Question

I have an array with 18 values in it from which I select random values using $array[rand(0,17)]. I put these randomly selected values next to each other on the page. Within the array are 6 sets of values that I do not want to be put next to each other on the page. Is there any way that I can detect when the pairs are together and select new values because of that

Answer 1

warning: Do you know for sure that you won’t get any degenerate cases where there are no possible orderings of the array? For example, if you won’t allow the pairs [1,2] or [2,1] and the array you get is [1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2], the you’re out of luck. There’s no way to display the array in the way you want, and a method like I describe below will never terminate.

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I would use shuffle($array) and then iterate through the shuffled array one item at a time, to find out whether any value is “incompatible” with the item before it. If so, just reshuffle the array and try again. You can’t predict how many tries it will take to get a shuffled array that works, but the amount of time it takes should be negligible.

To detect whether two values are compatible, I’d suggest making an array that contains all incompatible pairs. For example, if you don’t want to have the consecutive pairs 1 and 3 or 2 and 5, then your array would be:

$incompatible = array(
    array(1,3),
    array(2,5) );

Then you’d iterate over your shuffled array with something like:

for ($i=1; i<count($array)-1; i++;) {
    $pair = $array[i, i+1]; // this is why the for loop only goes to the next-to-last item
    if in_array($pair, $incompatible) { 
        // you had an incompatible pair in your shuffled array.
        // break out of the for loop, re-sort your array, and try again.
    }
} 
// if you get here, there were no incompatible pairs
// so go ahead and print the shuffled array!