A little known function, with not too many use cases, but a powerful and elegant tool to have in your belt.

The transpose function takes a multi-dimensional array, and switches the row indices with the column indices. A visualisation might explain things more clearly:

Consider we have a 2D array of columns 1 to 4 and rows A to D:

A1 A2 A3 A4
B1 B2 B3 B4
C1 C2 C3 C4
D1 D2 D3 D4

By transposing that array we end up with the following:

A1 B1 C1 D1
A2 B2 C2 D2
A3 B3 C3 D3
A4 B4 C4 D4

It's the same data, but the columns and rows have been switched around.

Use cases

We used this in a code kata last week, the exercise was to convert an ascii representation of digits and transform that to a string of numbers e.g.

 _     _  _ 
| |  | _| _|
|_|  ||_  _|

becomes "0123"

Each number is 4 lines tall and three characters wide (space, pipe or underscore only).

Here is the pseudo code and steps for how we solved this:

Split the ascii string into lines

[
    " _     _  _ ",
    "| |  | _| _|",
    "|_|  ||_  _|",
    "            ",
]

Map over each line, splitting the line into an array of strings of length 3

[
    [ " _ ", "   ", " _ ", " _ " ],
    [ "| |", "  |", " _|", " _|" ],
    [ "|_|", "  |", "|_ ", " _|" ],
    [ "   ", "   ", "   ", "   " ],
]

Transpose the array, so we have an array of ascii characters

[
    [ " _ ", "| |", "|_|", "   " ], // zero
    [ "   ", "  |", "  |", "   " ], // one
    [ " _ ", " _|", "|_ ", "   " ], // two
    [ " _ ", " _|", " |_", "   " ], // three
]

Lets map over and concat the above into an array of strings

[
    " _ | ||_|   ", // zero
    "     |  |   ", // one
    " _  _||_    ", // two
    " _  _| |_   ", // three
]

Now we have an array of chars in string form we can use this to lookup the numeric value from a hash map.

function lookup ($ascii)
{
    $asciiMap = [
        " _ | ||_|   ",
        "     |  |   ",
        " _  _||_    ",
        " _  _| _|   ",
        "   |_|  |   ",
        " _ |_  _|   ",
        " _ |_ |_|   ",
        " _   |  |   ",
        " _ |_||_|   ",
        " _ |_| _|   ",
    ];

    if (false !== $n = array_search($ascii, $asciiMap)) {
        return (string) $n;
    }
}

If we map our lookup function over our previous strings we get

[
    "0",
    "1",
    "2",
    "3",
]

Hopefully you can see now that we just need to concat the above array to return our final result

"0123"

et voilà!

Some more solutions to the kata above, implemented in Haskell, JavaScript and PHP. http://github.com/petemcfarlane/ascii-number-ocr-kata PRs in other languages welcome!

If you're using a language without the transpose function, you may have to write something like this:

$rows = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
];

function transpose ($rows)
{
    $temp = [];

    foreach ($rows as $r => $columns) {
        foreach ($columns as $c => $value) {
            $temp[$c][$r] = $value;
        }
    }

    return $temp;
}

or you could just pull in a function from another library, e.g. PHP 😉

Another use case for transpose, for cleaning up form input, by Adam Wathan: http://adamwathan.me/2016/04/06/cleaning-up-form-input-with-transpose/