Python While Loops, For Loops, and the range() Function

Before we continue, let us quickly review ideas you have already learned. These concepts will appear throughout this lesson.

Variables and Assignment

A variable stores a value. You set it with =.

i = 1       # i is a variable holding the integer 1
name = "Adaeze"

Conditions and Comparison Operators

A condition is a question that Python answers with True or False. You already know these from the if statement lesson.

The print() Function

You use print() to display a value or message on the screen.

print("Good morning Lagos!")
print(42)

Lists

A list is an ordered collection of items.

foods = ["jollof rice", "egusi soup", "suya"]

You already know all of this. Now let us put it to work inside loops!

Why Does a Loop Exist?

Imagine Emeka is a teacher at a secondary school in Abuja. He wants to print the name of every student in his class. The class has 30 students. Without a loop, he would have to write 30 separate print() lines. That is tedious, error-prone, and wastes time.

A loop is an instruction that tells Python:

"Run this block of code repeatedly. Keep going until I tell you to stop."

This is the core idea. Everything else in this lesson builds on top of it.

What Is a while Loop?

A while loop checks a condition before every repetition. As long as the condition is True, the loop body keeps running. The moment the condition becomes False, the loop stops.

Think of it like a security guard at a Lagos bank: "While the queue has customers, keep serving the next person."

The Syntax (Structure)

while condition:
    # code to repeat

• while · the keyword that starts the loop
• condition · any True/False expression (just like in an if statement)
• : · the colon that opens the loop body
• indented code · the instructions that run on every repetition

Simple Example 1: Count from 1 to 5

i = 1
while i < 6:
    print(i)
    i += 1

Expected Output:

1
2
3
4
5

Let us go line by line:

• i = 1 · We create a variable i and start it at 1. This is called the counter variable or loop variable.
• while i < 6: · Python checks: "Is i less than 6?" If yes, enter the loop. If no, skip it entirely.
• print(i) · Print the current value of i.
• i += 1 · This means i = i + 1. We increase i by 1. This is called incrementing the counter.
• After i += 1, Python goes back to while i < 6: and checks the condition again.
• When i reaches 6, the condition i < 6 becomes False, and the loop ends.

Very Important: You must update the counter variable inside the loop (i += 1). If you forget, i stays at 1 forever, and the condition i < 6 is always True. This creates an infinite loop · a loop that never stops. Your program will freeze!

Simple Example 2: Countdown from 5 to 1

Let us count down this time:

i = 5
while i > 0:
    print(i)
    i -= 1   # i -= 1 means i = i - 1 (decrement)
print("Liftoff!")

Expected Output:

5
4
3
2
1
Liftoff!

Notice: the print("Liftoff!") line is not indented, so it runs after the loop finishes.

💡 Thinking Prompt: What happens if you change i = 5 to i = 3? How many times will the loop run?

3.1 The break Statement

The break keyword immediately exits the loop · even if the condition is still True. It is like an emergency exit.

Real-life analogy: Imagine Ngozi is sampling bottles of groundnut oil in a warehouse. She checks each bottle. The moment she finds a bad one, she breaks · she stops checking and reports the problem immediately.

Example: Stop the loop when i equals 3

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1

Expected Output:

1
2
3

Line-by-line explanation:

• The loop starts with i = 1 and counts up.
• On each repetition, it prints i, then checks if i == 3.
• When i becomes 3, Python hits break and immediately exits the loop.
• The numbers 4 and 5 are never printed.

⚠️ Notice that i += 1 is after the if check. So when i is 3, Python prints 3, then hits break, and never reaches i += 1.

3.2 The continue Statement

The continue keyword skips the rest of the current repetition and jumps back to check the condition again. It does not exit the loop · it just skips one turn.

Real-life analogy: Amaka is reading through 10 exam papers. Whenever she sees a blank paper, she continues · she skips it and moves on to the next one, without stopping the whole process.

Example: Skip number 3, print everything else from 1 to 5

i = 0
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)

Expected Output:

1
2
4
5
6

Careful study of this code:

• i = 0 · We start at 0 this time.
• i += 1 · This is the first line inside the loop. It increments i before anything else.
• if i == 3: continue · When i is 3, Python jumps back to while i < 6: without reaching print(i).
• All other values of i reach print(i) and get printed.

