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Learn python : Python Loop

 Python for Loop


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.

Syntax of for Loop

for val in sequence:
	Body of for
Here, val is the variable that takes the value of the item inside the sequence on each iteration. Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Example: Python for Loop

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

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

# variable to store the sum
sum = 0

# iterate over the list
for val in numbers:
	sum = sum+val

# print the sum
# Output: The sum is 48
print("The sum is",sum)
output
The sum is 48

The range() function

We can generate a sequence of numbers using range() function. range(10) will generate numbers from 0 to 9 (10 numbers). We can also define the start, stop and step size as range(start,stop,step size). step size defaults to 1 if not provided. This function does not store all the values in memory, it would be inefficient. So it remembers the start, stop, step size and generates the next number on the go. To force this function to output all the items, we can use the function list(). The following example will clarify this.
# Print range
# Output: range(0, 10)
print(range(10))

# Print list of range
# from 0 to 10
# Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(list(range(10)))

# Print list of range between
# 2 to 8
# Output: [2, 3, 4, 5, 6, 7]
print(list(range(2,8)))

# Print list of range between
# 2 to 20 with a difference of 3
# Output: [2, 5, 8, 11, 14, 17]
print(list(range(2,20,3)))
output
range(0, 10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[2, 3, 4, 5, 6, 7]
[2, 5, 8, 11, 14, 17]
We can use the range() function in for loops to iterate through a sequence of numbers. It can be combined with the len() function to iterate though a sequence using indexing. Here is an example.
# Program to iterate
# through a list
# using indexing

# List of genre
genre = ['pop','rock','jazz']

# iterate over the list using index
# Output:
# I like pop
# I like rock
# I like jazz
for i in range(len(genre)):
	print("I like",genre[i])
output
I like pop
I like rock
I like jazz

for loop with else

A for loop can have an optional else block as well. The else part is executed if the items in the sequence used in for loop exhausts. break statement can be used to stop a for loop. In such case, the else part is ignored. Hence, a for loop's else part runs if no break occurs. Here is an example to illustrate this.
# Program to show
# the control flow
# when using else block
# in a for loop

# a list of digit
list_of_digits = [0,1,2,3,4,5,6]

# Try changing the input digit for varied results
input_digit = 3

# To take input from user, uncomment the code below
#input_digit = int(input("Enter a digit: "))

# search the input digit in our list
for i in list_of_digits:
	if input_digit == i:
		print("Digit is in the list")
		break
	else:
		print("Digit not found in list")
output
Digit not found in list
Digit not found in list
Digit not found in list
Digit is in the list
Here, we have a list of digits from 0 to 6. We ask the user to enter a digit and check if the digit is in our list or not. If the digit is present, for loop breaks prematurely. So, the else part does not run. But if the items in our list exhausts (digit not found in our list), the program enters the else part.

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Python while Loop

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 beforehand, the number of times to iterate.

Syntax of while Loop in Python

while test_expression:
    Body of while
In while loop, test expression is checked first. The body of the loop is entered only if the test_expression evaluates to True. After one iteration, the test expression is checked again. This process continues until the test_expression evaluates to False. In Python, the body of the while loop is determined through indentation. Body starts with indentation and the first unindented line marks the end. Python interprets any non-zero value as True. None and 0 are interpreted as False.

Example: Python while Loop

# Program to add natural
# numbers upto 
# sum = 1+2+3+...+n

# To take input from the user,
# n = int(input("Enter n: "))

n = 10

# initialize sum and counter
sum = 0
i = 1

while i <= n:
    sum = sum + i
    i = i+1    # update counter

# print the sum
print("The sum is", sum)
output
The sum is 55
In the above program, the test expression will be True as long as our counter variable i is less than or equal to n (10 in our program). We need to increase the value of counter variable in the body of the loop. This is very important (and mostly forgotten). Failing to do so will result in an infinite loop (never ending loop). Finally the result is displayed.

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while loop with else

Same as that of for loop, we can have an optional else block with while loop as well. The else part is executed if the condition in the while loop evaluates to False. The while loop can be terminated with a break statement. In such case, the else part is ignored. Hence, a while loop's else part runs if no break occurs and the condition is false. Here is an example to illustrate this.
# Example to illustrate
# the use of else statement
# with the while loop

counter = 0

while counter < 3:
    print("Inside loop")
    counter = counter + 1
else:
    print("Inside else")
output
Inside loop
Inside loop
Inside loop
Inside else
Here, we use a counter variable to print the string Inside loop three times. On the forth iteration, the condition in while becomes False. Hence, the else part is executed.


