function filterSelection(c) {
  var x, i;
  x = document.getElementsByClassName("column");
  if (c == "all") c = "";
  // Add the "show" class (display:block) to the filtered elements, and remove the "show" class from the elements that are not selected
  for (i = 0; i < x.length; i++) {
    w3RemoveClass(x[i], "show");
    if (x[i].className.indexOf(c) > -1) w3AddClass(x[i], "show");
  }
}