🔑 Why does i += 1 come before the if check here? Because if i += 1 were after continue, we would never increment i when i == 3. That would create an infinite loop! Always plan where your increment goes.

3.3 The else Statement in a while Loop

Python allows you to attach an else block to a while loop. The else block runs once · right after the loop condition becomes False (i.e., when the loop ends naturally).

Important rule: The else block does NOT run if the loop ends with break.

Example: Count from 1 to 5, then announce the end

i = 1
while i < 6:
    print(i)
    i += 1
else:
    print("i is no longer less than 6")

Expected Output:

1
2
3
4
5
i is no longer less than 6

Example: The else block does NOT run after break

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1
else:
    print("This will NOT print because break was used!")

Expected Output:

1
2
3

The else block is skipped entirely because break ended the loop.

💡 Real-world use: The while-else pattern is great for search tasks. Loop while searching; if found, break. The else block runs only if you finished searching without finding it · which means "not found."

What Is a for Loop?

A for loop is used to iterate over a sequence. "Iterate" means to go through each item, one by one.

Think of it like Fatimah checking every stall at a market. She visits each stall · no more, no less · and does something at each one.

A for loop works on:

• Lists
• Tuples
• Strings (every character is treated as a single item)
• Dictionaries
• Sets
• Ranges (we will cover this soon)

The key difference from while: A for loop does not need you to manage a counter variable manually. Python automatically picks up the next item from the sequence on each turn.

The Syntax

for variable in sequence:
    # code to run for each item

• for · keyword that starts the loop
• variable · a name you choose; Python puts the current item here on each turn
• in · keyword connecting the variable to the sequence
• sequence · the list, string, or range to loop through
• : · opens the loop body
• indented code · runs once for every item

Simple Example 1: Loop through a list of Nigerian foods

foods = ["jollof rice", "egusi soup", "suya", "puff puff"]
for food in foods:
    print(food)

Expected Output:

jollof rice
egusi soup
suya
puff puff

What is happening here?

• On Turn 1: Python sets food = "jollof rice" and runs print(food).
• On Turn 2: Python sets food = "egusi soup" and runs print(food).
• This continues until every item in the list has been visited.
• The loop ends automatically. No i += 1 needed!

Simple Example 2: Loop through names

names = ["Chidi", "Aisha", "Emeka", "Ngozi"]
for name in names:
    print("Hello,", name)

Expected Output:

Hello, Chidi
Hello, Aisha
Hello, Emeka
Hello, Ngozi

Strings are sequences of characters, so you can loop through them letter by letter.

Example: Print each letter in a city name

for letter in "Lagos":
    print(letter)

Expected Output:

L
a
g
o
s

On each turn, letter holds the next character in the string "Lagos". After the last character "s", the loop ends.

Example: Count vowels in a word

word = "Abuja"
vowels = "aeiouAEIOU"
count = 0

for letter in word:
    if letter in vowels:
        count += 1

print("Number of vowels:", count)

Expected Output:

Number of vowels: 3

(The vowels in "Abuja" are: A, u, a)

💡 Thinking Prompt: What would happen if you change "Abuja" to "Kano"? How many vowels does "Kano" have? Try to predict before running the code.

Just like in while loops, you can use break and continue inside for loops.

6.1 break in a for Loop

break exits the loop immediately when a condition is met.

Example 1: Stop when you find "banana"

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
    if fruit == "banana":
        break

Expected Output:

apple
banana

The loop prints "apple", then "banana", then hits break and stops. "cherry" is never printed.

Example 2: Break BEFORE printing

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    if fruit == "banana":
        break
    print(fruit)

Expected Output:

apple

Here, the if check comes before print. So when fruit == "banana", Python breaks before ever printing "banana".

💡 Thinking Prompt: Why does Example 1 print "banana" but Example 2 does not? What changed?

6.2 continue in a for Loop

continue skips the current item and moves to the next one.

Example: Print all fruits except "banana"

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    if fruit == "banana":
        continue
    print(fruit)

Expected Output:

apple
cherry

When fruit == "banana", Python hits continue, skips print(fruit), and immediately goes to the next item in the list · which is "cherry".

