4.2: Selection Control Structures

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Overview

In selection control structures, conditional statements are features of a programming language which perform different computations or actions depending on whether a programmer-specified Boolean condition evaluates to true or false.^[1]

Discussion

The basic attribute of a selection control structure is to be able to select between two or more alternate paths. This is described as either two-way selection or multi-way selection. A question using Boolean concepts usually controls which path is selected. All of the paths from a selection control structure join back up at the end of the control structure, before moving on to the next lines of code in a program.

If Then Else Control Structure

The if then else control structure is a two-way selection.

If age > 17
    Output "You can vote."
False:
    Output "You can't vote."
End

Language	Reserved Words
C++	`if`, `else`
C#	`if`, `else`
Java	`if`, `else`
JavaScript	`if`, `else`
Python	`if`, `elif`, `else`
Swift	`if`, `else`

Case Control Structure

The case control structure is a multi-way selection. Case control structures compare a given value with specified constants and take action according to the first expression to match.^[2]

Case of age
    0 to 17   Display "You can't vote."
    18 to 64  Display "You're in your working years."
    65 +      Display "You should be retired."
End

Language	Reserved Words
C++	`switch`, `case`, `break`, `default`
C#	`switch`, `case`, `break`, `default`
Java	`switch`, `case`, `break`, `default`
JavaScript	`switch`, `case`, `break`, `default`
Python	N/A
Swift	`switch`, `case`, `break` (optional), `default`

Python does not support a case control structure. There are workarounds, but they are beyond the scope of this book.

Key Terms

if then else: A two-way selection control structure.

case: A multi-way selection control structure.

References

cnx.org: Programming Fundamentals – A Modular Structured Approach using C++

Wikipedia: Conditional (computer programming) ↵
Wikipedia: Conditional (computer programming) ↵