Understanding Python For Loops: Iterating Over Sequences

Explore how to use the for loop in Python to iterate over sequences like lists, tuples, dictionaries, sets, and strings. Unlike traditional for loops in other languages, Python’s for loop functions more like an iterator method, allowing you to execute statements for each item in a collection efficiently.

Python For Loops

A for loop is used to iterate over a sequence (such as a list, tuple, dictionary, set, or string).

This is different from the for keyword in other programming languages. It works more like an iterator method found in object-oriented programming languages.

With the for loop, we can execute a set of statements once for each item in a list, tuple, set, etc.

Example: Print each fruit in a list


fruits = ["apple", "mango", "grape"]
for x in fruits:
  print(x)
            

apple
mango
grape

The for loop does not require an indexing variable to be set beforehand.

Looping Through a String

Even strings are iterable objects; they contain a sequence of characters.


for x in "orange":
  print(x)
            

o
r
a
n
g
e

The break Statement

The break statement allows us to stop the loop before it has looped through all the items.

Example: Exit the loop when x is "mango"

fruits = ["apple", "mango", "grape"]
for x in fruits:
  print(x)
  if x == "mango":
    break
            

apple
mango
Example: Exit the loop when x is "mango", but this time the break comes before the print

fruits = ["apple", "mango", "grape"]
for x in fruits:
  if x == "mango":
    break
  print(x)
            

apple

The continue Statement

The continue statement stops the current iteration of the loop and continues with the next.

Example: Do not print mango

fruits = ["apple", "mango", "grape"]
for x in fruits:
  if x == "mango":
    continue
  print(x)
            

apple
grape

The range() Function

To loop through a set of code a specified number of times, we can use the range() function.

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

Example: Using the range() function

for x in range(5):
  print(x)
            

0
1
2
3
4

Note that range(5) is not the values of 0 to 5, but the values 0 to 4.

Example: Using the start parameter

for x in range(3, 6):
  print(x)
            

3
4
5
Example: Increment the sequence with 3 (default is 1)

for x in range(2, 12, 3):
  print(x)
            

2
5
8
11

Else in For Loop

The else keyword in a for loop specifies a block of code to be executed when the loop is finished.

Example: Print all numbers from 0 to 4, and print a message when the loop has ended

for x in range(5):
  print(x)
else:
  print("Finally finished!")
            

0
1
2
3
4
Finally finished!

Note: The else block will NOT be executed if the loop is stopped by a break statement.

Example: Break the loop when x is 2, and see what happens with the else block

for x in range(5):
  if x == 2: break
  print(x)
else:
  print("Finally finished!")
            

0
1

Nested Loops

A nested loop is a loop inside a loop. The "inner loop" will be executed one time for each iteration of the "outer loop".

Example: Print each adjective for every fruit

adj = ["small", "medium", "large"]
fruits = ["apple", "mango", "grape"]

for x in adj:
  for y in fruits:
    print(x, y)
            

small apple
small mango
small grape
medium apple
medium mango
medium grape
large apple
large mango
large grape

The pass Statement

for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.

Example:

for x in [0, 1, 2]:
  pass