php function array_shift() to delete the last element in the array-PHP Tutorial-php.cn

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

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

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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)

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ParametersDescription

array Required. Specifies an array.

Technical detailsReturn value:

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

Use numeric key name:

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

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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) }

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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)); }

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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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