How to convert multidimensional array to one dimensional array in PHP

In PHP programming, we often use arrays, and multidimensional arrays are also a very common data type. Especially when dealing with complex, structured data, using multidimensional arrays can organize the data more clearly. However, in some cases, we need to convert a multi-dimensional array into a one-dimensional array, which is very convenient. This article will introduce how to convert a multi-dimensional array into a one-dimensional array in PHP.

1. What is a multidimensional array?

In PHP, arrays can have multiple dimensions. A one-dimensional array is equivalent to a list, in which each element has a subscript, and the corresponding value can be obtained through the subscript. Multidimensional arrays use an array to express a structure similar to a table or matrix, in which each element can be an array or another multidimensional array.

For example, the following two-dimensional array represents a student information table:

$students = array(
    array("name" => "张三", "age" => 18, "score" => array(90, 85, 94)),
    array("name" => "李四", "age" => 22, "score" => array(80, 88, 90)),
    array("name" => "王五", "age" => 20, "score" => array(92, 95, 90))
);

There are three elements in this array, each element is a one-dimensional array containing three key-value pairs Array with keys "name", "age", and "score". The value corresponding to "score" is a one-dimensional array containing three elements, representing the scores of the three exams.

2. Why do we need to convert multi-dimensional arrays into one-dimensional arrays?

Although multi-dimensional arrays are convenient when expressing complex data, in some cases they need to be converted into one-dimensional arrays, which can make it easier to operate and process the data.

For example, if you need to output the above student information table into a simple table, then you need to expand each student's information into one row:

| 姓名 | 年龄 | 成绩1 | 成绩2 | 成绩3 |
| 张三 | 18   | 90    | 85    | 94    |
| 李四 | 22   | 80    | 88    | 90    |
| 王五 | 20   | 92    | 95    | 90    |

At this time, you need to convert the multi-dimensional array Expand into a one-dimensional array, each row corresponds to a one-dimensional array.

3. How to convert a multi-dimensional array into a one-dimensional array?

In PHP, it is not difficult to convert a multi-dimensional array into a one-bit array. It can be achieved through recursive functions.

Here is a sample code that can flatten an array of arbitrary dimensions into a one-dimensional array:

function flatten($arr){
    $result = array();
    foreach($arr as $value){
        if(is_array($value)){
            $result = array_merge($result, flatten($value));
        }else{
            $result[] = $value;
        }
    }
    return $result;
}

What this function does is simple: if the element is an array, it calls itself recursively , otherwise the element is added to the resulting array. You can convert a multi-dimensional array into a one-dimensional array by calling this function, for example:

$students = array(
    array("name" => "张三", "age" => 18, "score" => array(90, 85, 94)),
    array("name" => "李四", "age" => 22, "score" => array(80, 88, 90)),
    array("name" => "王五", "age" => 20, "score" => array(92, 95, 90))
);

$result = flatten($students);

print_r($result);

The output result is:

Array
(
    [0] => 张三
    [1] => 18
    [2] => 90
    [3] => 85
    [4] => 94
    [5] => 李四
    [6] => 22
    [7] => 80
    [8] => 88
    [9] => 90
    [10] => 王五
    [11] => 20
    [12] => 92
    [13] => 95
    [14] => 90
)

As you can see, the original two-dimensional array has been flattened into a A one-dimensional array of 15 elements.

4. Problems encountered and solutions

When converting multi-dimensional arrays, you may encounter some problems. Below are some possible problems and corresponding solutions.

1. How to deal with array keys?

In the previous example, we just expanded the values ​​in the array into a one-dimensional array, ignoring the array keys. If you need to preserve the keys, you can use the following code:

function flatten2($arr){
    $result = array();
    foreach($arr as $key => $value){
        if(is_array($value)){
            $result = array_merge($result, flatten2($value));
        }else{
            $result[$key] = $value;
        }
    }
    return $result;
}

The only difference between this function and the previous function is that when the element is not an array, both the value and the key are added to the resulting array.

2. How to deal with multi-dimensional associative arrays?

When each element in a multidimensional array is an associative array, it can be expanded into a one-dimensional array, in which each element is an associative array. For example, to expand this array:

$students = array(
    array(
        "name" => "张三",
        "age" => 18,
        "score" => array("语文" => 90, "数学" => 85, "英语" => 94)
    ),
    array(
        "name" => "李四",
        "age" => 22,
        "score" => array("语文" => 80, "数学" => 88, "英语" => 90)
    ),
    array(
        "name" => "王五",
        "age" => 20,
        "score" => array("语文" => 92, "数学" => 95, "英语" => 90)
    )
);

into a one-dimensional array containing 15 elements, you can use the following code:

function flatten3($arr){
    $result = array();
    foreach($arr as $key => $value){
        if(is_array($value)){
            if(!empty($value)){
                foreach($value as $sub_key => $sub_value){
                    $result[$key . "_" . $sub_key] = $sub_value;
                }
            }else{
                $result[$key] = "";
            }
        }else{
            $result[$key] = $value;
        }
    }
    return $result;
}

The logic of this function is similar to the previous function, the only difference When the element is an associative array, the key and value are concatenated and stored as a new key in the result array.

3. How to deal with arrays containing null and empty arrays?

In the above example, if the original array contains null or empty arrays, they will be ignored after flattening because they are not elements in the array that need to be processed. If you want to retain these elements, you can directly add a judgment to the if statement:

function flatten4($arr){
    $result = array();
    foreach($arr as $key => $value){
        if(is_array($value)){
            if(!empty($value)){
                foreach($value as $sub_key => $sub_value){
                    $result[$key . "_" . $sub_key] = $sub_value;
                }
            }else{
                $result[$key] = array();
            }
        }else{
            $result[$key] = $value;
            if(is_null($value)){
                $result[$key] = null;
            }
        }
    }
    return $result;
}

The only difference between this function and the previous function is that when the element is null or an empty array, the corresponding key value is directly added to the result array.

5. Summary

Converting multi-dimensional arrays into one-dimensional arrays is very common in PHP programming. It allows us to process and operate data more conveniently. With recursive functions, we can easily flatten an array of arbitrary dimensions into a one-dimensional array. In practice, due to the different structures of multi-dimensional arrays, some edge cases may be encountered that require special handling. But in general, this process is not difficult, you just need to be familiar with the use of recursive functions.

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