Need help with the Next and Previous for SlideShow

Hello, Basically I am coding a simple JavaScript coded slideshow. Everything seems to work. However, what's happening is that I can go "previous" but I have to press twice to get it to work:

Here is my code:

var index = 0;
var img;
var slideName;
function moveToNextSlide() {
img = document.getElementById("img1");
slideName = "images/slide" + ++index + ".png";
img.src = slideName;
if (index == 3) {
index = 0;
}

}

function moveToPreviousSlide() {
if (index == 0) {
index = 3;
}
slideName = "images/slide" + index-- + ".png";
img.src = slideName;
}



More Info:
I am experimenting this with 3 images.

Thanks In Advance..

moveToPreviousSlide shouldn't work at all, I don't think.. you don't have img = document.getElementById("img1"); in it.

You have slide0.png through slide2.png?

moveToPreviousSlide shouldn't work at all, I don't think.. you don't have img = document.getElementById("img1"); in it.

You have slide0.png through slide2.png?

Hey, Thanks for input..

moveToPreviousSlide is working. I have made 'img' a global variable.

The slide names are slide1.png through slide3.png

Thanks

Is it working, now? I think the math MIGHT be off.

Is it working, now? I think the math MIGHT be off.

Hey, It wasn't working at the point you asked...

Here is the working code, for anyone in future looking at this post:
var index = 0;
var img;
var slideName;
function moveToNextSlide() {
img = document.getElementById("img1");
if (index == 3) {
index = 0;
slideName = "images/slide" + index + ".png";
}
slideName = "images/slide" + ++index + ".png";
img.src = slideName;
}

function moveToPreviousSlide() {
slideName = "images/slide" + --index + ".png";
if (index == 0) {
index = 3;
slideName = "images/slide" + index + ".png";
}
img.src = slideName;
}

That works, but it really kind of much bigger and uglier than needed.

Try something like this:

var MINSLIDE = 0;
var MAXSLIDE = 2; // adjust those as needed

var curSlide = MINSLIDE;

function moveSlide( byWhat )
{
curSlide += byWhat;
if ( curSlide > MAXSLIDE ) { curSlide = MINSLIDE; }
else if ( curSlide < MINSLIDE ) { curslide = MAXSLIDE; }
document.getElementById("img1") = "images/slide" + curSlide + ".png";
}

And now, instead of calling moveToNextSlide() or moveToPreviousSlide() you call moveSlide(1) and moveSlide(-1).

And the beauty of this is that you can even call moveSlide(0) to start everything up with the first slide.

Oh...and this works whether the slides are number 0 through N or 1 through N. Or even 73 through 122, for that matter.