Programming Fundamentals/Selection Control Structures

From Wikibooks, open books for an open world
Jump to navigation Jump to search

Overview[edit]

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

Discussion[edit]

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[edit]

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[edit]

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[edit]

if then else
A two-way selection control structure.
case
A multi-way selection control structure.

References[edit]