File: gawk.info,  Node: Walking Arrays,  Next: Library Functions Summary,  Prev: Group Functions,  Up: Library Functions

10.7 Traversing Arrays of Arrays
================================

*note 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 (*note Type Functions::) 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
     }