Coding Notes

2.4 For Loop

6 min readAndreas PatuzziAuroville Institute of Applied Technology

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.
  • Else: Runs only if the loop completes normally (not because of break).

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

Else

A for loop can have an else block, which works the same as with a while loop. It runs only if the loop finishes normally (without break).

for num in range(2, 10):
    for i in range(2, num):
        if num % i == 0:
            print(f"{num} is not a prime number because it is divisible by {i}")
            break
    else:
        print(f"{num} is a prime number")
2 is a prime number
3 is a prime number
4 is not a prime number because it is divisible by 2
5 is a prime number
6 is not a prime number because it is divisible by 2
7 is a prime number
8 is not a prime number because it is divisible by 2
9 is not a prime number because it is divisible by 3

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]

add other datatypes later

set, dictionary, generator expression, tuple find in Working Draft 2.

Examples

These examples show how you can use a for loop in more advanced scenarios. You can find details of the functions/methods used in the other chapters or online.

Dictionary

You can loop through dictionary keys, values, or both.

person = {"name": "Alice", "age": 30, "city": "New York"}
 
# Loop through keys
for key in person:
    print(f"Key: {key}, Value: {person[key]}")
 
# Loop through values
for value in person.values():
    print(f"Value: {value}")
 
# Loop through key-value pairs
for key, value in person.items():
    print(f"Key: {key}, Value: {value}")
Key: name, Value: Alice
Key: age, Value: 30
Key: city, Value: New York
Value: Alice
Value: 30
Value: New York
Key: name, Value: Alice
Key: age, Value: 30
Key: city, Value: New York

Multiple Lists

If you need to iterate through multiple lists at once, use zip().

names = ["Alice", "Bob", "Charlie"]
ages = [30, 25, 35]
print(list(zip(names, ages)))
 
for name, age in zip(names, ages):
    print(f"{name} is {age} years old")
[('Alice', 30), ('Bob', 25), ('Charlie', 35)]
Alice is 30 years old
Bob is 25 years old
Charlie is 35 years old

Enumerate List

To keep track of the index while looping through a list, use enumerate().

colors = ["red", "green", "blue"]
 
for index, color in enumerate(colors):
    print(f"Color {index}: {color}")
Color 0: red
Color 1: green
Color 2: blue

String

A string is also iterable, so you can loop through its characters.

word = "Python"
for letter in word:
    print(letter)
P
y
t
h
o
n

Nested Loops

You can nest loops, for example when working with 2D structures.

grid = ((1, 2, 3), (4, 5, 6), (7, 8, 9))
 
for row in grid:
    for x in row:
        print(f"value {x}", end=", ")
    print(f"from row {row}")
 
for i in range(3):
    for j in range(2):
        print(f"i={i}, j={j}")
value 1, value 2, value 3, from row (1, 2, 3)
value 4, value 5, value 6, from row (4, 5, 6)
value 7, value 8, value 9, from row (7, 8, 9)
i=0, j=0
i=0, j=1
i=1, j=0
i=1, j=1
i=2, j=0
i=2, j=1

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.
  • Explain the following code and its output:
for a, b in enumerate("Test"):
    print(a, b)
  • Explain else in combination with a for loop in your own words.
  • How can a for loop be used to sum all even numbers from 1 to 100? Explain and write the code.

Graph View