Lesson 4
For loop with range

1. For loop with range

In the previous lessons we dealt with sequential programs and conditions. Often the program needs to repeat some block several times. That's where the loops come in handy. There are for and while loop operators in Python, in this lesson we cover for.

for loop iterates over any sequence. For instance, any string in Python is a sequence of its characters, so we can iterate over them using for:

None

for character in 'hello':
    print(character)

Another use case for a for-loop is to iterate some integer variable in increasing or decreasing order. Such a sequence of integer can be created using the function range(min_value, max_value):

None

for i in range(5, 8):
    print(i, i ** 2)
print('end of loop')
# 5 25
# 6 36
# 7 49
# end of loop

Function range(min_value, max_value) generates a sequence with numbers min_value, min_value + 1, ..., max_value - 1. The last number is not included.

There's a reduced form of range() - range(max_value), in which case min_value is implicitly set to zero:

None

for i in range(3):
    print(i)
# 0
# 1
# 2

This way we can repeat some action several times:

None

for i in range(2 ** 2):
    print('Hello, world!')

Same as with if-else, indentation is what specifies which instructions are controlled by for and which aren't.

Range() can define an empty sequence, like range(-5) or range(7, 3). In this case the for-block won't be executed:

None

for i in range(-5):
    print('Hello, world!')

Let's have more complex example and sum the integers from 1 to n inclusively.

None

result = 0
n = 5
for i in range(1, n + 1):
    result += i
    # this ^^ is the shorthand for
    # result = result + i
print(result)

Pay attention that maximum value in range() is n + 1 to make i equal to n on the last step.

To iterate over a decreasing sequence, we can use an extended form of range() with three arguments - range(start_value, end_value, step). When omitted, the step is implicitly equal to 1. However, can be any non-zero value. The loop always includes start_value and excludes end_value during iteration:

None

for i in range(10, 0, -2):
    print(i)
# 10
# 8
# 6
# 4
# 2

2. setting the function print()

None

print(1, 2, 3)
print(4, 5, 6)
print(1, 2, 3, sep=', ', end='. ')
print(4, 5, 6, sep=', ', end='. ')
print()
print(1, 2, 3, sep='', end=' -- ')
print(4, 5, 6, sep=' * ', end='.')