11. For Loop

A for loop is a primary control flow structure used to repeat a code block a defined number of times. It is the preferred loop when the sequence of items to process is known in advance.

By operating over an iterable sequence, this structure automatically manages the count. This ensures the computational logic is more efficient and streamlined for clearly defined repetition tasks.

1. Loop Structure and Control Flow

1.1. Basic Structure and Iteration

A for loop repeats a block of code for a defined number of times or for each item in a sequence. It handles the iteration automatically, meaning you do not need to manually manage an index variable.

# General Structure
for variable in iterable_sequence:
    # repeated instructions (code block)

The loop's control flow is determined by the iterable_sequence. For every item in that sequence, the code block runs once. The loop variable (variable in the example) takes on the value of the current item in each turn.

1.2. The range() Function

The built-in range() function is the primary way to execute a for loop a specific number of times. It generates a sequence of numbers that the loop iterates through.

Function Form	Description
range(stop)	Generates a sequence from 0 up to, but not including, stop.
range(start, stop)	Generates a sequence from start up to, but not including, stop.
range(start, stop, step)	Generates a sequence from start up to, but not including, stop, incrementing or decrementing by step.

2. Python Shorts (Video)

A quick, visual explanation of the for loop.

If the video doesn't load, open directly on YouTube: Watch Python Shorts: for Loop ↗

3. Commented Examples

3.1. Iterating a Defined Range

# 1. Loop: Use for to display the numbers from 2 to 5
for n in range(2, 6):
    # 2. Output: Print the current number
    print(n)
# Output:
# 2
# 3
# 4
# 5

Explanation:

• The range(2, 6) function generates a sequence starting at 2 and stopping strictly before 6.
• The for loop runs for a known, defined number of iterations (4 times).
• The loop variable (n) automatically takes the values: 2, 3, 4, and 5.

3.2. Using the Step Parameter for Countdown

# 1. Loop: Use for a countdown, counting backwards by 5
for x in range(20, 1, -5):
    # 2. Output: Print the current number
    print(x)
# Output:
# 20
# 15
# 10
# 5

Explanation:

• The loop uses a negative step (-5) to count backward from the start value (20).
• The loop continues until the value reaches the stop boundary (1).
• This structure is used for sequences where the number is decreasing in a fixed interval.

3.3. Iterating Over a String

# 1. Setup: Initialize the variable word
word = "Python"
# 2. Loop: Iterate over each characters in word
for char in word:
    #3. Output: Print the current character
    print(char)
# Output:
# P
# y
# t
# h
# o
# n

Explanation:

• For loops are used to directly iterate through any iterable sequence (like a string).
• In each turn, the loop variable (char) automatically holds the value of the current item (letter).
• The loop executes once for every item in the sequence.

4. Short Practice Exercises

4.1. Predict the Output (Iterating a String)

What will be printed by the following code?

word = "LOOP"
for letter in word:
    print(letter)

Show solution

L
O
O
P

Explanation: The for loop iterates through the string, printing each character on a new line.

4.2. Predict the Output (Counting with Step)

What numbers will be printed by the following loop?

# Prints multiples of 3
for num in range(3, 16, 3):
    print(num)

Show solution

3
6
9
12
15  

Explanation: The sequence starts at 3, increases by a step of 3, and stops before the number 16 (exclusive), resulting in multiples up to 15.

4.3. Predict Final Value

What is the final value of the total variable after the following code snippet executes?

total = 0
for i in range(1, 8):
    total = total + i

Show solution

28

Explanation: The loop calculates the sum of numbers from 1 up to and including 7 (range(1, 8)), which is 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28.

4.4. Correct the Code (Reverse Count)

The following code is intended to count down by ones from 8 to 3 (inclusive). Correct the range() function so that it prints 8, 7, 6, 5, 4, 3.

# Incorrect countdown:
for i in range(8, 3, 1):
    print(i)

Show solution

# Correct countdown:
for i in range(8, 2, -1):
    print(i)

Explanation: For a countdown, the step must be negative (-1), and the stop value must be 1 less than the last desired number (stopping at 2 is necessary to include 3).

5. Google Colab: Try It Yourself

Practice for loops with this interactive Colab notebook:

👉 Open the For Loope Colab notebook ↗

First time using Google Colab? Read the quick beginner guide ↗

6. Mini Quiz

6.1. What is the main characteristic that distinguishes a for loop from a while loop?

A) The for loop runs as long as a condition is true.
B) The for loop repeats a known, defined number of times.
C) The for loop is only used for comparisons.
D) The for loop cannot contain if statements.

Show answer

B) The for loop repeats a known, defined number of times.

6.2. If you use range(1, 10, 2), what does the number '10' represent?

A) The starting number of the sequence.
B) The step size between numbers.
C) The number that the sequence will stop immediately before (exclusive).
D) The number of times the loop will run.

Show answer

C) The number that the sequence will stop immediately before (exclusive).

6.3. What is the output of the Python code for n in range(4): print(n)?

A) 1 2 3 4
B) 0 1 2 3
C) 0 1 2 3 4
D) 4

Show answer

B) 0 1 2 3

6.4. How do you create a sequence that counts backward (e.g., from 10 down to 1) using the range() function?

A) By making the start parameter negative.
B) By omitting the stop parameter.
C) By using a negative value for the step parameter.
D) By using the while loop instead.

Show answer

C) By using a negative value for the step parameter.

6.5. In the structure for variable in iterable:, what defines the instructions that will be repeated in each iteration?

A) The use of the range() function.
B) The statements placed outside the loop structure.
C) The statements that are correctly indented below the for statement.
D) The variable name used in the loop.

Show answer

C) The statements that are correctly indented below the for statement.

7. Common Mistakes

• Confusing for loop with while loop when iterations are unknown.
• Assuming the stop value in range() is inclusive (it is always exclusive).
• Forgetting the Colon (:) at the end of the for statement line.
• Trying to manually modify the for loop variable (n = n + 1), which is automatic.

8. Summary Diagram

mindmap
  root((For Loop))
    Concept
      Repeats a known number of times
      Iterates over a sequence
    range Function
      Defines start, stop, and step
      Used for counting
    Iteration
      Automatically manages the loop variable
      Used on strings, lists, or sequences
    Key Points
      Don't manually change the loop variable
      Stop value is exclusive

9. Optional Extensions

• Research and demonstrate how to use a for loop inside another for loop to print a multiplication table (e.g., 1x1 to 10x10).
• Explore the break and continue keywords and write a program using them to exit or skip iterations within a for loop.
• Write a Python program that initializes a variable to calculate the factorial of a number (e.g., 5! = 5 * 4 * 3 * 2 * 1) using a for loop.