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.