// Contains index of the current photo, initialized to the first photo (1)
var glbPhoto = 1;

// Array holding photo filenames
var photos = new Array ();

function showPhoto(index) {
  mainImage.src = photos[index - 1];
}

function showNext() {
  if (glbPhoto >= photos.length) {
      glbPhoto = 1;
      showPhoto (glbPhoto);
    }
   else {
    glbPhoto += 1;
    showPhoto (glbPhoto);
  }
}

function showPrevious() {
  if (glbPhoto <= 1) {
      glbPhoto = photos.length;
      showPhoto (glbPhoto);
    }
  else {
    glbPhoto += -1;
    showPhoto (glbPhoto);
  }
}

function addPhoto(filename) {
  // Add filenames and captions to their respective arrays.
  var len = photos.length;
  photos[len] = './jpg/' + filename;
}