For loop execution model in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for loop used for in Python?

A for loop is used to repeat a block of code multiple times, once for each item in a sequence like a list or string.

beginner

How does Python decide how many times to run the code inside a for loop?

Python runs the code once for each item in the sequence you give it. It stops when it reaches the end of the sequence.

beginner

What happens to the loop variable in each iteration of a for loop?

The loop variable takes the value of the next item in the sequence each time the loop runs.

beginner

Explain the flow of a for loop with an example:

for x in [1, 2, 3]: print(x)

The loop runs 3 times. First, x = 1 and prints 1. Then x = 2 and prints 2. Finally, x = 3 and prints 3. Then it stops.

beginner

Can a for loop run zero times? When?

Yes, if the sequence is empty (like an empty list), the for loop does not run at all.

What does the loop variable represent in a for loop?

A The total number of items in the sequence

B Each item in the sequence, one at a time

C The index of the last item only

D A fixed value throughout the loop

The loop variable takes each item from the sequence one by one during the loop.

How many times will this loop run?

for i in []: print(i)

A 0 times

B 1 time

C Depends on i

D Infinite times

The sequence is empty, so the loop does not run at all.

What happens after the for loop finishes all items?

A The loop variable resets to zero

B The program stops

C The loop restarts automatically

D The program continues with the next line after the loop

After the loop finishes, the program moves on to the next instruction after the loop.

Which of these is a valid sequence to use in a for loop?

A A boolean like True

B A number like 5

C A list like [1, 2, 3]

D A single character like 'a'

For loops need a sequence like a list, string, or tuple to loop over.

What is printed by this code?

for c in 'Hi': print(c)

A H then i on separate lines

B Hi on one line

C Nothing

D Error

The loop prints each character of the string 'Hi' on its own line.

Describe how a for loop works in Python, including what the loop variable does.

Think about how the loop variable changes each time.

Explain what happens if the sequence in a for loop is empty.

What does the loop do when there is nothing to loop over?

Practice

1. What does a for loop do in Python?

easy

A. Repeats a block of code for each item in a sequence

B. Runs code only once

C. Stops the program immediately

D. Creates a new variable

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop runs code repeatedly for each item in a list, string, or range. 'Repeats a block of code for each item in a sequence' matches this; others do not.

2. Final Answer:

   Repeats a block of code for each item in a sequence -> Option A

3. Quick Check:

   For loop = repeat for each item [OK]

Hint: For loops repeat actions for every item in a list [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2. Which of the following is the correct syntax for a for loop in Python?

easy

A. for (i=0; i<5; i++) print(i)

B. for i to range(5) print(i)

C. for i in range(5): print(i)

D. foreach i in range(5): print(i)

Solution

1. Step 1: Recall Python for loop syntax

   Python uses 'for variable in sequence:' followed by indented code. 'for i in range(5): print(i)' matches; others use wrong keywords or styles.

2. Final Answer:

   for i in range(5): print(i) -> Option C

3. Quick Check:

   Python for loop = for variable in sequence: [OK]

Hint: Use 'for variable in sequence:' syntax in Python [OK]

Common Mistakes:

• Using 'to' instead of 'in'
• Using C-style for loop syntax
• Using 'foreach' which is not Python

3. What is the output of this code?

for i in range(3):
    print(i * 2)

medium

A. 2 4 6

B. 0 1 2

C. 1 2 3

D. 0 2 4

Solution

1. Step 1: Trace the loop range and output

   range(3) gives i=0,1,2. Prints i*2: 0, 2, 4.

2. Final Answer:

   0 2 4 -> Option D

3. Quick Check:

   Multiply each i by 2 = 0 2 4 [OK]

Hint: Multiply loop variable by 2 for each iteration [OK]

Common Mistakes:

• Printing i instead of i*2
• Starting count from 1 instead of 0
• Confusing range(3) with range(1,3)

4. Find the error in this code:

for i in range(5)
    print(i)

medium

A. Indentation error on print line

B. Missing colon ':' after range(5)

C. range(5) is invalid syntax

D. Variable i is not defined

Solution

1. Step 1: Check for syntax errors in for loop

   Python requires ':' after the for statement. The code misses ':' after range(5).

2. Final Answer:

   Missing colon ':' after range(5) -> Option B

3. Quick Check:

   For loop needs ':' after header [OK]

Hint: Always add ':' after for loop header [OK]

Common Mistakes:

• Forgetting colon ':' after for statement
• Incorrect indentation
• Assuming range(5) is invalid

5. You want to create a list of squares for numbers 0 to 4 using a for loop. Which code correctly does this?

hard

A. squares = []
for i in range(5):
squares.append(i*i)
print(squares)

B. squares = [i*i for i in range(5)]
print(squares)

C. squares = []
for i in range(5):
squares = i*i
print(squares)

D. for i in range(5):
squares.append(i*i)
print(squares)

Solution

1. Step 1: Identify correct code for list of squares using for loop

   Initialize squares = [], then for i in range(5): append i*i, then print. squares = [] for i in range(5): squares.append(i*i) print(squares) works; others miss init, use comp, or overwrite.

2. Final Answer:

   squares = []\nfor i in range(5):\n squares.append(i*i)\nprint(squares) -> Option A

3. Quick Check:

   Initialize list, append squares in loop [OK]

Hint: Initialize list before loop, append inside loop [OK]

Common Mistakes:

• Not initializing list before appending
• Overwriting list variable inside loop
• Using list comprehension but question asks for for loop