Loops and Control Statements (continue, break and pass) in Python

Last Updated: 08-08-2017

Python programming language provides following types of loops to handle looping requirements.

While Loop
Syntax :

while expression:
    statement(s)

In Python, all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements.

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# prints Hello Geek 3 Times count = 0 while (count < 3):         count = count+1     print("Hello Geek")

Output:

Hello Geek
Hello Geek
Hello Geek

See this for an example where while loop is used for iterators. As mentioned in the article, it is not recommended to use while loop for iterators in python.

For in Loop
In Python, there is no C style for loop, i.e., for (i=0; i<n; i++). There is “for in” loop which is similar to for each loop in other languages.

Syntax:

for iterator_var in sequence:
    statements(s)

It can be used to iterate over iterators and a range.

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# Iterating over a list print("List Iteration") l = ["geeks", "for", "geeks"] for i in l:     print(i)         # Iterating over a tuple (immutable) print("\nTuple Iteration") t = ("geeks", "for", "geeks") for i in t:     print(i)         # Iterating over a String print("\nString Iteration")     s = "Geeks" for i in s :     print(i)         # Iterating over dictionary print("\nDictionary Iteration")    d = dict()  d['xyz'] = 123 d['abc'] = 345 for i in d :     print("%s  %d" %(i, d[i]))

Output:

List Iteration
geeks
for
geeks

Tuple Iteration
geeks
for
geeks

String Iteration
G
e
e
k
s

Dictionary Iteration
xyz  123
abc  345

We can use for in loop for user defined iterators. See this for example.

 

Nested Loops
Python programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.
Syntax:

for iterator_var in sequence:
    for iterator_var in sequence:
        statements(s)
        statements(s)

The syntax for a nested while loop statement in Python programming language is as follows:

while expression:
    while expression: 
        statement(s)
        statement(s)

A final note on loop nesting is that we can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa.

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from __future__ import print_function for i in range(1, 5):     for j in range(i):          print(i, end=' ')     print()

Output:

1
2 2
3 3 3
4 4 4 4

Loop Control Statements
Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements.

Continue Statement
It returns the control to the beginning of the loop.

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# Prints all letters except 'e' and 's' for letter in 'geeksforgeeks':      if letter == 'e' or letter == 's':          continue     print 'Current Letter :', letter     var = 10

Output:

Current Letter : g
Current Letter : k
Current Letter : f
Current Letter : o
Current Letter : r
Current Letter : g
Current Letter : k

Break Statement
It brings control out of the loop

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for letter in 'geeksforgeeks':         # break the loop as soon it sees 'e'      # or 's'     if letter == 'e' or letter == 's':          break    print 'Current Letter :', letter

Output:

Current Letter : e

Pass Statement
We use pass statement to write empty loops. Pass is also used for empty control statement, function and classes.

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# An empty loop for letter in 'geeksforgeeks':     pass print 'Last Letter :', letter

Output:

Last Letter : s