ITERATION/LOOPING.
Repeating identical or similar tasks without making errors is something that computers do well and people do poorly. Repeated execution of a set of statements is called iteration. Because iteration is so common, Python provides several language features to make it easier.
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 the time.
While Loop:
- In python, while loop is used to execute a block of statements repeatedly until a given a condition is satisfied. And when the condition becomes false, the line immediately after the loop in the program is executed.
- Syntax :
statement(x)
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.
Example:
# Python program to illustrate
# while loop
count = 0
while (count < 3):
count = count + 1
print("Hello ")
output:
hello
hello
hello
Using else statement with while loops: As discussed above, while
loop executes the block until a condition is satisfied. When the
condition becomes false, the statement immediately after the loop
is executed.
The else clause is only executed when your while condition
becomes false. If you break out of the loop, or if an exception is
raised, it won’t be executed.
If else like this:
if condition:
# execute these statements
else:
# execute these statements
and while loop like this is similar
while condition:
# execute these statements
else:
# execute these statements
#Python program to illustrate
# combining else with while
count = 0
while (count < 3):
count = count + 1
print("Hello ")
else:
print("In Else Block")
output:
Hello
Hello
Hello
In Else Block
Single statement while block: Just like the if block, if the while
block consists of a single statement we can declare the entire
loop in a single line as shown below:
# Python program to illustrate
# Single statement while the block
count = 0
while (count == 0): print("Hello Geek")
Note: It is suggested not to use this type of loops as it is a never-ending infinite loop where the condition is always true and you have to forcefully terminate the compiler.
See this for an example where while loop is used for iterators. As mentioned in the article, it is not recommended to use a while loop for iterators in python.for loop: For loops are used for sequential traversal. For example: traversing a list or string or array etc. In Python, there is no C style for loop, i.e., for (i=0; i<n; i++). There is a “for in” loop which is similar to for each loop in other languages. Let us learn how to use for in loop for sequential traversals.
Syntax:for iterator_variable in sequence:
statement(s)
It can be used to iterate over iterators and a range.
# Python program to illustrate
# Iterating over a list
print("List Iteration")
l = ["pari", "for","paritosh"]
for i in l:
print(i)
# Iterating over a tuple (immutable)
print("\nTuple Iteration")
t = ("paritosh","for","paritosh")
for i in t:
print(i)
# Iterating over a String
print("\nString Iteration")
s = "paritosh"
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
pari
for
Paritosh
Tuple iteration
Paritosh
for
Paritosh
string iteration
p
a
r
i
t
o
s
h
dictionary iteration
XYZ 123
ABC 345
Iterating by an index of sequences: We can also use the index of
elements in the sequence to iterate. The key idea is to first calculate
the length of the list and in iterate over the sequence within the
range of this length.
See the below example:
# Python program to illustrate
# Iterating by index
list = ["paritosh","for","paritosh"]
for index in range(len(list)):
print list[index]
output:
Paritosh
for
Paritosh
Using else statement with for loops: We can also combine else
statement with for loop like in while loop. But as there is no
condition in for loop based on which the execution will terminate
so the else block will be executed immediately after for block
finishes execution.
Below example explains how to do this:
# Python program to illustrate
# combining else with for
list = ["paritosh","for","paritosh"]
for index in range(len(list)):
print list[index]
else:
print "Inside Else Block"
output:
paritosh
for
paritosh
Inside Else Block
Nested Loops: Python programming language allows to use one
loop inside another loop. The following section shows a 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.
# Python program to illustrate
# nested for loops in Python
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 'paritosh singh':
if letter == 'e' or letter == 's':
continue
print 'Current Letter :', letter
var = 10
Break Statement: It brings control out of the loop.
for letter in 'Paritosh':
# break the loop as soon it sees 'e'
# or 's'
if letter == 'e' or letter == 's':
break
print 'Current Letter :', letter
Pass Statement: We use pass statement to write empty loops. Pass
is also used for empty control statements, functions and classes.
# An empty loop
for letter in 'paritosh':
pass
print 'Last Letter :', letter
Exercise: How to print a list in reverse order (from last to the first item) using while and for-in loops.
PURPOSE OF LOOPING:
loop in python is used to execute a block of statements or code several times until the given condition becomes false. We use for loop when we know the number of times to iterate. Here var will take the value from the sequence and execute it until all the values in the sequence are done.
