For Loop

8.1 For Loop

It's awesome that you can run a set of commands based on a condition. But what if I want to do something 100 times? How do we do that?

For this, Python has loops. And for our task, a for loop is perfect. The general form of the for statement is like this:

for variable in value_list:
    command1
    command1
    commandN

Where value_list is some set of values: a list, a string, a dictionary (more on them later).

The variable sequentially takes values from value_list, and for each of them, the block of commands is executed.

Example:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

This is how the loop works:

If there are 10 values in the list after the word in, then the block of commands will be executed 10 times.

For each value from the value_list, two things happen:

1. The loop variable (which is specified after the word for) is assigned the next value from the value_list.
2. Then the block of commands is executed.

Another example:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for x in numbers:
    print(x)

1
2
3
4
5
6
7
8
9
10

8.2 Specifying a Range — range

In the last example, you could see that printing 10 numbers is very simple. But what if we want to loop 100 times? Or 1000?

Of course, there's a solution for this. In Python, there's a special function range() for generating a sequence of numbers, which in combination with a for loop makes it a powerful tool for handling repetitive tasks.

The function range() can be used in several ways, depending on the number of arguments passed to it.

Basics of using range():

One argument: range(n) generates a sequence from 0 to n-1. This is useful when you need to perform an action a certain number of times.

for i in range(5):
    print(i)  # Prints numbers from 0 to 4

Two arguments: range(start, stop) generates a sequence from start to stop-1. This is useful for starting iteration from a specific number.

for i in range(1, 6):
    print(i)  # Prints numbers from 1 to 5 

Three arguments: range(start, stop, step) adds a third argument step, which determines the step between numbers in the sequence.

for i in range(0, 10, 2):
    print(i)  # Prints even numbers from 0 to 8  

Reverse loops: range() can generate numbers in reverse order, allowing for reverse iterations.

for i in range(10, 0, -1):
    print(i)  # Prints numbers from 10 to 1 

The range() function, in combination with the for loop, provides a flexible tool for controlling iterations in Python. Understanding its capabilities helps create more efficient and readable loops, which are an integral part of any program.

8.3 Looping through a List

If you want to perform a set of actions on different values, the range() function doesn't quite fit; you can always group these values in a list using square brackets. This operation generally looks like this:

for variable in [val1, val2, …, valN]:
    command1
    command1
    commandN

You simply list the values separated by commas, and the loop will execute for each of them, with the variable sequentially taking each value. Example:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
                    

apple
banana
cherry

You can specify numbers in the list, right in the loop after the word in:

for price in [0.99, 2.75, 0.25]:
    print(price)

0.99
2.75
0.25

You can list objects of any type in the list:

for data in [0.99, "apple", -1, True]:
    print(data)
                

0.99
apple
-1
True

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