Concatenate values of n arrays in php

Question

I have an unknown number of arrays, each containing an unknown number of words. I want to concatenate the values from each list so that all possible variations of the words are stored to a final array.

For example, if array 1 contains:

dog
cat

and array 2 contains:

food
tooth

and array 3 contains:

car
bike

I'd like the output to be:

dog food car
dog food bike
dog tooth car
dog tooth bike
cat food car
cat food bike
cat tooth car
cat tooth bike

There could be more than 3 lists, and each list will most likely have more than 2 words.

I'd like to do this in PHP.

I know how to do it if I know the number of lists, though it's probably not the most resource efficient method. But nested foreach loops works if you know the number of arrays. What if you don't? And what are some methods to solve this problem that will still work if, let's say, there are 100 arrays of 100 words each. Or 1000?

Thanks!

Answer 1

You can put all word arrays into one array and use a recursive function like this:

function concat(array $array) {
    $current = array_shift($array);
    if(count($array) > 0) {
        $results = array();
        $temp = concat($array);
        foreach($current as $word) {
          foreach($temp as $value) {
            $results[] =  $word . ' ' . $value;
          }
        }
        return $results;           
    }
    else {
       return $current;
    }
}

$a = array(array('dog', 'cat'), array('food', 'tooth'), array('car', 'bike'));

print_r(concat($a));

Which returns:

Array
(
    [0] => dog food car
    [1] => dog food bike
    [2] => dog tooth car
    [3] => dog tooth bike
    [4] => cat food car
    [5] => cat food bike
    [6] => cat tooth car
    [7] => cat tooth bike
)

But I guess this behaves badly for large arrays as the output array will be very big.

---

To get around this, you can output the combinations directly, using a similar approach:

function concat(array $array, $concat = '') {
    $current = array_shift($array);

    $current_strings = array();

    foreach($current as $word) {
            $current_strings[] = $concat . ' ' . $word;
    }

    if(count($array) > 0) {
        foreach($current_strings as $string) {
            concat($array, $string);
        }       
    }
    else {
      foreach($current_strings as $string) {
          echo $string . PHP_EOL;
      }   
    }
}

concat(array(array('dog', 'cat'), array('food', 'tooth'), array('car', 'bike')));

Which gives:

dog food car
dog food bike
dog tooth car
dog tooth bike
cat food car
cat food bike
cat tooth car
cat tooth bike

With this approach it is also easy to get the "sub-concatinations". Just insert echo $string . PHP_EOL; before concat($array, $string); and the output is:

 dog
 dog food
 dog food car
 dog food bike
 dog tooth
 dog tooth car
 dog tooth bike
 cat
 cat food
 cat food car
 cat food bike
 cat tooth
 cat tooth car
 cat tooth bike