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.
Example
Print each fruit in a fruit list:
fruits = [“apple”, “banana”, “cherry”]
for x in fruits:
print(x)
Output:
apple
banana
cherry
The for loop does not require an indexing variable to set beforehand.
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)
Output:
b
a
n
a
n
a
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
Output:
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)
Output:
apple
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)
Output:
apple
cherry
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)
Output:
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)
Output:
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)
Output:
2
5
8
11
14
17
20
23
26
29
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!”)
Output:
0
1
2
3
4
5
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 3, and see what happens with the else block:
for x in range(6):
if x == 3: break
print(x)
else:
print(“Finally finished!”)
#If the loop breaks, the else block is not executed.
Output:
0
1
2
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)
Output:
red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry
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
#having an empty for loop like this, would raise an error without the pass statement