[PHP source code reading] array_slice and array_splice functions, slicesplice_PHP tutorial

[PHP source code reading]array_slice and array_splice functions, slicesplice

array_slice and array_splice functions are used to take out a slice of the array, array_splice also replaces the original deleted slice position with a new slice function. Similar to the Array.prototype.splice and Array.prototype.slice methods in javascript.

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array_slice

<p>array array_slice ( array $array , int $offset [, int $length = NULL [, bool $preserve_keys = false ]] )</p>

Returns the subarray slice of the specified subscript offset and length in the array.

Parameter description

Suppose the length of the first parameter array is num_in.

offset

If offset is a positive number and less than length, the returned array will start from offset; if offset is greater than length, no operation will be performed and it will be returned directly. If offset is a negative number, offset = num_in offset, if num_in offset == 0, offset is set to 0.

length

If length is less than 0, then length will be converted to num_in - offset length; otherwise, if offset length > array_count, then length = num_in - offset. If length is still less than 0 after processing, it will be returned directly.

preserve_keys

The default is false. The original order of numeric key values ​​is not retained by default. If set to true, the original numeric key value order of the array will be retained.

Usage examples

<?<span>php
</span><span>$input</span> = <span>array</span>("a", "b", "c", "d", "e"<span>);

</span><span>$output</span> = <span>array_slice</span>(<span>$input</span>, 2);      <span>//</span><span> returns "c", "d", and "e"</span>
<span>$output</span> = <span>array_slice</span>(<span>$input</span>, -2, 1);  <span>//</span><span> returns "d"</span>
<span>$output</span> = <span>array_slice</span>(<span>$input</span>, 0, 3);   <span>//</span><span> returns "a", "b", and "c"</span>

<span>print_r</span>(<span>array_slice</span>(<span>$input</span>, 2, -1)); <span>//</span><span> array(0 => 'c', 1 => 'd');</span>
<span>print_r</span>(<span>array_slice</span>(<span>$input</span>, 2, -1, <span>true</span>)); <span>//</span><span> array(2 => 'c', 1 => 'd');</span>

Run steps

<p>处理参数：offset、length</p>
<p>移动指针到offset指向的位置</p>
<p>从offset开始，拷贝length个元素到返回数组</p>

The operation flow chart is as follows

php $input = array("red", "green", "blue", "yellow"); array_splice($input, 2); // $input becomes array("red", "green") $input = array("red", "green", "blue", "yellow"); array_splice($input, 1, -1); // $input becomes array("red", "yellow") $input = array("red", "green", "blue", "yellow"); array_splice($input, 1, count($input), "orange"); // $input becomes array("red", "orange") $input = array("red", "green", "blue", "yellow"); array_splice($input, -1, 1, array("black", "maroon")); // $input is array("red", "green", // "blue", "black", "maroon") $input = array("red", "green", "blue", "yellow"); array_splice($input, 3, 0, "purple"); // $input is array("red", "green", // "blue", "purple", "yellow");

Source code interpretation

In array_splice, there is this piece of code:

    <span>/*</span><span> Don't create the array of removed elements if it's not going
     * to be used; e.g. only removing and/or replacing elements </span><span>*/</span>
    <span>if</span> (return_value_used) { <span>//</span><span> 如果有用到函数返回值则创建返回数组，否则不创建返回数组</span>
        <span>int</span> size =<span> length;

        </span><span>/*</span><span> Clamp the offset.. </span><span>*/</span>
        <span>if</span> (offset ><span> num_in) {
            offset </span>=<span> num_in;
        } </span><span>else</span> <span>if</span> (offset < <span>0</span> && (offset = (num_in + offset)) < <span>0</span><span>) {
            offset </span>= <span>0</span><span>;
        }

        </span><span>/*</span><span> ..and the length </span><span>*/</span>
        <span>if</span> (length < <span>0</span><span>) {
            size </span>= num_in - offset +<span> length;
        } </span><span>else</span> <span>if</span> (((unsigned <span>long</span>) offset + (unsigned <span>long</span>) length) ><span> (unsigned) num_in)         {
            size </span>= num_in -<span> offset;
        }

        </span><span>/*</span><span> Initialize return value </span><span>*/</span><span>
        array_init_size(return_value, size </span>> <span>0</span> ? size : <span>0</span><span>);
        rem_hash </span>= &<span>Z_ARRVAL_P(return_value);
    }</span>

The array_splice function returns the deleted slice. The meaning of this code is that if array_splice needs to return a value, then create the return array, otherwise do not create it to avoid wasting space. This is also a little programming trick, return only when needed. For example, if $result = array_splice(...) is used in a function, return_value_used is true.

Summary

This is the end of this article. In daily programming, you should deal with the most special situations first, and then continue, just like you did when implementing these two functions, to avoid making redundant judgments; save new variables if necessary Only apply for new space when the time comes, otherwise it will cause waste.

Original article with limited writing style and limited knowledge. If there is anything wrong in the article, please let me know.

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