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Python Programming Looping Techniques, For Loop in Python Programming and While Loop in Python Programming | Python Programming

For loop in Python Programming

What is for loop in Python Programming?

The for loop in Python Programming 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.


Flowchart of for Loop

Flowchart of for Loop

Example: Python for Loop
# Program to find the sum of all numbers stored in a list

# List of numbers
numbers = [6538425411]

# variable to store the sum
sum = 0

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

# Output: The sum is 48
print("The sum is"sum)

when you run the program, the output will be:
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 a 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.
# Output: range(0, 10)
print(range(10))
# Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(list(range(10)))
# Output: [2, 3, 4, 5, 6, 7]
print(list(range(2, 8)))
# Output: [2, 5, 8, 11, 14, 17]
print(list(range(2, 20, 3)))   
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.

When you run the program, the output will be:
I like Classical
I like rock
I like Bollywood


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.
digits = [015]
for i in digits:
    print(i)
else:
    print("No items left.")
When you run the program, the output will be:
0
1

5

No items left.
Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the block of code in the else and prints
No items left.


While Loop in Python Programming

What is while loop in Python?

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 TrueNone and 0 are interpreted as False.


Flowchart of while Loop
Flowchart of while Loop


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)

When you run the program, the output will be:
Enter n: 10
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.



While loop with else


Same as that of for loop, we can have an optional else block with a 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 a 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 Looping Technique

Python programming offers two kinds of the 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 a while statement. If the condition of while loop is always True, we get an infinite loop.
Example #1: Infinite loop using while
1.  # An example of infinite loop
2.  # press Ctrl + c to exit from the loop
3.   
4.  while True:
5.     num = int(input("Enter an integer: "))
6.     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 the 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.

Flowchart of Loop With Condition at Top



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
print("The sum is",sum)

When you run the program, the output will be:
The sum is 55


Loop with the 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.
Flowchart of Loop with Condition in Middle
Flowchart of Loop with Condition in Middle

Example #3: Loop with condition in the middle
1.  # Program to illustrate a loop with the condition in the middle.
2.  # Take input from the user until a vowel is entered
3.   
4.  vowels = "aeiouAEIOU"
5.   
6.  # infinite loop
7.  while True:
8.     v = input("Enter a vowel: ")
9.     # condition in the middle
10.    if v in vowels:
11.        break
12.    print("That is not a vowel. Try again!")
13.  
14. 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 the 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.
Flowchart of Loop with Condition at Bottom
Flowchart of Loop with Condition at Bottom

Example #4: Loop with condition at the bottom
1.   
2.  # Python program to illustrate a loop with the condition at the bottom
3.  # Roll a dice until user chooses to exit
4.   
5.  # import random module
6.  import random
7.   
8.  while True:
9.     input("Press enter to roll the dice")
10.  
11.    # get a number between 1 to 6
12.    num = random.randint(1,6)
13.    print("You got",num)
14.    option = input("Roll again?(y/n) ")
15.  
16.    # condition
17.    if option == 'n':
18.        break
19.  
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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