Arithmetic built-ins

m4 provides three built-in macros for doing integer arithmetic. incr increments its numeric argument by 1. decr decrements by 1. To handle the common programming situation in which a variable is to be defined as ``one more than N'' you would use

   define(N, 100)
   define(N1, `incr(N)')

The more general mechanism for arithmetic is a built-in called eval, which is capable of arbitrary arithmetic on integers. Its operators in decreasing order of precedence are

   +    (unary)
   
     /  %
   +  
   ==  !=  <  <=  >  >=
   !  ~
   &
   |  ^
   &&
   ||

Parentheses may be used to group operations where needed. All the operands of an expression given to eval must ultimately be numeric. The numeric value of a true relation (like 1 > 0) is 1, and false is 0. The precision in eval is 32 bits on the UNIX^® operating system.

As a simple example, you can define M to be 2N+1 with

   define(M, `eval(2N+1)')

   define(N, 3)
   M(2)