Simple For loop issue - ios

Question

I have a two for loops in my Xcode project which are using to change the image in 9 UIImageViews. The images in these UIImageViews is downloaded from a server and then presented.
My for loops use 3 different integers to figure out what image to show:
next, current_photo and previous are integers.
I have two UIButtons which control showing the next and previous set of images.
When I want to show the next set of images, I made and use the following for loop:
NSArray *imageViews = #[picview_1, picview_2, picview_3, picview_4, picview_5, picview_6, picview_7, picview_8, picview_9];
for (next = current_photo; next < (current_photo+9); next++) {
NSString *next_set = [NSString stringWithFormat:#"%#%i.%#", SERVER_URL, next+1, FORMAT_TYPE];
NSData *imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString: next_set]];
UIImage *image = [[UIImage alloc] initWithData:imageData];
[[imageViews objectAtIndex:(next-9)] setImage:image];
}
current_photo = next;
This for loop works perfectly and all 9 UIImagesViews change image.
However, when I want to show the previous set of images in the 9 UIImageViews, the following for loop does NOT work properly for some reason:
NSArray *imageViews = #[picview_1, picview_2, picview_3, picview_4, picview_5, picview_6, picview_7, picview_8, picview_9];
for (previous = current_photo; previous > (previous-9); previous--) {
NSString *next_set = [NSString stringWithFormat:#"%#%i.%#", SERVER_URL, previous+1, FORMAT_TYPE];
NSData *imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString: next_set]];
UIImage *image = [[UIImage alloc] initWithData:imageData];
[[imageViews objectAtIndex:(previous-previous)] setImage:image];
}
current_photo = previous;
What is wrong with my for loop? Please explain.
Below is what happens when my app is opened:
Below is what happens when the next button is pressed:
And finally when pressing the back button the app just freezes.....
Why? Whats wrong? Please help.
Thanks for your time :)

Answer 1

For one thing:
[[imageViews objectAtIndex:(previous-previous)] setImage:image];
will always be setting the image at [0] and none of the others will have anything set for them.
Also
(previous = current_photo; previous > (previous-9); previous--)
will loop forever! Every time you do a previous--, the thing you are comparing against to know when to stop, previous-9 also goes down one.
I would recommend this:
for (previous = current_photo; previous > (current_photo-9); previous--) {
NSString *next_set = [NSString stringWithFormat:#"%#%i.%#", SERVER_URL, previous+1, FORMAT_TYPE];
NSData *imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString: next_set]];
UIImage *image = [[UIImage alloc] initWithData:imageData];
[[imageViews objectAtIndex:(8 - (current_photo - previous))] setImage:image];
}