2.4 For Loop

A for loop is used to iterate over a collection of elements or a range of numbers.

Summary

• Basic Usage: Iterates over all elements in a collection or range automatically.
• Break: Exits the loop immediately.
• Continue: Skips the rest of the current iteration and moves to the next iteration.

Basic

A for loop is useful when you need to iterate over a collection of elements or perform an action a set number of times.

for element in collection:
    # Code to execute for each element

A for loop does not use a condition. Instead, it iterates through the collection using the elements one by one. So the first iteration uses, the first element, and the second iteration uses the second element, … Below you can see a for loop iterating repeating 5 times using range() and then iterating through all elements of a list.

for num in range(5):
    print(f"Number: {num}")
 
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

Number: 0
Number: 1
Number: 2
Number: 3
Number: 4
I like apple
I like banana
I like cherry

Break

The keyword break works exactly the same as with a while loop and is used to immediately exit the loop.

for num in range(10):
    if num == 5:
        break
    print(num)

0
1
2
3
4

Continue

The keyword continue works exactly the same as with a while loop and is used to skip the rest of the current iteration and move to the next one.

for num in range(10):
    if num%2 == 0:
        continue
    print(num)

1
3
5
7
9

List Comprehension

List comprehensions provide a shorter and more readable way to create lists using a for loop in a single line. Instead of using a normal for loop to build a new list, list comprehension lets you do it directly. The syntax to do this is [new_element for element in iterable], as you can see in the example below. When calculating the new element you can use operators, functions, …

# For loop version
squares1 = []
for n in range(5):
    squares.append(n**2)
print(squares1) # -> [0, 1, 4, 9, 16]
 
# List comprehension version
squares2 = [n**2 for n in range(5)]
print(squares2) # -> [0, 1, 4, 9, 16]

Both ways have the exactly same result, the only difference is that the second one is shorter.

Condition

You can add a condition to include only certain elements with the syntax [new_element for element in iterable if condition]. An element will only be added to the list, if the condition is True. If the condition is False, it will be ignored

# For loop version
evens1 = []
for i in range(10):
    if i % 2 == 0:
        evens.append(i)
print(evens1) # -> [0, 2, 4, 6, 8]
 
# List comprehension version
evens2 = [i for i in range(10) if i % 2 == 0]
print(evens2) # -> [0, 2, 4, 6, 8]

Questions

• Explain for in your own words.
• What are the differences and similarities between for and while loops? When should you use which kind?
• Explain break and continue in your own words.
• Are break and continue potentially more or less useful within a for loop than in a while loop? Explain why with reasons and examples.
• What happens if a for loop runs on an empty collection?
• Is it possible that a for loop is infinite? Why? Give reasons and examples.
• How can a for loop be used to sum all even numbers from 1 to 100? Explain and write the code.