Why Does range() Exist?

Imagine you want to print the numbers from 1 to 1000. You definitely do not want to type out a list of 1000 numbers! The range() function solves this: it generates a sequence of numbers automatically.

What is range()? range() is a built-in Python function that returns an immutable sequence of numbers. "Immutable" means the sequence cannot be changed after it is created. It has its own special data type, also called range.

The Syntax

range(start, stop, step)

7.1 range() with One Argument

When you call range() with only one number, that number is the stop value. The sequence starts at 0 and goes up to but not including the stop value.

x = range(10)
print(list(x))

Expected Output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

⚠️ range(10) gives you numbers 0 to 9, not 0 to 10! The stop value is exclusive · it is not included.

💡 Tip: To display the contents of a range, we wrap it in list(). This converts it to a regular list that print() can show. You will learn more about this below.

7.2 range() with Two Arguments

When you call range() with two numbers, the first is start and the second is stop.

x = range(3, 10)
print(list(x))

Expected Output:

[3, 4, 5, 6, 7, 8, 9]

The sequence starts at 3 and goes up to 9 (not 10).

7.3 range() with Three Arguments

When you call range() with three numbers, the third is the step · how much to add each time.

x = range(2, 30, 3)
print(list(x))

Expected Output:

[2, 5, 8, 11, 14, 17, 20, 23, 26, 29]

Starting at 2, add 3 each time, stop before 30.

7.4 Using range() in a for Loop

This is the most common use of range(): to run a loop a specific number of times.

Example 1: Print numbers 0 to 5

for x in range(6):
    print(x)

Expected Output:

0
1
2
3
4
5

Example 2: Print numbers 2 to 5 (using start and stop)

for x in range(2, 6):
    print(x)

Expected Output:

2
3
4
5

Example 3: Print even numbers from 0 to 20 (using step)

for x in range(0, 21, 2):
    print(x)

Expected Output:

0
2
4
6
8
10
12
14
16
18
20

Example 4: Count down using a negative step

for x in range(10, 0, -1):
    print(x)

Expected Output:

10
9
8
7
6
5
4
3
2
1

When the step is negative, the sequence counts down. Start must be greater than stop for this to work.

8.1 Converting a range to a List

The range object itself is not a list · it just represents the numbers. To display all the values at once, convert it to a list using list().

print(list(range(5)))
print(list(range(1, 6)))
print(list(range(5, 20, 3)))

Expected Output:

[0, 1, 2, 3, 4]
[1, 2, 3, 4, 5]
[5, 8, 11, 14, 17]

8.2 Slicing a range

You can access individual elements of a range using index notation (just like a list), and you can also slice it.

r = range(10)
print(r[2])     # the item at index 2
print(r[:3])    # a new range from index 0 up to (not including) 3

Expected Output:

2
range(0, 3)

• r[2] gives you the value at position 2, which is 2 (since the range starts at 0).
• r[:3] gives you a new range object · range(0, 3) · not a list.

8.3 Testing Membership with in

You can check whether a number is inside a range using the in operator.

r = range(0, 10, 2)
print(list(r))
print(6 in r)
print(7 in r)

Expected Output:

[0, 2, 4, 6, 8]
True
False

• 6 in r is True because 6 is in the sequence [0, 2, 4, 6, 8].
• 7 in r is False because 7 is not in that sequence (we skip odd numbers with step 2).

8.4 Getting the Length of a range

Use len() to find out how many numbers are in a range.

r = range(0, 10, 2)
print(len(r))

Expected Output:

5

The range [0, 2, 4, 6, 8] has 5 numbers, so len(r) returns 5.

Just like with while, you can add an else block to a for loop. It runs once after the loop has finished iterating through all items · but only if the loop was not ended by a break.

Example 1: Normal completion · else runs

for x in range(6):
    print(x)
else:
    print("Finally finished!")

Expected Output:

0
1
2
3
4
5
Finally finished!

Example 2: Loop broken early · else does NOT run

for x in range(6):
    if x == 3:
        break
    print(x)
else:
    print("Finally finished!")

Expected Output:

0
1
2

The else block is skipped because break ended the loop at x == 3.

