Python Flow Control: If-else and Loop with practical examples

if…else Statement In Python

In the computer programming, the if statement is a contingent statement. It is used to conduct a block of code only when a appointed condition is met. For example

If we need to assign several grades to students based on their scores.

  1. If a student scores upon 90, assign grade A
  2. If a student scores upon 75, assign grade B
  3. If a student scores upon 65, assign grade C

This conditional tasks can be achieved using the if statement

if Statement In Python

An if statement conduct, a block of code only if the mark condition is met.

if condition:
    # body of if statement

Now, if the condition of the if statement below:

  • True - the body of the if statement conduct.
  • False - the body of the if statement is skipped from conduction.

Now see an example-

Screenshot-2024-01-30-031233

Example: if Statement In Python

number = 10

# check if number is greater than 0
if number > 0:
    print('Number is positive')

Sample Output:

Number is positive
This statement always executes

In the upon example, we have maked a variable named number. advertising the test condition,

number > 0

if the number is greater than 0, then the condition evaluates True. Hence, the body of the if statement executes.

Sample Output 2

Let’s alternative the value of the number to a negative integer, say -5.

number = -5

Then, when we run the program, the output will be below-

This statement always executes

This is from the value of the number is less than 0. Here, the condition evaluates to False. Also, the body of the if statement is scamper.

if…else Statement In Python

An if statement can have an elective else clause. The else statement conduct if the condition in the if statement evaluates to False.

if condition:
    # body of if statement

else:
    # body of else statement

Below, if the condition under the if statement evaluates to

  • True - the body of if conducts, and the body of else is scamper.
  • False - the body of else conducts, and the body of if is scamper

Now see an example-

Screenshot-2024-01-30-034115

Example: if…else Statement In Python

number = 10

if number > 0:
    print('Positive number')

else:
    print('Negative number')

print('This statement always executes')

Sample Output :

Positive number
This statement always executes

In the upon example, we have maked a variable named number. Look that the test condition,

Now the value of the number is 10, then condition evaluates to True. Here, code under the body of if is evolved.

if…elif…else Statement In Python

In if...else statement is used to conduct a block of code among two substitutes.

Only, if we need to make a option between more than two alternatives, we conduct the if...elif...else statement.

if condition1:
    # code block 1

elif condition2:
    # code block 2

else: 
    # code block 3

Below,

  • if condition1 - This checks if condition1 is True. Suppose it is, then program conducts code block 1.
  • elif condition2 - If condition1 is not True, then program checks condition2. If condition2 is True, it executes code block 2.
  • else - If neither condition1 nor condition2 is True, the program incapacity to executing code block 3

Now see an example-

Screenshot-2024-01-30-041218

Example: if…elif…else Statement In Python

number = 0

if number > 0:
    print('Positive number')

elif number <0:
    print('Negative number')

else:
    print('Zero')

print('This statement is always executed')

Output :

Zero
This statement is always executed

In the upon example, we have maked a variable named number.

While the value of the number is 0, both the test situations evaluate to False.

Here, the statement under the body of else is evolved.

if Statements Nested In Python

It is probable to include an if statement under another if statement. For example-

number = 5

# outer if statement
if number >= 0:
    # inner if statement
    if number == 0:
      print('Number is 0')
    
    # inner else statement
    else:
        print('Number is positive')

# outer else statement
else:
    print('Number is negative')

Output :

Number is positive
Screenshot-2024-01-30-042136

for Loop In Python

In system programming, loops are used to repeat a block of code.

languages = ['Swift', 'Python', 'Go']

# access items of a list using for loop
for i in languages:
    print(i)

**Output : **

Swift
Python
Go

In the upon example, we have maked a list called languages. So the list has 3 elements, the loop iterates 3 times.

The value of i is below

  • Swift is of the first iteration.
  • Python is of the second iteration.
  • Go is of the third iteration.
for val in sequence:
    # statement(s)

Now , val accesses every item of the sequence on every iteration. The loop continues still we reach the last item in the order.

Flowchart of Python for Loop In Python program

Screenshot-2024-02-02-010702

Example: Loop Through a String In python

language = 'Python'

# iterate over each character in language
for x in language:
    print(x)

Output :

P
y
t
h
o
n

for Loop with Python range() In Python

In Python Program, the range() function returns a order of numbers. For example-

values = range(4)

Above, range(4) returns a order of 0, 1, 2 ,and 3.

