在PHP中向多维数组添加元素

Question

I'm iterating over an array of courses, which all contain category IDs in the form of strings. I'm trying ro group them all by category using a matrix, but I'm having trouble doing that.

Here's an example of what I want:

$courses_by_code = [[2/11]=>[course1],[course2]], [[3/12]=>[course3], [course4]]

So my questions are:

How do I add an element that has a key that's already in the matrix to the array that corresponds to that key?

How do I create a new line in the matrix in case I find a new key?

Answer 1

I am not sure I understood 100%, but from what I understood you should do something like:

$keyThatMightExist // '2/11' for example
if(isset($courses_by_code[$keyThatMightExist])) {
    $courses_by_code[$keyThatMightExist][] = $newCourseToAdd;
} else {
    $courses_by_code[$keyThatMightExist] = array($newCourseToAdd);
}

Answer 2

Let's start with PHP's Arrays documentation:

An array in PHP is actually an ordered map. A map is a type that associates values to keys.

Something that will be invaluable to you when working with arrays is the print_r function. var_dump is also useful. You will be able to see array structure as its stored in PHP. Here's some useful things to know with arrays.

Let's answer some questions now:

How do I add an element that has a key that's already in the matrix to the array that corresponds to that key? How do I create a new line in the matrix in case I find a new key?

$courses = []; // initialize
$courses['2/11'][] = 'course1';
$courses['2/11'][] = 'course2';
$courses['3/12'][] = 'course3';
$courses['3/12'][] = 'course4';

The empty [] you see indicates that I'm adding more elements to the key 2/11 . If I wanted I could also name those keys, but I'm not going to do that. Using print_r (described above) I will now print the array out in a human-readable format. Note that with print_r you generally want to surround the output with <pre> tags, as such:

echo "<pre>";
print_r($courses);
echo "</pre>";

And here is my output:

Array
(
    [2/11] => Array
        (
            [0] => course1
            [1] => course2
        )

    [3/12] => Array
        (
            [0] => course3
            [1] => course4
        )

)

Here is how I can access these elements:

echo $courses['2/11'][1]; // this is course2
echo "<br>";
print_r($courses['3/12']);    // this is the entire '3/12' array

Result:

course2
Array
(
    [0] => course3
    [1] => course4
)