Conditionals

Last updated on 2026-10-21 | Edit this page

Overview

Questions

  • How can my programs do different things based on data values?

Objectives

  • Write conditional statements including if, elif, and else branches.
  • Trace the execution of conditionals.
  • Apply conditionals inside loops.

Booleans can be True or False

We’ve discussed the following primitive data types so far: strings, ints, and floats. Let’s add bools or Booleans to that list.

Booleans can be True or False. They are returned by conditional statements formed with operators like ==, <, or >.

PYTHON 

print(3 > 5) # Should return False

OUTPUT 

False

The == sign and = have very different meanings in Python. The == sign tests whether or not two values are equal.

PYTHON 

y = 4 # Assignment
print (y == 4) # Is y (4) equal to 4?

OUTPUT 

True

Use if statements to control whether or not a block of code is executed

Today, we will discuss control flow, one of the most powerful aspects of programming languages.

  • An if statement controls whether or not a block of code is executed.
  • The first line of an if statement starts with if and ends with a colon.
  • The body of an if statement is indented, just like a for loop.

PYTHON 

temp_f = 95
if temp_f > 90: # The indented block runs if (temp_f > 90) is True
    print("Specimen is too hot")

OUTPUT 

Specimen is too hot

If the condition tested in the if statement evalutates to True, then the block of code underneath the if statement executes. If it evaluates to False, the code block is skipped.

Conditionals can be used with loops

Conditionals can be used used inside loops to do different things with different items in a collection.

PYTHON 

grades = [70, 85, 90, 42, 95, 63]
for grade in grades:
    if grade >= 70:
        print("This is a passing grade", grade)

OUTPUT 

This is a passing grade 70
This is a passing grade 85
This is a passing grade 90
This is a passing grade 95

Let’s combine this with the accumulator pattern to answer a more sophisticated question. Rather than print the passing grades, let’s print the number of students who passed.

PYTHON 

passed = 0 # Initialize to zero
for grade in grades:
    if grade >= 70:
        passed = passed + 1
print(passed, "students passed the class.")

OUTPUT 

4 students passed the class.

Use else to execute a block of code when an if condition is not true.

Else executes a block of code when an if condition is not True. It is always paired with an if statement.

  • else is indented like the if it is paired with.
  • Not every if needs an else but every else needs an if.

PYTHON 

temp = 32
if temp > 90:
    print("It's too hot")
else:
    print("It's freezing.")

OUTPUT 

It's freezing.
Challenge

If, else, and indentation

When using conditional statements, if variable is shorthand for if variable == True.

In the example below, how many print statements will print if no_gas is True? How many if no_gas is False?

PYTHON 

no_gas = _____
if no_gas:
    print("Getting gas")
    no_gas = False
else:
    print("Buying snacks")
print("Resumed driving")

If no_gas is True, the “Getting gas” and “Resumed driving” statements will print.

OUTPUT 

Getting gas
Resumed driving

If no_gas is False, the “Buying snacks” and “Resumed driving” statements will print.

OUTPUT 

Buying snacks
Resumed driving

Though the value of no_gas was changed within the code block under the if statement, the else statement was skipped over. Which block of code executes in an if/else pair is decided at the the time the if is tested.

Use elif to specify additional tests

We can also chain several tests together using elif, which is short for “else if”. The following Python code uses elif to print the sign of a number.

PYTHON 

num = -3
if num > 0:
    print(num, 'is positive')
elif num == 0:
    print(num, 'is zero')
else:
    print(num, 'is negative')

OUTPUT 

-3 is negative

Conditions are tested once, in order

  • Exactly one condition of an if/else pair or an if/elif/else trio will execute.
  • Consecutive if statements are tested independently and in order.

PYTHON 

grade = 85
if grade >= 90:
    print('grade is A')
elif grade >= 80:
    print('grade is B')
elif grade >= 70:
    print('grade is C')

OUTPUT 

grade is B

Python does not go back and re-evaluate a statement if the values change.

PYTHON 

velocity = 10.0
if velocity > 20.0:
    print('moving too fast')
else:
    print('adjusting velocity')
    velocity = 50.0

OUTPUT 

adjusting velocity

Use and and or to combine tests

We can also combine tests using and and or. and is only true if both parts are true:

PYTHON 

if (1 > 0) and (-1 >= 0):
    print('both parts are true')
else:
    print('at least one part is false')

OUTPUT 

at least one part is false

or is true if at least one part is true:

PYTHON 

if (1 < 0) or (1 >= 0):
    print('at least one test is true')

OUTPUT 

at least one test is true
Challenge

Tracing Execution

What does this program print?

PYTHON 

pressure = 71.9
if pressure > 50.0:
    pressure = 25.0
elif pressure <= 50.0:
    pressure = 0.0
print(pressure)

OUTPUT 

25.0
Challenge

Trimming Values

Fill in the blanks so that this program creates a new list containing zeroes where the original list’s values were negative and ones where the original list’s values were positive.

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = ____
for value in original:
    if ____:
        result.append(0)
    else:
        ____
print(result)

OUTPUT 

[0, 1, 1, 1, 0, 1]

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = []
for value in original:
    if value < 0.0:
        result.append(0)
    else:
        result.append(1)
print(result)
Challenge

Computing absolute value using conditionals

Let’s say you want to compute the absolute value \(|x|\) of each float x in a list. Append each value to a new list called result in the same order as the original input. Do not change the values in original.

I have provided a skeleton to get you started.

PYTHON 

original = [-1.5, 0.2, 0.4, -1.3]
result = ___
for value in original:
    if value < ___: # if negative
        result.append(value * __)
    else: # if positive
        result.append(____)
print(result)

PYTHON 

original = [-1.5, 0.2, 0.4, -1.3]
result = [] # Initialize an empty list
for value in original:
    if value < 0: # if negative
        result.append(value * -1)
    else: # if positive
        result.append(value) # abs val of a positive number is itself 
print(result)

OUTPUT 

[1.5, 0.2, 0.4, 1.3]
Key Points
  • Use if statements to control whether or not a block of code is executed.
  • Conditionals are often used inside loops.
  • Use else to execute a block of code when an if condition is not true.
  • Use elif to specify additional tests.
  • Conditions are tested once, in order.
  • X and Y is only true if both X and Y are true.
  • X or Y is true if either X or Y, or both, are true.