Saying It With Style

The C language offers some additional "shortcut" operators designed to make programs more compact. Sometimes using these operators can obscure the meaning of the program, but with appropriate comments, use of these operators can give your programs more of a "professional" look.

These operators include:

-Autoincrement and autodecrement operators-Compound assignment operators-Comma operator-Conditional expression operator

These operators (and others) fit into the full C precedence table

Autoincrement and Autodecrement Operators

     i++           =>     i = i + 1

     i--           =>     i = i - 1

These are post increment and post decrement operators. In general, they take the form:

     <Lvalue>++

     <Lvalue>--

We can also have pre increment and pre decrement operators.

     ++i           =>     i = i + 1

     --i           =>     i = i - 1

The difference is that the "post" operators evaluate to the value of i before the update; while the "pre" operators evaluate the the value if i after the update.

For example:

     i = j = 1;

     if( i++ > 1) ...

is false, but

     if( ++j > 1) ...

is true.

Compound Assignment Operators

The autoincrement/decrement operators always add a constant 1 to the variable. We can make this an arbitrary value with the compound assignment operators:

     k += 5        =>    k = k + 5

     w -= 10       =>    w = w - 10

     x *= b        =>    x = x * b

     y /= a+b      =>    y = y / (a+b)

     x %= 10       =>    x = x % 10

     (also <<, >>, &, ^, | operators)

In general, these take the form

     <Lvalue> <op>= <expression>

equivalent to

     <Lvalue> = <Lvalue> <op> ( <expression> )

Comma Operator

The comma operators is a way to combine two expressions to act as one, of the form:

     <expression 1> , <expression2>

This "comma" expression evaluates the the value of the last expression. For example:

     f( a, (t = 4, t += 2), 9)

calls the function, f(), passing three values: a, 6, and 9.

The comma operator is most often used in for loops. For example, we can rewrite our factorial function more compactly as:

     int factorial( int n)

     {  int prod;

        int j;

             for( (prod=1,j=1); j <= n; j++)

                   prod *= j;

             return prod;

Conditional Expression Operator

We have seen the if statement:

     if ( <condition> ) <statement 1> [else <statement 2>]

but this "statement" has no value. C provides a conditional expression which can conditionally evaluate to a value:

     <condition> ? <expression 1> : <expression 2>

which evaluates to <expression 1> if <condition> is true, otherwise to <expression 2>.

We might use this expression in place of a function call for absolute:

     x = (x < 0) ? -x : x;

or better, define a macro:

     #define ABSOLUTE(x)  (((x) < 0) ? -(x) : (x))

and write

     diff = ABSOLUTE(diff);

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