Python loops

Python programming language provides the following types of loops to handle looping requirements. Python provides three ways for executing the loops. While all the ways provide similar basic functionality, they differ in their syntax and condition checking time.

Python For loop

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

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

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

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

The continue Statement

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

Example

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

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

for x in range(7):
  print(x)

Note that range(6) is not the values of 0 to 6, but the values 0 to 6.

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

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):

Example

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

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

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.

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

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

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

Python while loop

The while loop in Python is used to iterate over a block of code as long as the test expression (condition) is true.

We generally use this loop when we don’t know the number of times to iterate beforehand.

Syntax of while Loop in Python

while expression:
    Body

Let’s see it with an example

i=0
while i < 5:
    print(i)
    i=i+1

Here, the loop will run until the condition gets false and it will exit the loop.

#Output
0
1
2
3
4

While loop with else

Same as with for loops, while loops can also have an optional else block.

The else part is executed if the condition in the while loop evaluates to False.

The while loop can be terminated with a break statement. In such cases, the else part is ignored. Hence, a while loop’s else part runs if no break occurs and the condition is false.

Here is an example to illustrate this

counter = 0

while counter < 3:
    print("Inside loop")
    counter = counter + 1
else:
    print("Inside else")

Output

Inside loop
Inside loop
Inside loop
Inside else

Here, we use a counter variable to print the string Inside loop three times.

On the fourth iteration, the condition in while becomes False. Hence, the else part is executed.

Conclusion

Hence we studied different types of loops in python with there definition and implementation along with it.

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