💡 Real-world use: This pattern is common in search tasks. You loop through a list looking for something. If you find it, you break. The else block runs only if you went through the entire list and never found it.

A nested loop is a loop inside another loop. The inner loop runs completely for every single step of the outer loop.

Real-life analogy: Think of Damilola visiting every floor of a building in Lagos, and on each floor she knocks on every office door. For each floor (outer loop), she knocks on every door (inner loop).

The Syntax

for outer_item in outer_sequence:
    for inner_item in inner_sequence:
        # this code runs for every combination

Example 1: Print combinations of colours and items

colours = ["red", "green", "blue"]
items = ["bag", "shoe"]

for colour in colours:
    for item in items:
        print(colour, item)

Expected Output:

red bag
red shoe
green bag
green shoe
blue bag
blue shoe

For every colour (3 colours), Python goes through all items (2 items). Total combinations: 3 × 2 = 6 lines.

Example 2: Multiplication table for 3 using nested loops

for i in range(1, 4):        # outer loop: i goes 1, 2, 3
    for j in range(1, 6):    # inner loop: j goes 1, 2, 3, 4, 5
        print(i, "x", j, "=", i * j)
    print("---")             # printed after each inner loop finishes

Expected Output:

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
1 x 4 = 4
1 x 5 = 5
---
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
2 x 4 = 8
2 x 5 = 10
---
3 x 1 = 3
3 x 2 = 6
3 x 3 = 9
3 x 4 = 12
3 x 5 = 15
---

A for loop (or while loop) cannot be empty. If you write a loop with no body, Python will give you a SyntaxError. But sometimes, while you are planning your code, you want to create a loop body you have not written yet. The pass statement acts as a placeholder.

for x in [0, 1, 2]:
    pass

This runs without any output and without any errors. pass tells Python: "There is nothing to do here · just continue."

💡 This is useful when you are building a program in stages. You sketch out the loop first with pass, then come back to fill in the body later.

Mistake 1: Forgetting to Increment in a while Loop

# WRONG — infinite loop!
i = 1
while i < 6:
    print(i)
    # i never changes! i stays at 1 forever

# CORRECT
i = 1
while i < 6:
    print(i)
    i += 1   # always update the counter!

How to spot it: Your program prints the same number over and over and never stops. Press Ctrl + C to force-quit.

Mistake 2: Off-by-One Error with range()

# WRONG — only prints 1 to 4, not 1 to 5
for i in range(1, 5):
    print(i)

# CORRECT — to include 5, stop must be 6
for i in range(1, 6):
    print(i)

Rule to remember: range(start, stop) includes start but excludes stop. If you want to include a number n, write range(1, n + 1).

Mistake 3: Wrong Indentation in Loop Body

# WRONG — the print is outside the loop!
for i in range(3):
    pass
print(i)   # this only runs once, after the loop

# CORRECT — print is inside the loop
for i in range(3):
    print(i)   # this runs 3 times

In Python, indentation is everything. The loop body must be indented (4 spaces or 1 tab) consistently.

Mistake 4: Expecting continue to Exit the Loop

# WRONG thinking — continue does NOT exit the loop
for i in range(5):
    if i == 3:
        continue   # this SKIPS 3, but the loop keeps going
    print(i)

# If you want to EXIT at 3, use break:
for i in range(5):
    if i == 3:
        break
    print(i)

Mistake 5: Confusing range(n) with a List

# WRONG — range is not a list; you cannot do this directly
r = range(5)
print(r)   # Output: range(0, 5) — NOT [0, 1, 2, 3, 4]

# CORRECT — convert to a list to see all values
print(list(r))   # Output: [0, 1, 2, 3, 4]

Mistake 6: Empty for Loop Body

# WRONG — this causes a SyntaxError
for x in range(5):
    # nothing here

# CORRECT — use pass as a placeholder
for x in range(5):
    pass

Mistake 7: Expecting else to Run After break

# WRONG expectation
for x in range(5):
    if x == 3:
        break
else:
    print("This runs!")   # This does NOT run after break

# The else block only runs if the loop completed WITHOUT break

End of Lesson 14 · Next up: Python Functions