10.7 Traversing Arrays of Arrays

Arrays of Arrays described how gawk provides arrays of arrays. In particular, any element of an array may be either a scalar or another array. The isarray() function (see section Getting Type Information) lets you distinguish an array from a scalar. The following function, walk_array(), recursively traverses an array, printing the element indices and values. You call it with the array and a string representing the name of the array:

function walk_array(arr, name,      i)
{
    for (i in arr) {
        if (isarray(arr[i]))
            walk_array(arr[i], (name "[" i "]"))
        else
            printf("%s[%s] = %s\n", name, i, arr[i])
    }
}

It works by looping over each element of the array. If any given element is itself an array, the function calls itself recursively, passing the subarray and a new string representing the current index. Otherwise, the function simply prints the element’s name, index, and value. Here is a main program to demonstrate:

BEGIN {
    a[1] = 1
    a[2][1] = 21
    a[2][2] = 22
    a[3] = 3
    a[4][1][1] = 411
    a[4][2] = 42

    walk_array(a, "a")
}

When run, the program produces the following output:

$ gawk -f walk_array.awk
-| a[1] = 1
-| a[2][1] = 21
-| a[2][2] = 22
-| a[3] = 3
-| a[4][1][1] = 411
-| a[4][2] = 42

The function just presented simply prints the name and value of each scalar array element. However, it is easy to generalize it, by passing in the name of a function to call when walking an array. The modified function looks like this:

function process_array(arr, name, process, do_arrays,   i, new_name)
{
    for (i in arr) {
        new_name = (name "[" i "]")
        if (isarray(arr[i])) {
            if (do_arrays)
                @process(new_name, arr[i])
            process_array(arr[i], new_name, process, do_arrays)
        } else
            @process(new_name, arr[i])
    }
}

The arguments are as follows:

arr

The array.

name

The name of the array (a string).

process

The name of the function to call.

do_arrays

If this is true, the function can handle elements that are subarrays.

If subarrays are to be processed, that is done before walking them further.

When run with the following scaffolding, the function produces the same results as does the earlier version of walk_array():

BEGIN {
    a[1] = 1
    a[2][1] = 21
    a[2][2] = 22
    a[3] = 3
    a[4][1][1] = 411
    a[4][2] = 42

    process_array(a, "a", "do_print", 0)
}

function do_print(name, element)
{
    printf "%s = %s\n", name, element
}