If-Else Absolute Value

One way to compute an absolute value is:

if ( value < 0 )
  abs = -value;
else
  abs = value;

This is awkward for such a simple idea. The following does the same thing in one statement:

abs = (value < 0 ) ? -value : value ;

The right side of the = uses a conditional operator. In general, it looks like this:

true-or-false-condition ? value-if-true : value-if-false

1. The entire expression evaluates to a single value.
2. That value will be one of two values:
   • If the condition is true, then the expression between ? and : says what value to use.
   • If the condition is false, then the expression between : and the end says what value to use.
3. If used in an assignment statement (as above), that value is assigned to the variable.
4. An expression using a conditional operator can be part of larger expressions.

Here is how it works with the above example:

double value = -34.569;
double abs;

// compute absolute value of value
abs = (value < 0 )   ? -value : value ;
      -------------     ------
      1. condition      2.  this is evaluated,
         is true             to +34.569
                      
----
3.  The +34.569 is assigned to abs

The conditional expression is a type of expression---that is, it asks for a value to be computed but does not by itself change any variable. In the above example, the variable value is not changed.

int a = 7, b = 21;

a > b ? a : b