Python For Loops

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 language, 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.

Example

Print each fruit in a fruit list:

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

Run example »
C:\Users\My Name>python demo_for.py
apple
banana
cherry

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

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Looping Through a String

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

Example

Loop through the letters in the word "banana":

for x in "banana":
  print(x)

Run example »
C:\Users\My Name>python demo_for_string.py
b
a
n
a
n
a

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The break Statement

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

Example

Exit the loop when x is "banana":

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

Run example »
C:\Users\My Name>python demo_for_break.py
apple
banana

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)

Run example »
C:\Users\My Name>python demo_for_break2.py
apple

---

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The continue Statement

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

Example

Do not print banana:

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

Run example »
C:\Users\My Name>python demo_for_continue.py
apple
cherry

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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(6):
  print(x)

Run example »
C:\Users\My Name>python demo_for_range.py
0
1
2
3
4
5

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):

Example

Using the start parameter:

for x in range(2, 6):
  print(x)

Run example »
C:\Users\My Name>python demo_for_range2.py
2
3
4
5

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):

Example

Increment the sequence with 3 (default is 1):

for x in range(2, 30, 3):
  print(x)

Run example »
C:\Users\My Name>python demo_for_range3.py
2
5
8
11
14
17
20
23
26
29

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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 5, and print a message when the loop has ended:

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

Run example »
C:\Users\My Name>python demo_for_else.py
0
1
2
3
4
5
Finally finished!

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

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

Run example »
C:\Users\My Name>python demo_for_nested.py
red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry