loops in python
Published: 2021-12-01 14:43:09

Python programming language provides 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.

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 program is executed. 
Syntax :

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while expression:
    statement(s)
        3. 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 Geek")
Output: 

Hello Geek
Hello Geek
Hello Geek

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 are 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 Geek")
else:
    print("In Else Block")
Output: 

Hello Geek
Hello Geek
Hello Geek
In Else Block

Single statement while block: Just like the if block, if the while block consists of a single statement the we can declare the entire loop in a single line as shown below:

# Python program to illustrate
# Single statement while 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.
for in 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 “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_var in sequence:
    statements(s)

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


# Python program to illustrate
# Iterating over range 0 to n-1
 
n = 4
for i in range(0, n):
    print(i)
Output :

0
1
2
3


# Python program to illustrate
# 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]))