How to generate random number in a dynamic world

Besides the random number by Loadrunner parameter itself, usually you need to create a Random number of which range is based on the correlated result, for example, you want to check a check-box which number is dynamically changed:

int checkbox_num1=0;
char Randomitem[10];
char RandomStatus[10];
char SRanStatus[10];
...

web_reg_save_param("Checkbox_name1",
  "LB=TableSelectCheckbox-_dash_",
  "RB=)\" >",
  "ORD=ALL",
  LAST);

web_submit_data(...
              .... );

checkbox_num1 = atoi(lr_eval_string("{Checkbox_name1_count}"));

    srand(time(NULL));

 if (checkbox_num1>0) {
 
 itoa(rand() % checkbox_num1 +1,Randomitem,10);

 sprintf(RandomStatus,"{Checkbox_name1_%s}",Randomitem);

 sprintf(SRanStatus, "%s", lr_eval_string(RandomStatus));

 lr_save_string(SRanStatus, "CheckboxORD");

 }

 else if (checkbox_num1==0) {//there is no any suitable value found

  lr_exit(LR_EXIT_ITERATION_AND_CONTINUE, LR_AUTO);

 }