If–else execution flow in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an if statement do in Python?

It checks a condition and runs a block of code only if that condition is true.

beginner

What is the purpose of the else block?

The else block runs code when the if condition is false.

beginner

How does Python decide which block to run in an if–else structure?

Python checks the if condition first. If true, it runs the if block. If false, it runs the else block.

beginner

What happens if the if condition is true and there is an else block?

Only the if block runs. The else block is skipped.

beginner

Can you have an if statement without an else?

Yes. The else block is optional. If missing, nothing happens when the if condition is false.

What will this code print?

if 5 > 3:
    print('Yes')
else:
    print('No')

A Yes

B No

C Nothing

D Error

5 is greater than 3, so the if condition is true and 'Yes' is printed.

If the if condition is false and there is no else, what happens?

A The program crashes

B Nothing happens

C The code inside <code>if</code> runs

D The program asks for input

If the condition is false and no else block exists, Python skips the if block and continues.

Which keyword starts the block that runs when the if condition is false?

A elif

B if

C else

D then

The else keyword starts the block that runs when the if condition is false.

What is the correct way to write an if–else statement in Python?

A if condition: do_something else: do_something_else

B if condition then do_something else do_something_else

C if (condition) { do_something } else { do_something_else }

D if condition do_something else do_something_else

Python uses colons and indentation to define blocks, as in option D.

What will this code print?

if False:
    print('A')
else:
    print('B')

A Error

B A

C Nothing

D B

The condition is false, so the else block runs and prints 'B'.

Explain how Python decides which code to run in an if–else statement.

Think about the condition and the two possible paths.

Describe what happens if the if condition is false and there is no else block.

Consider what Python does when no alternative code is given.

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