var num=0;

imgArray = [
  ['pdch1.jpg'],
  ['pd1_1_1.gif'],
  ['pd1_1_2.gif'],
  ['pd1_1_3.gif'],
  ['pd1_1_4.gif'],
  ['pd1_1_5.gif'],
  ['pd1_1_6.gif'],
  ['pd1_1_7.gif'],
  ['pd1_1_8.gif'],
  ['pd1_1_9.gif'],
  ['pd1_1_10.gif'],
  ['pd1_1_11.gif'],
  ['pd1_1_12.gif'],
  ['pd1_1_13.gif'],
  ['pd1_1_14.jpg'],
  ['pd1_1_15.gif'],
  ['pd1_1_16.gif'],
  ['pd1_1_17.jpg'],
  ['pd1_1_18.jpg'],
  ['pd1_1_19.jpg'],
  ['pd1_1_20.gif'],
  ['pd1_1_21.gif'],
  ['pd1_1_22.gif'],
  ['pd1_1_23.gif'],
  ['pd1_1_24.gif'],
  ['pd1_1_25.jpg'],
  ['pd1_1_26.jpg'],
  ['pd1_1_27.jpg'],
  ['pd1_1_28.jpg'],
  ['pd1_1_29.jpg'],
  ['pd1_1_30.jpg'],
  ['pd1_1_31.jpg'],
  ['pd1_1_32.jpg'],
  ['pd1_1_33.jpg'],
]

function slideshow(slide_num) {
  document.getElementById('mypic').src=imgArray[slide_num][0];
  document.getElementById('mypic').alt=imgArray[slide_num][1];
}

function slideshowUp() {
  num++;
  num = num % imgArray.length;
  slideshow(num);
}

function slideshowBack() {
  num--;
  if (num < 0) {num=imgArray.length-1;}
  num = num % imgArray.length;
  slideshow(num);
}