$(document).ready(function(){
  var sourceBtns = [];
  var selectedSource;
  var selectedSourceMath;

  //this part works
  for(i=1;i<=4;i++){
    sourceBtns.push(document.getElementById("source"+i));
  }
    console.log(sourceBtns);

  //this version works
  for(i=0;i<=sourceBtns.length-1;i++){
    sourceBtns[i].addEventListener("click",function(){
      selectedSource = this.id.replace("source","");
      console.log(selectedSource);
      selectedSourceMath = selectedSource + 10;
      console.log(selectedSourceMath);
    });
  }
  //this prints "1" through "4" to the console at page load
  //then doesn't respond to click events at all
  /*
  for(i=0;i<=sourceBtns.length-1;i++){
    sourceBtns[i].addEventListener("click",assignSource());
  }
  function assignSource(){
    selectedSource = this.id.replace("source","");
    console.log(selectedSource);
  }*/
});