Python break and continue

In Python, break and continue statements can alter the flow of a normal loop. Loops iterate over a block of code until test expression is false, but sometimes we wish to terminate the current iteration or even the whole loop without cheking test expression. The break and continue statements are used in these cases.Python break statement The break statement terminates the loop containing it. Control of the program flows to the statement immediately after the body of the loop. If break statement is inside a nested loop (loop inside another loop), break will terminate the innermost loop.

Syntax of break

break

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Example: Python break

# Use of break statement inside loop

for val in "string":
    if val == "i":
        break
    print(val)

print("The end")
output
s
t
r
The end
In this program, we iterate through the "string" sequence. We check if the letter is "i", upon which we break from the loop. Hence, we see in our output that all the letters up till "i" gets printed. After that, the loop terminates.

Python continue statement

The continue statement is used to skip the rest of the code inside a loop for the current iteration only. Loop does not terminate but continues on with the next iteration.

Syntax of Continue

continue

Example: Python continue

# Program to show
# the use of continue
# statement inside loops

for val in "string":
    if val == "i":
        continue
    print(val)

print("The end")
output
s
t
r
n
g
The end
This program is same as the above example except the break statement has been replaced with continue. We continue with the loop, if the string is "i", not executing the rest of the block. Hence, we see in our output that all the letters except "i" gets printed.


Python Looping Techniques

Python programming offers two kinds of loop, the for loop and the while loop. Using these loops along with loop control statements like break and continue, we can create various forms of loop.

The infinite loop

We can create an infinite loop using while statement. If the condition of while loop is always True, we get an infinite loop.

Example #1: Infinite loop using while

# An example of infinite loop
# press Ctrl + c to exit from the loop

while True:
   num = int(input("Enter an integer: "))
   print("The double of",num,"is",2 * num)

Output

Enter an integer: 3
The double of 3 is 6
Enter an integer: 5
The double of 5 is 10
Enter an integer: 6
The double of 6 is 12
Enter an integer:
Traceback (most recent call last):

Loop with condition at the top

This is a normal while loop without break statements. The condition of the while loop is at the top and the loop terminates when this condition is False.

Example #2: Loop with condition at the top

# Program to illustrate a loop with condition at the top

# Try different numbers
n = 10

# Uncomment to get user input
#n = int(input("Enter n: "))

# initialize sum and counter
sum = 0
i = 1

while i <= n:
   sum = sum + i
   i = i+1    # update counter

# print the sum
# Output: The sum is 55
print("The sum is",sum)
output
The sum is 55

Loop with condition in the middle

This kind of loop can be implemented using an infinite loop along with a conditional break in between the body of the loop.

Example #3: Loop with condition in the middle

# Program to illustrate a loop with condition in the middle. 
# Take input from the user untill a vowel is entered

vowels = "aeiouAEIOU"

# infinite loop
while True:
   v = input("Enter a vowel: ")
   # condition in the middle
   if v in vowels:
       break
   print("That is not a vowel. Try again!")

print("Thank you!")

Output

Enter a vowel: r
That is not a vowel. Try again!
Enter a vowel: 6
That is not a vowel. Try again!
Enter a vowel: ,
That is not a vowel. Try again!
Enter a vowel: u
Thank you!

Loop with condition at the bottom

This kind of loop ensures that the body of the loop is executed at least once. It can be implemented using an infinite loop along with a conditional break at the end. This is similar to the do...while loop in C.

Example #4: Loop with condition at the bottom

# Python program to illustrate a loop with condition at the bottom
# Roll a dice untill user chooses to exit

# import random module
import random

while True:
   input("Press enter to roll the dice")

   # get a number between 1 to 6
   num = random.randint(1,6)
   print("You got",num)
   option = input("Roll again?(y/n) ")

   # condition
   if option == 'n':
       break

Output

Press enter to roll the dice
You got 1
Roll again?(y/n) y
Press enter to roll the dice
You got 5
Roll again?(y/n) n

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