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gawk: Type Functions

 
 9.1.7 Getting Type Information
 ------------------------------
 
 'gawk' provides two functions that let you distinguish the type of a
 variable.  This is necessary for writing code that traverses every
 element of an array of arrays (⇒Arrays of Arrays), and in other
 contexts.
 
 'isarray(X)'
      Return a true value if X is an array.  Otherwise, return false.
 
 'typeof(X)'
      Return one of the following strings, depending upon the type of X:
 
      '"array"'
           X is an array.
 
      '"regexp"'
           X is a strongly typed regexp (⇒Strong Regexp
           Constants).
 
      '"number"'
           X is a number.
 
      '"string"'
           X is a string.
 
      '"strnum"'
           X is a number that started life as user input, such as a field
           or the result of calling 'split()'.  (I.e., X has the strnum
           attribute; ⇒Variable Typing.)
 
      '"unassigned"'
           X is a scalar variable that has not been assigned a value yet.
           For example:
 
                BEGIN {
                    # creates a[1] but it has no assigned value
                    a[1]
                    print typeof(a[1])  # unassigned
                }
 
      '"untyped"'
           X has not yet been used yet at all; it can become a scalar or
           an array.  The typing could even conceivably differ from run
           to run of the same program!  For example:
 
                BEGIN {
                    print "initially, typeof(v) = ", typeof(v)
 
                    if ("FOO" in ENVIRON)
                        make_scalar(v)
                    else
                        make_array(v)
 
                    print "typeof(v) =", typeof(v)
                }
 
                function make_scalar(p,    l) { l = p }
 
                function make_array(p) { p[1] = 1 }
 
    'isarray()' is meant for use in two circumstances.  The first is when
 traversing a multidimensional array: you can test if an element is
 itself an array or not.  The second is inside the body of a user-defined
 function (not discussed yet; ⇒User-defined), to test if a
 parameter is an array or not.
 
      NOTE: Using 'isarray()' at the global level to test variables makes
      no sense.  Because you are the one writing the program, you are
      supposed to know if your variables are arrays or not.  And in fact,
      due to the way 'gawk' works, if you pass the name of a variable
      that has not been previously used to 'isarray()', 'gawk' ends up
      turning it into a scalar.
 
    The 'typeof()' function is general; it allows you to determine if a
 variable or function parameter is a scalar, an array, or a strongly
 typed regexp.
 
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