You can use range() with a for loop to iterate a particular number of times. For example-

# iterate from i = 0 to i = 3
for i in range(4):
    print(i)

Output :

0
1
2
3

You used the for loop to iterate over a range from 0 to 3.

while Loop In Python

In Python Program, we use the while loop to revolving a block of code until a certain agreement is met. For example-

number = 1

while number <= 3:
    print(number)
    number = number + 1

Output :

1
2
3

In the upon example, you have used a while loop to print the numbers from 1 to 3. The loop runs as long as the agreement number <= 3 is satisfied.

while condition:
    # body of while loop

Below,

  1. The while loop evaluates the agreement.
  2. If the agreement is true, body of while loop is evolved. The agreement is evaluated again.
  3. This process continues until the agreement is False.
  4. Once the agreement evaluates to False, the loop terminates

Flowchart of while Loop In Python

Screenshot-2024-02-02-012212

Example: while Loop In Python

# calculate the sum of five numbers entered by user
sum = 0
count = 0

while count < 5:
    input_value = int(input('Enter a number: '))
    sum += input_value
    count += 1

print('The sum is:', sum)

Output :

Enter a number: 3
Enter a number: 2
Enter a number: 1
Enter a number: 4
Enter a number: 6
The sum is: 16

There is how the above program works in every iteration of the loop below:

Screenshot-2024-02-02-012455

Example: while Loop to Display Game Level In python

# variables to store game information
current_level = 1
final_level = 4
game_completed = True

# iterate until the final level is reached
while current_level <= final_level:
    if game_completed:
        print(f'Welcome to Level {current_level}')
        current_level += 1

print('Game Over!')

Output :

Welcome to Level 1
Welcome to Level 2
Welcome to Level 3
Welcome to Level 4
Game Over!

In the upon example, we have used a while loop to check the present level of the player and display it.

Infinite while Loop

If the agreement of a while loop is all-time True, the loop runs for infinite times, forming an infinite while loop. For example below-

age = 32

# the test condition is always True
while age > 18:
    print('You can vote')

Output :

You can vote
You can vote
You can vote
.
.
.

break and continue In Python

In the programming, the break and continue statements are used to innovate the flow of loops:

  • break passage the loop entirely
  • continue skips the present iteration and revenue to the next one

break Statement In Python

In the break statement terminates the loop instantly when it’s encountered.

break

Working break Statement In Python

Screenshot-2024-02-02-014212

Example: break Statement with for Loop In Python

You can use the break statement with the for loop to terminate the loop when a certain agreement is met. For example-

for i in range(5):
    if i == 3:
        break
    print(i)

Output :

0
1
2

In the upon example,

if i == 3:
    break

terminates the loop when i is equal to 3. here, the output doesn’t add values after 2.

continue Statement In Python

The continue announcement skips the current iteration of the loop and the control flow of the program goes to the after iteration.

continue

Working of continue Statement in Python Program

Screenshot-2024-02-02-034549

Example: continue Statement with for Loop In Python

You can use the continue statement with the for loop to skip the present iteration of the loop and jump to the after iteration. For example-

for i in range(5):
    if i == 3:
        continue
    print(i)

Output : In python

In Python programming, the pass statement is a null statement which can be used as a placeholder for future code.

0
1
2
4

in the upon example-

if i == 3:
    continue

skips the present iteration when i is equal to 3, and run the next iteration. Here, the output has all the values without 3.

pass Statement In python

In Python programming, the pass statement is a null statement what can be used as a placeholder for in future code.

If we have a loop or a function which is not perfect yet, but you want to perfect it in the future. In like cases, we can use the pass statement.

So the syntax of the pass statement is:

pass

Using pass With Conditional Statement In python

n = 10

# use pass inside if statement
if n > 10:
    pass

print('Hello')

There, notice that you have used the pass statement under the if statement .

Still, nothing happens when the pass is accomplished. Now It results in no operation (NOP).

If we didn’t use pass or just put a comment as like below-

n = 10

if n > 10:
    # write code later

print('Hello')

Below, we will show an error message: IndentationError: expected an indented block

Use of pass Statement inside Function or Class In python

We can do the equivalent thing in an empty function or class as well. For example-

def function(args):
    pass

class Example:
    pass
Previous
Python File Handling Essential Tips and Tricks for Mastery