#Use Taskscheduler to replace below for loop
#  for (int 1=0; i< 8; i++) {
#     iterate thru array[i]
#     delay(1000)
#     do some other work
#  }

#include <TaskScheduler.h>
Scheduler runner;
void t1Callback();
Task t1(TASK_IMMEDIATE,8,&t1Callback); 
//int global_index=0;    //workaround with 8*2

void t1Callback()
{
    int counter=t1.getRunCounter();
    Serial.print("t1: ");
    Serial.println(counter);    //counter=1,3,5,7
    //iterate thru array using [counter]
    //does not work,need counter=1,2,3,4,5,6,7,8
    t1.setCallback(&t1_1Callback);
    t1.delay(1000);
    //global_index++;
}

void t1_1Callback()
{

    int counter=t1.getRunCounter();
    Serial.print("t1_1: ");
    Serial.println(counter);    //counter=2,3,6,8
    //do some other work
    t1.setCallback(&t1Callback);
}

void setup() {
  Serial.begin(9600);
  runner.init();
  runner.addTask(t1);
  t1.enable();
}

void loop() {
  runner.execute();
}