void callback(char* topic, byte* payload, unsigned int length) {
for (int i=0;i<length;i++) {
payload[length] = '\0';

cstring = (char *) payload;
}
// Serial.println(topic);  //This is the topic
// Serial.println(cstring); //this is the payload
request = atof(cstring);

if (strstr(topic, "setpoint") != NULL) {
  for (unsigned i = 0; i < sizeof(sensor_list) / sizeof(Sensor); i++) {
    char row = i;
    if (strstr(topic, row) != NULL) {
      setpoint = atof(cstring);
      sensor_list[i].setpoint = setpoint;
    }
  }

}