lr_paramarr_random - which id was used

how to get ID of used random dataset.? We populate array with follwoing function:

web_reg_save_param_ex("ParamName=parray", "LB=\myBoundary\":","RB=,\"","Ordinal=ALL",LAST);
This provides data like designed:

parray_1 = 1000
parray_2 = 1230
...
parray_10 = 1022
parray_count = 10

In order to proceed next steps in script we use function lr_paramarr_random in order to populate variable "pToBeUsedInScript" with this

do {
            lr_save_string(lr_paramarr_random("parray"),"pToBeUsedInScript");             
        } while (atoi(strstr(lr_eval_string( "{pToBeUsedInScript}" ),"1300")) ==1230);

This will filter data that is outdated for our process.

Unfortunately next step needs number of row as integer for posting data. We want to use atoi(<used_array_id>) but how to get this ?