Skip to main content

3.0 Python: For Loops

3.1 Python For Loops​

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

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

3.1.1 Example​

Print each fruit in a fruit list:

fruits = ["apple", "banana", "cherry"]

for x in fruits:
  print(x)

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

3.2 Looping Through a String​

Even strings are iterable objects, they contain a sequence of characters:

3.2.1 Example​

Loop through the letters in the word "banana":

for x in "banana":
  print(x)

3.3 The break Statement​

With the break statement we can stop the loop before it has looped through all the items:

3.3.1 Example​

Exit the loop when x is "banana":

fruits = ["apple", "banana", "cherry"]

for x in fruits:
    print(x)
    if x == "banana":
        break

3.3.2 Example​

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

fruits = ["apple", "banana", "cherry"]

for x in fruits:
    if x == "banana":
        break
    print(x)

3.4 The continue Statement​

With the continue statement we can stop the current iteration of the loop, and continue with the next:

3.4.1 Example​

Do not print banana:

fruits = ["apple", "banana", "cherry"]

for x in fruits:
    if x == "banana":
        continue
    print(x)

3.5 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.

3.5.1 Example​

Using the range() function:

for x in range(6):
    print(x)
note

That range(6) is not the values of 0 to 6, but the values 0 to 5.

The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):

3.5.2 Example​

Using the start parameter:

for x in range(2, 6):
    print(x)

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):

3.5.3 Example​

Increment the sequence with 3 (default is 1):

for x in range(2, 30, 3):
    print(x)

3.6 Else in For Loop​

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

3.6.1 Example​

Print all numbers from 0 to 5, and print a message when the loop has ended:

for x in range(6):
    print(x)
else:
    print("Finally finished!")
note

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

3.6.2 Example​

Break the loop when x is 3, and see what happens with the else block:

for x in range(6):
    if x == 3:
        break
    print(x)
else:
    print("Finally finished!")

3.7 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":

3.7.1 Example​

Print each adjective for every fruit:

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for f in fruits:
    for a in adj:
        print(f, a)

3.8 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.

3.8.1 Example​

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