How to segment php array

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

array_slice

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

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

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, 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.

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Usage examples

<?php
$input = array("a", "b", "c", "d", "e");
 
$output = array_slice($input, 2);   // returns "c", "d", and "e"
$output = array_slice($input, -2, 1); // returns "d"
$output = array_slice($input, 0, 3);  // returns "a", "b", and "c"
 
print_r(array_slice($input, 2, -1)); // array(0 => &#39;c&#39;, 1 => &#39;d&#39;);
print_r(array_slice($input, 2, -1, true)); // array(2 => &#39;c&#39;, 1 => &#39;d&#39;);

Running steps

·Processing parameters: offset, length

·Move the pointer to the position pointed by offset

· Starting from offset, copy length elements to the return array

The operation flow chart is as follows:

array_splice

array array_splice(array &$input,int $offset[, int $length = 0 [, mixed $replacement = array() ]])

Delete length elements starting from offset in the input. If there is a replacement parameter, use the replacement array to replace the deleted elements.

Parameter description

The offset and length parameters in the array_splice function are used the same as in the array_slice function.

replacement

If this parameter is set, the function will use the replacement array for replacement.

If offset and length specify that no elements need to be removed, then replacement will be inserted at the offset position.

If replacement has only one element, you don’t need array() to wrap it.

Usage example

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

Source code interpretation

In array_splice, there is such a piece of code:

/* Don't create the array of removed elements if it's not going
  * to be used; e.g. only removing and/or replacing elements */
 if (return_value_used) { // 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
   int size = length;
 
   /* Clamp the offset.. */
   if (offset > num_in) {
     offset = num_in;
   } else if (offset < 0 && (offset = (num_in + offset)) < 0) {
     offset = 0;
   }
 
   /* ..and the length */
   if (length < 0) {
     size = num_in - offset + length;
   } else if (((unsigned long) offset + (unsigned long) length) > (unsigned) num_in)     {
     size = num_in - offset;
   }
 
   /* Initialize return value */
   array_init_size(return_value, size > 0 ? size : 0);
   rem_hash = &Z_ARRVAL_P(return_value);
 }

## 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 just like you did when implementing these two functions. Then continue to avoid making unnecessary judgments; only apply for new space when you need to save new variables, otherwise it will cause waste.

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