php - Duplicate each string in a flat array N times

Solution:

str_repeat creates a single string, not an array.

Use array_fill() to create an array with N copies of a value. Then append the array to the result.

foreach ($chars as $char) {
    $result = array_merge($result, array_fill(0, 3, $char));
}

Solution:

Since you know the number of repeats, you can get the desired output using a foreach and for

1. Loop through the original array
2. For $repeat + 1 times; push the char to $result

<?php

$chars = ["a", "b" , "c"];
$result = [];
$repeat = 3;

foreach ($chars as $char) {
    for ($i = 0; $i < $repeat; $i++) {
        $result[] = $char;
    }
}
    
var_dump($result);

array(9) {
  [0]=>
  string(1) "a"
  [1]=>
  string(1) "a"
  [2]=>
  string(1) "a"
  [3]=>
  string(1) "b"
  [4]=>
  string(1) "b"
  [5]=>
  string(1) "b"
  [6]=>
  string(1) "c"
  [7]=>
  string(1) "c"
  [8]=>
  string(1) "c"
}

Try it online!

@Barmar's answer has the same 'idea', but uses

• array_merge instead off $result[] = ;
• array_fill instead off an extra for.

Solution:

You can get by without the loops as array_fill will take an array:

$result = array_merge(...array_fill(0, 3, $chars));

Or you could use a string:

$result = str_split(str_repeat(implode($chars), 3));

If needed just sort to get the order that you show.

Solution:

I'll add a few more techniques for completeness. Solutions that leverage the magic number 3 will be more scalable, but I'll also show some less scalable approaches which repeat a variable or an action 3 times.

• A body-less loop pushing each element into a new array 3 times: (Demo)

  $result = [];
  foreach (array_chunk($chars, 1) as [0 => $result[], 0 => $result[], 0 => $result[]]);
  var_export($result);
  

  
  

• A functional loop calling array_fill() and array_merge() on every iteration: (Demo)

   var_export(
       array_reduce(
           $chars,
           fn($result, $v) => array_merge($result, array_fill(0, 3, $v)),
           []
       )
   );
  

  
  

• Transpose and flatten 3 copies of the input array: (Demo)

   var_export(
       array_merge(...array_map(null, $chars, $chars, $chars))
   );
  

  or: (Demo)

   var_export(
       array_merge(...array_map(null, ...array_fill(0, 3, $chars)))
   );
  

  
  

• Looped calls of array_fill() with its returned array spread as parameters of array_push(): (Demo)

   $result = [];
   foreach ($chars as $char) {
       array_push($result, ...array_fill(0, 3, $char));
   }
   var_export($result);
  

  
  

• A for() loop calling array_fill() and array_splice() on every iteration: (Demo)

   for ($i = count($chars) - 1; $i >= 0; --$i) {
       array_splice($chars, $i, 1, array_fill(0, 3, $chars[$i]));
   }
   var_export($chars);