Python has two primitive loop commands:
while loopsfor loopsWith the while loop we can execute a set of statements as long as a condition is true.
# Print i as long as i is less than 6:
i = 1
while i < 6:
print(i)
i += 1
Output:
1
2
3
4
5
Note: remember to increment i, or else the loop will continue forever.
The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.
With the break statement we can stop the loop even if the while condition is true:
# Exit the loop when i is 3:
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
Output:
1
2
3
With the continue statement we can stop the current iteration, and continue with the next:
# Continue to the next iteration if i is 3:
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
Output:
1
2
4
5
6
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.
# 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.
Even strings are iterable objects, they contain a sequence of characters:
# Loop through the letters in the word "banana":
for x in "banana":
print(x)
Output:
b
a
n
a
n
a
With the break statement we can stop the loop before it has looped through all the items:
# Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
if x == "banana":
break
Output:
apple
banana
# 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
With the continue statement we can stop the current iteration of the loop, and continue with the next:
# Do not print banana:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x == "banana":
continue
print(x)
Output:
apple
cherry
range() FunctionTo 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.
# 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):
# 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):
# 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
A nested loop is a loop inside a loop.
The "inner loop" will be executed one time for each iteration of the "outer loop":
# 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
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.
for x in [0, 1, 2]:
pass