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.
# prints Hello prutor 3 Times
count = 0
while (count < 3):
count = count+1
print("Hello prutor")
Output:
Hello prutor
Hello prutor
Hello prutor
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.
# Iterating over a list
print("List Iteration")
l = ["prutor", 'DOT', "Ai"]
for i in l:
print(i)
# Iterating over a tuple (immutable)
print("nTuple Iteration")
t = ("prutor", 'DOT', "Ai")
for i in t:
print(i)
# Iterating over a String
print("nString Iteration")
s = "prutor"
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
prutor
dot
ai
Tuple Iteration
prutor
dot
ai
String Iteration
p
r
u
t
o
r
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.
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.
# Prints all letters except 'e' and 's'
for letter in 'prutor':
if letter == 'e' or letter == 's':
continue
print 'Current Letter :', letter
var = 10
Output:
Current Letter : p
Current Letter : r
Current Letter : u
Current Letter : t
Current Letter : o
Current Letter : r
Current Letter : a
Break Statement
It brings control out of the loop
for letter in 'prutor':
# 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.
# An empty loop
for letter in 'prutor':
pass
print 'Last Letter :', letter
Output:
Last Letter : s