If–else execution flow in Python - Mini Project: Build & Apply

If-else Execution Flow

📖 Scenario: You are building a simple program to check if a person is old enough to vote. Voting age is 18 years or older.

🎯 Goal: Create a program that uses if-else statements to decide if a person can vote based on their age.

📋 What You'll Learn

Create a variable called age with the exact value 20

Create a variable called voting_age and set it to 18

Use an if-else statement to check if age is greater than or equal to voting_age

Print You can vote! if the person is old enough

Print You cannot vote yet. if the person is too young

💡 Why This Matters

🌍 Real World

Programs often need to make decisions based on conditions, like checking age for voting or permissions.

💼 Career

Understanding if-else statements is fundamental for any programming job because it controls the flow of decisions.

1

Create the age variable

Create a variable called age and set it to the number 20.

Python

# Create a variable called age and set it to 20
# Your code here

Hint

Use = to assign the value 20 to the variable age.

2

Set the voting age

Create a variable called voting_age and set it to 18.

Python

age = 20
# Create a variable called voting_age and set it to 18
# Your code here

Hint

Voting age is usually 18. Assign this number to voting_age.

3

Write the if-else statement

Use an if-else statement to check if age is greater than or equal to voting_age. Use the exact variables age and voting_age in your condition.

Python

age = 20
voting_age = 18
# Write an if-else statement to check if age >= voting_age
# Your code here

Hint

Use if age >= voting_age: to start the condition. Then add else: for the other case.

4

Display the result

Run the program and print the output. The program should print You can vote! because age is 20.

Python

age = 20
voting_age = 18
if age >= voting_age:
    print("You can vote!")
else:
    print("You cannot vote yet.")
# Run the program to see the output

Hint

Just run the program. It will print the correct message based on the age.

Practice

1. What does an if-else statement do in Python?

easy

A. It chooses between two paths based on a condition.

B. It repeats code multiple times.

C. It defines a function.

D. It creates a list.

Solution

1. Step 1: Understand the purpose of if-else

   An if-else statement lets the program decide which code to run based on a condition being true or false.

2. Step 2: Compare with other options

   Repeating code is done by loops, functions define reusable code blocks, and lists store multiple items, so these are not correct.

3. Final Answer:

   It chooses between two paths based on a condition. -> Option A

4. Quick Check:

   If-else = choose path [OK]

Hint: If-else picks one of two paths based on condition [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else creates data structures
• Mixing if-else with function definitions

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

easy

A. if x > 0:
print('Positive')
else:
print('Non-positive')

B. if x > 0 then print('Positive') else print('Non-positive')

C. if (x > 0) { print('Positive'); } else { print('Non-positive'); }

D. if x > 0
print('Positive')
else
print('Non-positive')

Solution

1. Step 1: Recall Python if-else syntax

   Python uses a colon after the condition and indentation for the code blocks.

2. Step 2: Check each option

   if x > 0: print('Positive') else: print('Non-positive') uses colons and indentation correctly. if x > 0 then print('Positive') else print('Non-positive') uses 'then' which is not Python syntax. if (x > 0) { print('Positive'); } else { print('Non-positive'); } uses braces and semicolons, which are for other languages. if x > 0 print('Positive') else print('Non-positive') misses colons and indentation.

3. Final Answer:

   if x > 0: print('Positive') else: print('Non-positive') -> Option A

4. Quick Check:

   Colon + indent = correct if-else [OK]

Hint: Remember colons and indentation for if-else in Python [OK]

Common Mistakes:

• Using 'then' keyword
• Forgetting colons after if and else
• Not indenting code blocks

3. What will be the output of this code?

age = 18
if age >= 18:
    print('Adult')
else:
    print('Minor')

medium

A. Minor

B. Adult

C. 18

D. No output

Solution

1. Step 1: Evaluate the condition age >= 18

   Since age is 18, the condition age >= 18 is true.

2. Step 2: Determine which block runs

   Because the condition is true, the code inside the if block runs, printing 'Adult'.

3. Final Answer:

   Adult -> Option B

4. Quick Check:

   Condition true -> print 'Adult' [OK]

Hint: Check if condition is true or false to pick output [OK]

Common Mistakes:

• Assuming >= means less than
• Printing else block by mistake
• Confusing output with variable value

4. Find the error in this code:

num = 5
if num > 0
    print('Positive')
else:
    print('Non-positive')

medium

A. Variable 'num' is not defined

B. Wrong indentation on print statements

C. Missing colon ':' after 'if num > 0' line

D. 'else' should be 'elif' here

Solution

1. Step 1: Check syntax of if statement

   The if statement must end with a colon ':' to mark the start of the block.

2. Step 2: Identify the missing colon

   The code misses the colon after 'if num > 0', causing a syntax error.

3. Final Answer:

   Missing colon ':' after 'if num > 0' line -> Option C

4. Quick Check:

   Colon needed after if condition [OK]

Hint: Always put colon after if condition [OK]

Common Mistakes:

• Forgetting colon after if
• Confusing else with elif
• Incorrect indentation

5. You want to write a program that prints 'Even' if a number is even, and 'Odd' if it is odd. Which code correctly uses if-else to do this?

hard

A. num = 4
if num % 2:
print('Even')
else:
print('Odd')

B. num = 4
if num / 2 == 0:
print('Even')
else:
print('Odd')

C. num = 4
if num % 2 != 0:
print('Even')
else:
print('Odd')

D. num = 4
if num % 2 == 0:
print('Even')
else:
print('Odd')

Solution

1. Step 1: Understand how to check even numbers

   A number is even if dividing by 2 leaves no remainder, so num % 2 == 0 is true for even numbers.

2. Step 2: Check each option's condition

   num = 4 if num % 2 == 0: print('Even') else: print('Odd') correctly uses num % 2 == 0. num = 4 if num / 2 == 0: print('Even') else: print('Odd') uses division instead of modulo, which is wrong. num = 4 if num % 2: print('Even') else: print('Odd') treats nonzero remainder as even, which is incorrect. num = 4 if num % 2 != 0: print('Even') else: print('Odd') reverses the logic.

3. Final Answer:

   num = 4 if num % 2 == 0: print('Even') else: print('Odd') -> Option D

4. Quick Check:

   Modulo equals zero means even [OK]

Hint: Use 'num % 2 == 0' to check even numbers [OK]

Common Mistakes:

• Using division instead of modulo
• Reversing even and odd logic
• Checking truthiness of modulo without comparison