File: gawk.info,  Node: Creating Arrays,  Prev: Flattening Arrays,  Up: Array Manipulation

17.4.12.4 How To Create and Populate Arrays
...........................................

Besides working with arrays created by 'awk' code, you can create arrays
and populate them as you see fit, and then 'awk' code can access them
and manipulate them.

   There are two important points about creating arrays from extension
code:

   * You must install a new array into 'gawk''s symbol table immediately
     upon creating it.  Once you have done so, you can then populate the
     array.

     Similarly, if installing a new array as a subarray of an existing
     array, you must add the new array to its parent before adding any
     elements to it.

     Thus, the correct way to build an array is to work "top down."
     Create the array, and immediately install it in 'gawk''s symbol
     table using 'sym_update()', or install it as an element in a
     previously existing array using 'set_array_element()'.  We show
     example code shortly.

   * Due to 'gawk' internals, after using 'sym_update()' to install an
     array into 'gawk', you have to retrieve the array cookie from the
     value passed in to 'sym_update()' before doing anything else with
     it, like so:

          awk_value_t val;
          awk_array_t new_array;

          new_array = create_array();
          val.val_type = AWK_ARRAY;
          val.array_cookie = new_array;

          /* install array in the symbol table */
          sym_update("array", & val);

          new_array = val.array_cookie;    /* YOU MUST DO THIS */

     If installing an array as a subarray, you must also retrieve the
     value of the array cookie after the call to 'set_element()'.

   The following C code is a simple test extension to create an array
with two regular elements and with a subarray.  The leading '#include'
directives and boilerplate variable declarations (*note Extension API
Boilerplate::) are omitted for brevity.  The first step is to create a
new array and then install it in the symbol table:

     /* create_new_array --- create a named array */

     static void
     create_new_array()
     {
         awk_array_t a_cookie;
         awk_array_t subarray;
         awk_value_t index, value;

         a_cookie = create_array();
         value.val_type = AWK_ARRAY;
         value.array_cookie = a_cookie;

         if (! sym_update("new_array", & value))
             printf("create_new_array: sym_update(\"new_array\") failed!\n");
         a_cookie = value.array_cookie;

Note how 'a_cookie' is reset from the 'array_cookie' field in the
'value' structure.

   The second step is to install two regular values into 'new_array':

         (void) make_const_string("hello", 5, & index);
         (void) make_const_string("world", 5, & value);
         if (! set_array_element(a_cookie, & index, & value)) {
             printf("fill_in_array: set_array_element failed\n");
             return;
         }

         (void) make_const_string("answer", 6, & index);
         (void) make_number(42.0, & value);
         if (! set_array_element(a_cookie, & index, & value)) {
             printf("fill_in_array: set_array_element failed\n");
             return;
         }

   The third step is to create the subarray and install it:

         (void) make_const_string("subarray", 8, & index);
         subarray = create_array();
         value.val_type = AWK_ARRAY;
         value.array_cookie = subarray;
         if (! set_array_element(a_cookie, & index, & value)) {
             printf("fill_in_array: set_array_element failed\n");
             return;
         }
         subarray = value.array_cookie;

   The final step is to populate the subarray with its own element:

         (void) make_const_string("foo", 3, & index);
         (void) make_const_string("bar", 3, & value);
         if (! set_array_element(subarray, & index, & value)) {
             printf("fill_in_array: set_array_element failed\n");
             return;
         }
     }

   Here is a sample script that loads the extension and then dumps the
array:

     @load "subarray"

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

     BEGIN {
         dumparray("new_array", new_array);
     }

   Here is the result of running the script:

     $ AWKLIBPATH=$PWD gawk -f subarray.awk
     -| new_array["subarray"]["foo"] = bar
     -| new_array["hello"] = world
     -| new_array["answer"] = 42

(*Note Finding Extensions:: for more information on the 'AWKLIBPATH'
environment variable.)