php function array_shift() to delete the last element in the array

Delete the first element (red) in the array and return the deleted element:

<?php
    $a=array("a"=>"red","b"=>"green","c"=>"blue");
    echo array_shift($a);print_r ($a);
?>

Definition and usage

array_shift() Function is used to delete the first element in the array and returns the deleted element.

Note: If the key name is numeric, all elements will get new key names, starting from 0 and increasing by 1 (see example below).

Syntax

array_shift(array)

Parameters Description

array Required. Specifies an array.

Technical details Return value:

Returns the value of removing elements from the array, or NULL if the array is empty.

Use numeric key name:

<?php
$a=array(0=>"red",1=>"green",2=>"blue");
echo array_shift($a);
print_r ($a);
?>

A large php array (1w+), when using array_shfit and array_pop to retrieve array elements, the performance gap is particularly large, array_shift is unbearably slow, and array_pop That's pretty quick.

Let’s not talk about the answer first, let’s look at the code:

$arr = array(    0=>123,    3=>132,    2=>987,);array_shift($arr);//array_pop($arr);var_dump($arr);
输出会有什么不同呢，
array_shift后，输出为：
array(2) {
  [0]=>
  int(132)
  [1]=>
  int(987)
}
array_pop后，输出为：
array(2) {
  [0]=>
  int(123)
  [3]=>
  int(132)
}

What’s the difference?

Yes, after the array_shift operation, the key value of the array has changed. This is why array_shift is slow. Because the array_shift operation array will rebuild the numeric key value from 0 again. Every time you remove an element, you have to iterate through all the elements in the array. array_pop will not. One is O(1) and the other is O(n) complexity. When the array is large, the effect becomes obvious.

C code implementation of the corresponding function in php:

/* {{{ void _phpi_pop(INTERNAL_FUNCTION_PARAMETERS, int off_the_end) */
static void _phpi_pop(INTERNAL_FUNCTION_PARAMETERS, int off_the_end)
{
zval *stack,/* Input stack */
 **val;/* Value to be popped */
char *key = NULL;
uint key_len = 0;
ulong index;
if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "a", &stack) == FAILURE) {
return;
}
if (zend_hash_num_elements(Z_ARRVAL_P(stack)) == 0) {
return;
}
/* Get the first or last value and copy it into the return value */
if (off_the_end) {
zend_hash_internal_pointer_end(Z_ARRVAL_P(stack));
} else {
zend_hash_internal_pointer_reset(Z_ARRVAL_P(stack));
}
zend_hash_get_current_data(Z_ARRVAL_P(stack), (void **)&val);
RETVAL_ZVAL(*val, 1, 0);
/* Delete the first or last value */
zend_hash_get_current_key_ex(Z_ARRVAL_P(stack), &key, &key_len, &index, 0, NULL);
if (key && Z_ARRVAL_P(stack) == &EG(symbol_table)) {
zend_delete_global_variable(key, key_len - 1 TSRMLS_CC);
} else {
zend_hash_del_key_or_index(Z_ARRVAL_P(stack), key, key_len, index, (key) ? HASH_DEL_KEY : HASH_DEL_INDEX);
}
//就是下面这里，遍历所有元素，对是数字键的元素重新赋值。pop的off_the_end是1,shift的off_the_end是0
/* If we did a shift... re-index like it did before */
if (!off_the_end) {
unsigned int k = 0;
int should_rehash = 0;
Bucket *p = Z_ARRVAL_P(stack)->pListHead;
while (p != NULL) {
if (p->nKeyLength == 0) {//键值是数字
if (p->h != k) {
p->h = k++;
should_rehash = 1;
} else {
k++;
}
}
p = p->pListNext;
}
Z_ARRVAL_P(stack)->nNextFreeElement = k;
if (should_rehash) {
//因为重新给键赋值，hash过后的位置可能不一样了，就得重新hash后，放到对应的位置。
zend_hash_rehash(Z_ARRVAL_P(stack));
}
} else if (!key_len && index >= Z_ARRVAL_P(stack)->nNextFreeElement - 1) {
Z_ARRVAL_P(stack)->nNextFreeElement = Z_ARRVAL_P(stack)->nNextFreeElement - 1;
}
zend_hash_internal_pointer_reset(Z_ARRVAL_P(stack));
}

PHP_FUNCTION(array_pop)
{
_phpi_pop(INTERNAL_FUNCTION_PARAM_PASSTHRU, 1);
}
PHP_FUNCTION(array_shift)
{
_phpi_pop(INTERNAL_FUNCTION_PARAM_PASSTHRU, 0);
}

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