// set initial states for as many variables as needed
state = {
    shown_first_index: null,
    shown_second_index: null,
  }

/* change variables with setState.
Here, shown_second_index gets the value of shown_first_index and shown_first_index gets a new value.
Note: when setState is finished it calls the render function again */
  handle_select = (id) => {
    this.setState({
      shown_first_index: id,
      shown_second_index: this.state.shown_first_index,
    })
  }