positive/negative Modulo to get items in range

Question

I looked at https://math.stackexchange.com/questions/519845/modulo-of-a-negative-number and Modulo operation with negative numbers but I still wonder how to use a negative modulo value to get items in a range.

Here is a simplified use case before my question bellow, I have a slideshow with 3 slides (slidesCount = 3):

slide indexes:    0     1     2

Now I would like to access the right slide from these numbers:

    -2   -1       0   1   2       3   4

           should match slide:

     1    2       0   1   2       0   1

So with index % slidesCount I cover the cases:

                  0   1   2       3   4

but not negative values. -1 % 3 returns -1 so is slidesCount + index % slidesCount the correct expression if index is negative?

First of, is there a simpler/smarter way to write:

index = index % slidesCount + (index < 0 ? slidesCount : 0)

Now my question is for a slideshow of 3 visible items per slide, where the last slide may have only one item (index 9 bellow) so from these numbers:

  -3 -2 -1         0 1 2   3 4 5   6 7 8   9       10 11 12

I want to match slides:

      9              0       3       6     9           0

I hope the following diagram makes sense! Please help me get the correct equation out of it with minimum ifs:

     -3 -2 -1               0 1 2   3 4 5   6 7 8   9             10 11 12
    |________|             |______________________________________________|
        ||                              ||                           ||
        ||                              Math.floor( i / visibleSlides )
Math.ceil(i / visibleSlides)            ||                           ||
        ||                              ||                           ||
        \/                              \/                           \/

        -1                    0       1       2     3                 4
       |___|                 |_______________________|              |___|
        ||                              ||                           ||
slidesCnt + i % visibleSlides       i % visibleSlides                || ??
        ||                              ||                           ||
        \/                              \/                           \/

         3                    0       1       2     3                 0

                                        || i * visibleSlides
                                        \/ 

         9                    0       3       6     9                 0

the case of items par slide needs another control structure, maybe consider splitting it to another question? — towc, Aug 22 '18 at 08:25
and you can do `index = (index + slidesCount) % slidesCount` — towc, Aug 22 '18 at 08:26
my main question as said in the post is regarding the items per slides, i cannot divide it into another question — antoni, Aug 22 '18 at 08:34

score 0 · Accepted Answer · answered Aug 25 '18 at 07:11

in the end, this is how I solved the problem:

// Prevents out of range.
// e.g. -1, -2, 12, 13
let newIndex = (index + slidesCount) % slidesCount

// Calculates how many items are missing on last slide. 
// e.g. if 10 slides with 3 visible slides, 2 missing slides on last slide.
let lastSlideItems = slidesCount % visibleSlides || visibleSlides
let missingItems = visibleSlides - lastSlideItems

// If index is negative adjust by adding the missing slides.
newIndex += index < 0 ? missingItems : 0

// Always get the first item of the series of visible items.
// e.g. if index is 8 and 3 visible slides, newIndex will be 6. 
newIndex = Math.floor(newIndex / visibleSlides) * visibleSlides

positive/negative Modulo to get items in range

1 Answers1