[Ada-tut] Control Structures: Case

[wtd]

Background

We can match a value against many others with a long succession of "elsif" clauses, but that gets tiresome, and we end up duplicating code, which, as I've written in the debugging thread, is a very bad thing when it comes to maintaining a program.

So instead we use a case structure. Most languages implement this in one way or another. Those familiar with C, C++, or Java may be familiar with the "switch" statement.

How Ada85 answers the problem

Ada95's "case" construct is functionally the same, but provides some conveniences. Conditions can be ranges of numbers, or several distinct choices, and the "break" statement at the end of each branch is unnecessary.

Some code already?

Sure, keep your shorts on!

In my tutorial on if and else in Ada, I had a simple example which I'll reimplement here with a case.

The old version

code:

with Ada.Text_IO; use Ada.Text_IO; procedure Simple_Program is begin    if 8 = 8 then       Put_Line("All is well with the universe.");    elsif 8 = 7 then       Put_Line("Close enough for government work.");    else       Put_Line("The laws of mathematics went out the proverbial window.");    end if; end Simple_Program;

The new version

code:

with Ada.Text_IO; use Ada.Text_IO; procedure Simple_Program is begin    case 8 is       when 8 => Put_Line("All is well with the universe.");       when 7 => Put_Line("Close enough for government work.");       when others => Put_Line("The laws of mathematics went out the proverbial window.");    end case; end Simple_Program;

Dissected

There isn't much there that shouldn't be pretty straightforward. The "when others" case is essentially equivalent to "else".

Using a range

Let's say that anything from 7 to 5 is "close enough for government work". Representing this in a C-style "switch" would be quite onerous, but Ada95 makes it easy.

code:

with Ada.Text_IO; use Ada.Text_IO; procedure Simple_Program is begin    case 8 is       when 8 => Put_Line("All is well with the universe.");       when 5 .. 7 => Put_Line("Close enough for government work.");       when others => Put_Line("The laws of mathematics went out the proverbial window.");    end case; end Simple_Program;

Multiple values, but not a range...

If 8 equals 4 or 2, then things are whacky, but since 8 is a multiple of 4 and 2, at least things aren't quite as bad.

code:

with Ada.Text_IO; use Ada.Text_IO; procedure Simple_Program is begin    case 8 is       when 8 => Put_Line("All is well with the universe.");       when 5 .. 7 => Put_Line("Close enough for government work.");       when 4 | 2 => Put_Line("Still screwed up, but mathematically intriguing.");       when others => Put_Line("The laws of mathematics went out the proverbial window.");    end case; end Simple_Program;

Summary

Hopefully everything here is fairly well-explained. Questions as always are welcome.