + 1)

In this case, i goes from 0 to n - 1, so the print() adds one.

Examples

To make it very clear, the following table shows the successive values the loop variable would take for some different ranges:

range	values
range(8)	0 1 2 3 4 5 6 7
range(0)	``
range(1)	0
range(-5)	``
range(1, 5)	1 2 3 4
range(0, 8, 2)	0 2 4 6
range(0, 7, 2)	0 2 4 6
range(6, 0, -1)	6 5 4 3 2 1

The program that calculates the factorial of a natural number can be written like this with a for loop:

from yogi import read

n = read(int)
f = 1
for i in range(2, n + 1):
    f = f * i
print(f)

The program that draws a regular n-gon would be like this:

import turtle
import yogi

size = yogi.read(int)
sides = yogi.read(int)
angle = 360 / sides

for i in range(sides):
    turtle.forward(size)
    turtle.right(angle)

turtle.done()

Obviously, for loops can also be nested. This program draws a square rotated several times:

import turtle
import yogi

size = yogi.read(int)
rotations = yogi.read(int)
angle = 360 / rotations

for i in range(rotations):
    for j in range(4):
        turtle.forward(size)
        turtle.right(90)
    turtle.right(angle)

turtle.done()

As you have seen, although it may initially seem harder to understand, these codes are more compact and quicker to write and easy to read. The reason is that all the control elements of the loop are found in one single place. Therefore, we recommend you get used to for in range loops and use them when your algorithms follow the very common pattern mentioned above. Use while loops only for iterations with more original schemes. On the other hand, Python's for loop can do even more things, which we will see later.