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gawk: Calling Built-in

 9.1.1 Calling Built-in Functions
 To call one of 'awk''s built-in functions, write the name of the
 function followed by arguments in parentheses.  For example, 'atan2(y +
 z, 1)' is a call to the function 'atan2()' and has two arguments.
    Whitespace is ignored between the built-in function name and the
 opening parenthesis, but nonetheless it is good practice to avoid using
 whitespace there.  User-defined functions do not permit whitespace in
 this way, and it is easier to avoid mistakes by following a simple
 convention that always works--no whitespace after a function name.
    Each built-in function accepts a certain number of arguments.  In
 some cases, arguments can be omitted.  The defaults for omitted
 arguments vary from function to function and are described under the
 individual functions.  In some 'awk' implementations, extra arguments
 given to built-in functions are ignored.  However, in 'gawk', it is a
 fatal error to give extra arguments to a built-in function.
    When a function is called, expressions that create the function's
 actual parameters are evaluated completely before the call is performed.
 For example, in the following code fragment:
      i = 4
      j = sqrt(i++)
 the variable 'i' is incremented to the value five before 'sqrt()' is
 called with a value of four for its actual parameter.  The order of
 evaluation of the expressions used for the function's parameters is
 undefined.  Thus, avoid writing programs that assume that parameters are
 evaluated from left to right or from right to left.  For example:
      i = 5
      j = atan2(++i, i *= 2)
    If the order of evaluation is left to right, then 'i' first becomes
 six, and then 12, and 'atan2()' is called with the two arguments six and
 12.  But if the order of evaluation is right to left, 'i' first becomes
 10, then 11, and 'atan2()' is called with the two arguments 11 and 10.
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