Section 2.3 Selection

Selection is making a choice based on a true or false decision. For example, every time you login, the system checks your password and either grants access or denies it based on whether the password is correct.

In flowcharts, selection is represented by a diamond shape with a condition inside. The flowchart branches into different paths based on the outcome of the condition.

If the homework is submitted on time:
---
Record it as On Time.
---
Else (Otherwise):
---
Record it as Late.

This more complex flowchart for choosing a pet. Which path would you choose in this flowchart?

Watch the following video about conditional statements using the words if and else.

Almost all programming languages have a keyword called if which is used to implement selection (also called conditional statements). If-statements are followed by a block of statements that are only executed if the condition is true. In languages like C, C++, Java, JavaScript, and the AP Pseudocode above, the block of code is marked by using curly braces {}. But Python uses colon (:) after the if condition and indentation to mark the block of statements under the if; the parentheses around the condition are optional in Python and usually left out. The syntax for a Python if-statement is below:

if condition:
   block of statements

The condition in an if-statement is any Boolean expression (like we learned in the last lesson) that evaluates to either true or false, for example number > 0. If you forget to indent the statements under the if statement, you will get an indentation error. Try it in the code below.

Python also has a very useful operator called in that checks if a substring is inside a string. Try it in the code above instead of using "==" to check if part of your name is in the input string. For example,

if "B" in name:
    print("Welcome all names with B!")

x = 10
if x >= 10:
    print("A")
print("B")

x = 10
if x > 10:
    print("A")

If you want to choose between two actions, use an if-else statement. Although curly braces are used in some programming languages, in Python, the syntax for an if-else statement uses a : and indentation like below:

if condition:
   first block of statements
else:
   second block of statements

If the condition in an if-else statement is true, the first block of statements executes. If the condition is false, the second block of statements executes. In the activecode activity above, we wrote two if statements to check for positive and negative numbers, but we could have used an if-else statement to make the code more efficient.

if number > 0:
   print("The number is positive.")
else:
   print("The number is negative or 0.")

response = input("Terminate the program (Y/N)?")
---
if response == "Y":
---
if response = "Y":  #paired
---
 print("The program is now ending.")
---
else:
---
 print("The program will continue to run.")

Remember to use ==, not =, in the Boolean condition of an if statement to test a variable. One = assigns, two == tests!

x = 9
if x % 2 == 0:
    print("Even")
else:
    print("Odd")

If statements can be nested inside other if statements forming nested conditionals. The inner if statement is only evaluated if the outer if statement’s condition is true. For example, the following code which could be written with nested if statements or an and logical operator. Notice that in Python, the nested if statement is indented within the block of the outer if statement to indicate that it is inside it.

# Nested if statements
if userLoggedIn:
    if userHasPermission:
        print("Access granted.")

# Or using an and logical operator
if userLoggedIn and userHasPermission:
    print("Access granted.")

Nested if statements inside of else blocks can be used to make multi-way selections. A single if-else statement allows us to select between 2 branches of code. But with nested if-else-if statements, we can make multiple branches. Python has a shortcut keyword for this called elif which stands for "else if". Here’s an example of how it is used to chain together many conditions with the pattern if-elif-elif-...-else to branch the code into many directions. This is the same example of grade determination that we saw before, but this pattern of coding is shorter and easier to read. The extra tests for the maximum of each grade range is not needed because the tests are chained together. If the grade is not greater than or equal to 90, then it must be less than 90 when you reach the second test.

if grade >= 90:
    print("A")
elif grade >= 80:
    print("B")
elif grade >= 70:
    print("C")
elif grade >= 60:
    print("D")
else:
    print("F")

Here is a flowchart showing the if-elif-else multiple branching.

if x < 2:
---
if x > 2:  #paired
---
    message = "Hello"
---
elif x > 2:
---
else x > 2:  #paired
---
    message = "Hey"
---
else:
---
    message = "Hi"
---
    message == "Hi"  #paired

Click on next to trace the execution of nested conditional statements.

if x > 0 and y > 0:
    print("positive")
elif x > 0 or y > 0:
    print("positive1")
else:
    print("negative1")

If your class started the Lights Off app in MIT’s App Inventor, you can finish your app in section 2.14.4 Iteration 3 which uses if blocks and 2.14.5 Reflection in the App InventorLights Off App Lesson.