Loops

If statements are control flow statements which helps us to run a particular code only when a certain condition is satisfied. For example, you want to print a message on the screen only when a condition is true then you can use if statement to accomplish this in programming.

Syntax
if condition:
   block of code



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.

Python has two primitive loop commands:

• while loops
• for loops

The for loop in Python is used to iterate over a sequence (list, tuple, string) or other iterable objects. Iterating over a sequence is called traversal.In Python, there is no C style for loop, i.e., for (i=0; i<n; i++)

Syntax of for Loop

for val in sequence:

    Body of for

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

Program to find the sum of all numbers stored in a list

numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]

sum = 0

for val in numbers:

    sum = sum+val

print("The sum is", sum)

The Range() Function

If you do need to iterate over a sequence of numbers, the built-in function range() comes in handy. It generates arithmetic progressions:

for i in range(10):
       print(i)


The given end point is never part of the generated sequence; range(10) generates 10 values, the legal indices for items of a sequence of length 10. It is possible to let the range start at another number, or to specify a different increment (even negative; sometimes this is called the ‘step’):

for i in range(5, 10):
    print(i)

for i in  range(0, 10, 3):
    print(i)

for i in  range(-10, -100, -30):
    print(i)

To iterate over the indices of a sequence, you can combine range() and len() as follows:

a = ['Mary', 'had', 'a', 'little', 'lamb']

for i in range(len(a)):

    print(i, a[i])

 

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 'p' and 'o'
for letter in 'python-is-Langauage':
    if letter == 'p' or letter == 'o':
        continue
    print('Current Letter :', letter)


Break Statement: It returns the control to the beginning of the loop.
 # break the loop as soon it sees 'o'  or 'p'
for letter in 'python-is-Langauage':
    if letter == 'o' or letter == 'P':
        break
    print('Current Letter :', letter)

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                                             ,.

while test expression:

     body of while

 
Print i as long is less than 6

i = 1

while  i < 6:

    print(i)

    i +=1

The break Statement

i = 1

while  i < 6:

    print(i)

    if ( i == 2):

       break 

    i +=1

The else Statement

With the else statement we can run a block of code once when the condition no longer is true:

Print a message once the condition is false:
i = 1
while i < 6:
  print(i)
  i += 1
else:
  print("i is no longer less than 6")