If statement execution flow in Python

Introduction

An if statement helps your program decide what to do based on a condition. It lets your code choose different paths like making a decision in real life.

When you want to check if a number is positive or negative and act differently.

When you want to check if a user entered the correct password before allowing access.

When you want to do something only if it is raining outside.

When you want to give a discount only if the customer buys more than 5 items.

Syntax

Python

if condition:
    # code to run if condition is True
elif another_condition:
    # code to run if another_condition is True
else:
    # code to run if none of the above conditions are True

The if line checks a condition that is either True or False.

The elif and else parts are optional and help check more conditions or run code if none match.

Examples

This checks if the temperature is above 30 and prints a message if true.

Python

if temperature > 30:
    print("It's hot outside!")

This checks multiple conditions to print different grades.

Python

if score >= 90:
    print("You got an A")
elif score >= 80:
    print("You got a B")
else:
    print("Keep trying!")

This chooses between two actions based on whether it is raining.

Python

if is_raining:
    print("Take an umbrella")
else:
    print("No umbrella needed")

Sample Program

This program checks the temperature and prints a message based on its value.

Python

temperature = 25
if temperature > 30:
    print("It's hot outside!")
elif temperature > 20:
    print("The weather is nice.")
else:
    print("It's a bit cold.")

Important Notes

Indentation (spaces before code) is very important in Python to show which code belongs to the if or else.

Only one block of code runs in an if-elif-else chain, the first condition that is True.

If no condition is True and there is no else, nothing happens.

Summary

If statements let your program make choices based on conditions.

Use if, elif, and else to check multiple conditions.

Indent your code properly to show which parts belong to each condition.

Practice

1. What does an if statement do in Python?

easy

A. It stores data in a list.

B. It repeats code multiple times.

C. It runs code only if a condition is true.

D. It defines a new function.

Solution

1. Step 1: Understand the purpose of if

   An if statement checks a condition and runs code only if that condition is true.

2. Step 2: Compare with other options

   Repeating code is done by loops, storing data is done by lists, and defining functions uses def.

3. Final Answer:

   It runs code only if a condition is true. -> Option C

4. Quick Check:

   if runs code if condition true [OK]

Hint: If checks condition; runs code only when true [OK]

Common Mistakes:

• Confusing if with loops
• Thinking if stores data
• Mixing if with function definition

2. Which of the following is the correct syntax for an if statement in Python?

easy

A. if x > 5 then:

B. if x > 5:

C. if (x > 5) {}

D. if x > 5 then

Solution

1. Step 1: Recall Python if syntax

   Python uses a colon : after the condition and no parentheses or 'then'.

2. Step 2: Check each option

   if x > 5: uses if x > 5: which is correct. Others use 'then' or braces which are not Python syntax.

3. Final Answer:

   if x > 5: -> Option B

4. Quick Check:

   Python if ends with colon [OK]

Hint: Python if ends with colon, no 'then' or braces [OK]

Common Mistakes:

• Adding 'then' after condition
• Using braces {} like other languages
• Forgetting the colon at the end

3. What will be the output of this code?

age = 20
if age < 18:
    print("Child")
elif age < 65:
    print("Adult")
else:
    print("Senior")

medium

A. Adult

B. Senior

C. Child

D. No output

Solution

1. Step 1: Check the value of age

   The variable age is 20.

2. Step 2: Evaluate conditions in order

   First condition age < 18 is false (20 is not less than 18). Second condition age < 65 is true (20 is less than 65), so it prints "Adult" and skips the else.

3. Final Answer:

   Adult -> Option A

4. Quick Check:

   20 is less than 65, prints Adult [OK]

Hint: Check conditions top to bottom; first true runs [OK]

Common Mistakes:

• Printing Child for age 20
• Ignoring elif and jumping to else
• Thinking no output if first condition false

4. Find the error in this code:

score = 75
if score >= 90
    print("Excellent")
elif score >= 60:
    print("Pass")
else:
    print("Fail")

medium

A. score variable not defined

B. Wrong indentation on print statements

C. Using elif instead of else if

D. Missing colon after first if condition

Solution

1. Step 1: Check syntax of if statement

   The first if line is missing a colon : at the end, which is required in Python.

2. Step 2: Verify other parts

   Indentation and elif usage are correct. The variable score is defined.

3. Final Answer:

   Missing colon after first if condition -> Option D

4. Quick Check:

   Every if needs a colon [OK]

Hint: Check colons after all if/elif/else lines [OK]

Common Mistakes:

• Forgetting colon after if condition
• Confusing elif with else if syntax
• Incorrect indentation of print lines

5. You want to print "Positive", "Zero", or "Negative" based on a number's value. Which code correctly uses if, elif, and else to do this?

hard

A. if num > 0:
print("Positive")
elif num == 0:
print("Zero")
else:
print("Negative")

B. if num > 0:
print("Positive")
if num == 0:
print("Zero")
else:
print("Negative")

C. if num > 0:
print("Positive")
else if num == 0:
print("Zero")
else:
print("Negative")

D. if num > 0:
print("Positive")
elif num < 0:
print("Zero")
else:
print("Negative")

Solution

1. Step 1: Understand correct if-elif-else structure

   Use if for first condition, elif for the second, and else for all other cases.

2. Step 2: Check each option

   if num > 0: print("Positive") elif num == 0: print("Zero") else: print("Negative") correctly uses elif and else. if num > 0: print("Positive") if num == 0: print("Zero") else: print("Negative") uses two separate if statements which can cause multiple prints. if num > 0: print("Positive") else if num == 0: print("Zero") else: print("Negative") uses invalid syntax else if. if num > 0: print("Positive") elif num < 0: print("Zero") else: print("Negative")'s elif num < 0 will print "Zero" for negative numbers and "Negative" for zero incorrectly.

3. Final Answer:

   if num > 0: print("Positive") elif num == 0: print("Zero") else: print("Negative") -> Option A

4. Quick Check:

   Use if, elif, else for exclusive conditions [OK]

Hint: Use if, elif, else for clear exclusive choices [OK]

Common Mistakes:

• Using multiple separate ifs causing multiple outputs
• Writing else if instead of elif
• Overlapping conditions causing wrong output