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gawk: Function Calls

 
 6.4 Function Calls
 ==================
 
 A "function" is a name for a particular calculation.  This enables you
 to ask for it by name at any point in the program.  For example, the
 function 'sqrt()' computes the square root of a number.
 
    A fixed set of functions are "built in", which means they are
 available in every 'awk' program.  The 'sqrt()' function is one of
 these.  Built-in for a list of built-in functions and their
 descriptions.  In addition, you can define functions for use in your
 program.  User-defined for instructions on how to do this.
 Finally, 'gawk' lets you write functions in C or C++ that may be called
 from your program (Dynamic Extensions).
 
    The way to use a function is with a "function call" expression, which
 consists of the function name followed immediately by a list of
 "arguments" in parentheses.  The arguments are expressions that provide
 the raw materials for the function's calculations.  When there is more
 than one argument, they are separated by commas.  If there are no
 arguments, just write '()' after the function name.  The following
 examples show function calls with and without arguments:
 
      sqrt(x^2 + y^2)        one argument
      atan2(y, x)            two arguments
      rand()                 no arguments
 
      CAUTION: Do not put any space between the function name and the
      opening parenthesis!  A user-defined function name looks just like
      the name of a variable--a space would make the expression look like
      concatenation of a variable with an expression inside parentheses.
      With built-in functions, space before the parenthesis is harmless,
      but it is best not to get into the habit of using space to avoid
      mistakes with user-defined functions.
 
    Each function expects a particular number of arguments.  For example,
 the 'sqrt()' function must be called with a single argument, the number
 of which to take the square root:
 
      sqrt(ARGUMENT)
 
    Some of the built-in functions have one or more optional arguments.
 If those arguments are not supplied, the functions use a reasonable
 default value.  Built-in for full details.  If arguments are
 omitted in calls to user-defined functions, then those arguments are
 treated as local variables.  Such local variables act like the empty
 string if referenced where a string value is required, and like zero if
 referenced where a numeric value is required (User-defined).
 
    As an advanced feature, 'gawk' provides indirect function calls,
 which is a way to choose the function to call at runtime, instead of
 when you write the source code to your program.  We defer discussion of
 this feature until later; see Indirect Calls.
 
    Like every other expression, the function call has a value, often
 called the "return value", which is computed by the function based on
 the arguments you give it.  In this example, the return value of
 'sqrt(ARGUMENT)' is the square root of ARGUMENT.  The following program
 reads numbers, one number per line, and prints the square root of each
 one:
 
      $ awk '{ print "The square root of", $1, "is", sqrt($1) }'
      1
      -| The square root of 1 is 1
      3
      -| The square root of 3 is 1.73205
      5
      -| The square root of 5 is 2.23607
      Ctrl-d
 
    A function can also have side effects, such as assigning values to
 certain variables or doing I/O. This program shows how the 'match()'
 function (String Functions) changes the variables 'RSTART' and
 'RLENGTH':
 
      {
          if (match($1, $2))
              print RSTART, RLENGTH
          else
              print "no match"
      }
 
 Here is a sample run:
 
      $ awk -f matchit.awk
      aaccdd  c+
      -| 3 2
      foo     bar
      -| no match
      abcdefg e
      -| 5 1
 

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