POSIX Array – Portable Check for Array in Awk

arrayawkgawkmawkposix

Gawk has "isarray":

if (isarray(x))
  print "is array"
else
  print "is scalar"

However Mawk and "gawk –posix" do not:

fatal: function 'isarray' not defined

This can cause problems:

x
x[1]
fatal: attempt to use scalar 'x' as an array

Or:

x[1]
x
fatal: attempt to use array 'x' in a scalar context

Can Awk detect an array without using the "isarray" function?

Best Answer

I also don't think it's possible.

But I'll add that with busybox awk, variables can be both arrays and scalar. There it's OK to do:

a = "foo"; a["foo"] = "bar"

When a variable has been used as an array though, length() returns the number of elements in the array, even if it also has been defined as a scalar (though you can use length(var "") to get the length of the scalar), except when the variable has been passed as an argument to a function and assigned as a scalar there (could be considered as a bug):

$ busybox awk 'BEGIN{a[1] = 1; a = "foo"; print length(a), length(a"")}'
1 3
$ busybox awk 'function f(x) {x = "xxx"; print x[1], length(x)}
               BEGIN{a[1]=1; x = "yyy"; print a[1], length(a); f(a)}'
1 1
1 3

Too bad as otherwise it would have been easy to define a isarray() function there. We can still tell if a variable is an array with at least one element with

function isnonemptyarray(x) {
  return length(x) > 0 && length(x "") == 0
}

(assuming the variable hasn't been defined both as an array and scalar)

In anycase, that's busybox awk specific. length() can't be used on arrays portably. One can define a portable array_length() function with:

function array_length(a, tmp1, tmp2) {
  tmp1 = 0
  for (tmp2 in a) tmp1++
  return tmp1
}

But that can't be used portably on non-array variables.

Related Solutions

Mawk – Walking Multidimensional Arrays in Mawk

mawk (and nawk) provide only synthesised multi-dimensional arrays.

gawk provides (since 4.0, thx manatwork) true multi-dimensional arrays, though the man page (IMHO) misdirects a little: immediately after introducing if ((i,j) in array) it follows with "The in construct may also be used in a for loop to iterate over all the elements of an array." (fixed since v4.1.1!).

But, for ((i,j) in array) is not the way to iterate over these, the gawk way is (as you used originally):

 for (i in array)
     for (j in array[i])
         print array[i][j]

With nawk/mawk you are stuck with synthesised multi-dimensional arrays, so

for (ij in A) {
    split(ij,xx,SUBSEP);
    printf("A[%s,%s]=%s\n",xx[1],xx[2],A[ij])
}

Now, your next problem is going to be ordering, array indexes are implicitly string type, and arrays are implicitly unordered. Unless you have separate knowledge of the indexes, as would be the case with a simple non-sparse array with consecutive integer indexes from 0..N. gawk offers a solution for an ordered in.

If you know the indexes of a synthesised array, then you can use A[i,j] (which is treated as A[i SUBSEP j]), or for/in and some string splitting to rebuild the list of i and j, or if ((i,j) in A) (test for presence, without autovivification of indexes).

In gawk you cannot use (i,j) in arr where arr is a true multi-dimensional array, you need to break it into two (or however many dimensions) for loops as above. To be fully correct though, the inner loop(s) should contain an isarray() condition, since it's not required that every element of arr[i] in turn be an array, gawk is happy to allow scalars too.

I know of no mawk specific documentation other than the man page, it aims to be a standard new awk (i.e. nawk) implementation (so no true multi-dimensional arrays, no index sorting, and no isarray()).

POSIX Awk sort array

Here is an example:

function arr_sort(ary,   x, y, z) {
  for (x in arr) {
    y = arr[x]
    z = x - 1
    while (z && arr[z] > y) {
      arr[z + 1] = arr[z]
      z--
    }
    arr[z + 1] = y
  }
}

Source

Best Answer

Related Solutions

Mawk – Walking Multidimensional Arrays in Mawk

POSIX Awk sort array

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