Conditionals and Loops

Conditionals and Loops#

In this chapter, we’re diving into conditionals and loops in Python. Conditionals are a fundamental concept in programming, allowing your code to make decisions based on certain criteria. Loops let you repeat actions multiple times. Together, these enable your programs to respond differently to different inputs or situations, making your code more dynamic and versatile.

Understanding Comparison Operators#

Before we can make decisions, we need to be able to compare values. Python provides several comparison operators for this purpose:

Operator	Meaning	Example	Result
`>`	Greater than	`5 > 3`	`True`
`<`	Less than	`5 < 3`	`False`
`>=`	Greater than or equal to	`5 >= 5`	`True`
`<=`	Less than or equal to	`5 <= 3`	`False`
`==`	Equal to	`5 == 5`	`True`
`!=`	Not equal to	`5 != 3`	`True`

Warning

Notice that we use double equals == for comparison. A single = is for assignment (storing a value in a variable). This is a very common source of bugs for beginners!

These operators allow you to ask questions about the relationship between values. Each comparison returns either True or False — these are called boolean values.

print(5 > 3)

True

print(5 < 3)

False

x = 10
print(x == 10)

True

The `if` Statement#

The if statement is the simplest form of a conditional. It checks a condition: if the condition is True, it executes a block of code. If the condition is False, it skips that code.

x = int(input('Enter a number: '))

if x > 0:
    print('x is positive')

Enter a number: 2
x is positive

Let’s break down what’s happening:

We ask the user for a number and store it in x
The if statement checks whether x > 0 is True
If it is, Python executes the indented code below (print('x is positive'))
If x > 0 is False, Python skips the indented code entirely

Note

Notice the colon : after the condition, and the indentation of the code block. Both are required in Python!

Multiple `if` Statements#

We can check multiple conditions by using multiple if statements:

x = int(input('What is x? '))
y = int(input('What is y? '))

if x < y:
    print('x is less than y')

if x > y:
    print('x is greater than y')

if x == y:
    print('x equals y')

What is x? 2
What is y? 2
x equals y

Notice how this works:

First, the condition x < y is evaluated — if True, it prints the message
Then, x > y is evaluated — if True, it prints the message
Finally, x == y is evaluated — if True, it prints the message

Each if statement is checked independently. This flow of decisions is called control flow.

The `elif` Statement#

The code above works, but it’s inefficient. If x is less than y, we already know it can’t be greater than or equal to y. Why check those conditions at all?

This is where elif (short for “else if”) comes in:

x = int(input("What's x? "))
y = int(input("What's y? "))

if x < y:
    print("x is less than y")
elif x > y:
    print("x is greater than y")
elif x == y:
    print("x is equal to y")

What's x? 2
What's y? 2
x is equal to y

Now the program is smarter:

First, it checks x < y
If that’s True, it prints the message and skips all remaining conditions
If that’s False, it moves to the first elif and checks x > y
And so on…

This is more efficient because once a condition is True, Python doesn’t bother checking the rest.

The `else` Statement#

There’s one more improvement we can make. Think about it logically: if x is not less than y, and x is not greater than y, then x must equal y. We don’t need to check!

The else statement acts as a “catch-all” for when none of the previous conditions were True:

x = int(input('What is x? '))
y = int(input('What is y? '))

if x < y:
    print('x is less than y')
elif x > y:
    print('x is greater than y')
else:
    print('x equals y')

What is x? 2
What is y? 2
x equals y

The else block runs when all previous conditions are False. It doesn’t have a condition of its own — it’s simply “everything else.”

Tip

Use else when you have a clear “default” case. It makes your code cleaner and ensures you always handle every possibility.

The `or` Operator#

Sometimes you want to check if at least one of multiple conditions is true. The or operator does exactly this:

x = int(input('What is x? '))
y = int(input('What is y? '))

if x < y or x > y:
    print('x is not equal to y')
else:
    print('x equals y')

What is x? 2
What is y? 5
x is not equal to y

The condition x < y or x > y is True if either x < y is true or x > y is true (or both).

But wait — there’s a simpler way to check if two values are not equal. The != operator does this directly:

x = int(input('What is x? '))
y = int(input('What is y? '))

if x != y:
    print('x is not equal to y')
else:
    print('x equals y')

What is x? 2
What is y? 5
x is not equal to y

Much cleaner! The != operator returns True if the values are different.

The `and` Operator#

The and operator checks if both conditions are true:

score = int(input("Give me your score: "))

if score >= 90 and score <= 100:
    print('Grade A')
elif score >= 80 and score < 90:
    print('Grade B')
elif score >= 70 and score < 80:
    print('Grade C')
elif score >= 60 and score < 70:
    print('Grade D')
else:
    print('Fail')

Give me your score: 90
Grade A

The condition score >= 90 and score <= 100 is True only if both parts are true — the score must be at least 90 and at most 100.

Chained Comparisons#

Python has a nice shortcut for range checks. Instead of score >= 90 and score <= 100, you can write:

if 90 <= score <= 100:
    print('Grade A')

This reads almost like mathematical notation: “if 90 ≤ score ≤ 100”.

