Conditionals

Conditionals#

Conditionals are a code structure that help you control whether a certain line of code executes. We’ll discuss three ways in which you can control code execution: if, elif (which stands for ‘else if’), and else.

`if` statements#

Conditional statements begin with an if statement. They can optionall have elif and else, which we’ll get to in just a second, but they must have an if statement and that if statement must come first. The code within an if statement will execute if the condition following if evaluates as True.

Conditionals are statements that check for a condition, using the if statement, and then only execute a set of code if the condition evaluates as True.

The general syntax for a conditional if statments requires an if followed by a condition and a :. Subsequent lines that you want to execute if the condition are true go on indented lines beneath the if conditional statement:

if condition:
    # execute this code

For example, if we define condition to store the boolean True, the conditional if statement, will then execute the print() statement within the condition.

condition = True

if condition:
    print('This code executes if the condition evaluates as True.')

This code executes if the condition evaluates as True.

Note that if condition were to store the Boolean False, this conditional statement would produce no output, as the conditional if statement did not evaluate as True. The code within the conditional is only executed if the condition evaluates as True.

condition = False

if condition:
    print('This code executes if the condition evaluates as True.')

`else` statements#

After an if statement you can add an optional else statement. The code within this conditional will execute if the if conditional statement did not evaluate as True.

After an if, you can use an else that will run if the conditional(s) above have not run.

For example, below we see that condition is defined as False. Therefore, the if condtional statement evaluates as False. The code within the if conditional is ignored, moving onto the else statement. The code within that statement is evaluated, printing the code within the else statement.

condition = False

if condition:
    print('This code executes if the condition evaluates as True.')
else: 
    print('This code executes if the condition evaluates as False')

This code executes if the condition evaluates as False

Note that the syntax for both the if and the else statements require a colon at the end of the line and the lines of code within the if or else statement to be indented. Python requires this because visually makes it clear to readers of your code which lines of code will be executed under which conditions.

Futher, it’s important to note that only one condition can be met, so either the code for the if statement or the code for the else statement will execute, but not both.

`elif` statements#

The final type of conditional statement is the elif statement. These too are optional, and are read as “else if”. After an if statment, an elif statement can be used to check additional conditions. As with if statements, the code within an elif statement will only execute if the condition specified evaluates as True.

After an if statement, you can have any number of elif`s (meaning 'else if') to check other conditions.

Here, we have two variables condition_1 storing False and condition_2 storing True. We then see an if, elif and else statement. As before, the code for only one of these conditions will execute. If the if condition evaluates as True, the code within that statement will execute. Else, if (elif) the condition in the elif statement evaluates as True, the code within the elif statement will execute. Finally, if neither the if or elif evaluate as True, the code within the else statment will execute.

As before, the syntax requires colons after each if, elif, and else statement and indentation for the lines of code within each statement.

condition_1 = False
condition_2 = True

if condition_1:
    print('This code executes if condition_1 evaluates as True.')
elif condition_2:
    print('This code executes if condition_1 did not evaluate as True, but condition_2 does.')
else: 
    print('This code executes if both condition_1 and condition_2 evaluate as False')

This code executes if condition_1 did not evaluate as True, but condition_2 does.

In the example above, as condition_1 is False, the code beneath the if statement does not execute. However, the elif condition (condition_2) evaluates as True, printing the string within the elif statement.

`elif` without an `else`#

An else statement is not required, but if both the if and the elif condtions are not met (both evaluate as False), then nothing is returned.

For example below both conditions evaluate as False and thus the code passes silently, returning nothing.

condition_1 = False
condition_2 = False

if condition_1:
    print('This code executes if condition_1 evaluates as True.')
elif condition_2:
    print('This code executes if condition_1 did not evaluate as True, but condition_2 does.')

`elif` after an `else` does not make sense#

Finally, the order will always be if-elif-else…with only the if being required. If the elif is at the end…it will never be tested, as the else will have already returned a value once reached (and thus Python will throw an error).

## THIS CODE WILL PRODUCE AN ERROR
condition_1 = False
condition_2 = False

if condition_1:
    print('This code executes if condition_1 evaluates as True.')
else: 
    print('This code executes if both condition_1 and condition_2 evaluate as False')
elif condition_2:
    print('This code executes if condition_1 did not evaluate as True, but condition_2 does.')

  File "<ipython-input-5-aedfc0cec5db>", line 9
    elif condition_2:
       ^
SyntaxError: invalid syntax

In the example above, there is an elif at the end. By definition, there is no way the elif will ever be tested. Accordingly, Python indicates that there is a SyntaxError.

Conditionals With Value Comparisons#

Beyond providing Booleans directly, conditionals can be used with any expression that evaluates as a Boolean. The operators discussed in an earlier chapter can be used in your conditional statements.

Any expression that can be evaluated as a boolean, such as value comparisons, can be used with conditionals.

For example, below the condition language == "Python" evaluates as True because the string “Python” has been declared in the variable language. Thus, the code prints “Yay!”

language = "Python"

if language == "Python":
    print("Yay!")
elif language == "English":
    print("Not a programming language.")
else:
    print("Get yourself a programming language!")

Yay!

Beyond testing for variable equality, mathematical operators can be very helpful when working with condtionals. For example, you can have different code execute depending upon the value stored in a variable.

Here we have an example where number stores the integer 4. The conditional tests whether the value stored in number is less than 5. As 4 is less than 5, the code within the if statement executes, printing ‘if statement execution’.

number = 4
 
if number < 5:
    print('if statement execution')
elif number > 5:
    print('elif statement execution')

if statement execution

Summary#

To finish here, we’ll summarize a number of the key points in this chapter:

All conditionals start with an if, can have an optional and variable number of elif’s and an optional else statement
Conditionals can take any expression that can be evaluated as True or False.
At most one component (if / elif / else) of a conditional will run
The order of conditional blocks is always if then elif(s) then else
Code is only ever executed if the condition is met

Exercises#

Q1. Replace --- below with something that will print ‘True’

math = ---

if math:
    print('True')

Q2. Replace --- below with something that will print ‘False’.

my_value = ---

if my_value:
    print('True')
else: 
    print('False')

Q3. What will the following code snippet print out?

if False:
    print("John")
elif True:
    print("Paul")
elif True:
    print("George")
else:
    print("Ringo")

A) John
B) Paul, George, Ringo
C) Paul
D) Paul, George
E) Ringo

Q4. What will the following code snippet print out?

if 1 + 1 == 2:
    print("I did Math")
elif 1/0:
    print("I broke Math")
else:
    print("I didn't do math")

A) I did Math
B) I broke Math
C) I didn’t do math
D) This code won’t execute.

Q5. What will the following code snippet print out?

conditional = False
python = "great"

if conditional:
    if python == "great":
        print("Yay Python!")
    else:
        print("Oh no.")
else:
    print("I'm here.")

A) Yay Python!
B) Oh no.
C) I’m here.
D) This code won’t execute.

Q6. What would be the output of running the following code?

condition = 5 < 7

if condition: 
    print('SO GOOD!')
else:
    print('NOT SO GOOD.')

A) SO GOOD!
B) NOT SO GOOD.
C) True
D) False
E) SyntaxError

Conditionals

Contents

Conditionals#

if statements#

else statements#

elif statements#

elif without an else#

elif after an else does not make sense#