Further Simplification#

We can simplify the grading program even more. Since we use elif, once we’ve determined the score isn’t an A, we know it’s below 90. So we don’t need to check the upper bound:

score = int(input("Give me your score: "))

if score >= 90:
    print('Grade A')
elif score >= 80:
    print('Grade B')
elif score >= 70:
    print('Grade C')
elif score >= 60:
    print('Grade D')
else:
    print('Fail')

Give me your score: 85
Grade B

This works because:

If score is 85, the first check score >= 90 is False
Python moves to the next check: score >= 80 is True
Python prints “Grade B” and skips all remaining conditions

Loops and Iteration#

Loops are powerful constructs that allow you to repeat a block of code multiple times. Python offers two types of loops:

for loops: iterate over a sequence (like a list or range)
while loops: continue as long as a condition remains True

The Problem: Repetition#

Imagine you want to print “meow” three times:

print('meow')
print('meow')
print('meow')

meow
meow
meow

This works, but what if you wanted to print it 500 times? Typing print('meow') 500 times would be tedious and error-prone. This is exactly what loops are for — automating repetition.

The `while` Loop#

A while loop repeats as long as a condition is True:

i = 3

while i != 0:
    print('meow')
    i = i - 1

meow
meow
meow

Let’s trace through this step by step:

Iteration	`i` at start	`i != 0`?	Action	`i` at end
1	3	`True`	Print “meow”	2
2	2	`True`	Print “meow”	1
3	1	`True`	Print “meow”	0
4	0	`False`	Exit loop	—

The formal flow of a while loop:

Evaluate the condition
If False, exit the loop and continue with the next statement
If True, execute the body, then go back to step 1

Warning

The loop body must eventually make the condition False, otherwise you get an infinite loop — your program will run forever! The variable that controls when the loop ends is called the iteration variable.

Here’s another example — a countdown:

n = 5

while n > 0:
    print(n)
    n = n - 1

print('Blastoff!')

5
4
3
2
1
Blastoff!

You can almost read this as English: “While n is greater than 0, print n and reduce n by 1. When n reaches 0, print Blastoff!”

The `for` Loop#

A for loop iterates through a sequence of items. To understand for loops, let’s first introduce lists — a way to store multiple values:

for i in [0, 1, 2]:
    print('meow')

meow
meow
meow

Here, [0, 1, 2] is a list containing three items. The for loop goes through each item:

i is set to 0, and “meow” is printed
i is set to 1, and “meow” is printed
i is set to 2, and “meow” is printed
No more items, so the loop ends

Notice how clean this is compared to the while loop! We don’t need to manage a counter variable ourselves.

The `range()` Function#

What if you wanted to iterate 1000 times? Typing out [0, 1, 2, ..., 999] would be impractical. The range() function generates a sequence of numbers for you:

for i in range(3):
    print('meow')

meow
meow
meow

range(3) generates the numbers 0, 1, and 2 (three numbers total, starting from 0). This is equivalent to [0, 1, 2] but much more convenient for large ranges.

Note

range(n) produces numbers from 0 to n-1 (not including n). So range(5) gives you 0, 1, 2, 3, 4.

You can also specify a starting point:

for i in range(1, 6):  # 1 to 5
    print(i)

Or even a step size:

for i in range(0, 10, 2):  # 0 to 9, stepping by 2
    print(i)

When to Use `for` vs `while`#

Use a for loop when you know in advance how many times you want to iterate, or when you’re going through a collection of items
Use a while loop when you want to repeat until some condition changes, especially when you don’t know in advance how many iterations you’ll need

Exercises#

Exercise 4

Exercise 1: Even or Odd

Write a program that asks the user for a number and prints whether it’s even or odd.

Hint: A number is even if dividing it by 2 leaves no remainder. Use the modulo operator %.

Solution to Exercise 4

n = int(input("Enter a number: "))

if n % 2 == 0:
    print("Even")
else:
    print("Odd")

Exercise 5

Exercise 2: Age Category

Write a program that asks for a person’s age and prints their life stage:

0-12: Child
13-19: Teenager
20-64: Adult
65+: Senior

Solution to Exercise 5

age = int(input("Enter your age: "))

if age <= 12:
    print("Child")
elif age <= 19:
    print("Teenager")
elif age <= 64:
    print("Adult")
else:
    print("Senior")

Exercise 6

Exercise 3: Countdown

Write a program using a while loop that counts down from a number entered by the user to 1, then prints “Blast off!”.

For example, if the user enters 5, it should print:

5
4
3
2
1
Blast off!

Solution to Exercise 6

n = int(input("Start countdown from: "))

while n > 0:
    print(n)
    n = n - 1

print("Blast off!")

Exercise 7

Exercise 4: Sum of Numbers

Write a program using a for loop that calculates the sum of all numbers from 1 to n, where n is entered by the user.

For example, if the user enters 5, the sum is 1+2+3+4+5 = 15.

Solution to Exercise 7

n = int(input("Enter n: "))
total = 0

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

print(f"The sum from 1 to {n} is {total}")

Conditionals and Loops

Contents

Conditionals and Loops#

Understanding Comparison Operators#

The if Statement#

Multiple if Statements#

The elif Statement#

The else Statement#

The or Operator#

The and Operator#