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Improve the efficiency of finding array elements
1.php in_array method description
php finds whether an array element exists, generally using the in_array method.
bool in_array ( mixed $needle , array $haystack [, bool $strict = FALSE ] )
Parameter description:
needle
The value to be searched for. If needle is a string, the comparison is case-sensitive.
haystack
Array used for comparison
strict
If the value of the third parameter strict is TRUE, the in_array() function will also check whether the type of needle is the same as that in haystack
Return value
Returns TRUE if needle is found, FALSE otherwise.
2.Efficiency of finding elements in in_array
When the compared array haystack is large, in_array efficiency will be very low
Example: Use in_array to perform 1000 comparisons on an array of 100,000 elements
<?php
$arr = array();
// 创建10万个元素的数组
for($i=0; $i<100000; $i++){
$arr[] = $i;
}
// 记录开始时间
$starttime = getMicrotime();
// 随机创建1000个数字使用in_array比较
for($j=0; $j<1000; $j++){
$str = mt_rand(1,99999);
in_array($str, $arr);
}
// 记录结束时间
$endtime = getMicrotime();
echo 'run time:'.(float)(($endtime-$starttime)*1000).'ms<br>';
/**
* 获取microtime
* @return float
*/
function getMicrotime(){
list($usec, $sec) = explode(' ', microtime());
return (float)$usec + (float)$sec;
}
?>
run time:2003.6449432373ms
Use in_array to determine whether an element exists. Compare 1000 times in an array of 100,000 elements. The running time takes about 2 seconds
3. Methods to improve the efficiency of finding elements
We can first use array_flip to perform key-value exchange, and then use the isset method to determine whether the element exists, which can improve efficiency.
Example: Use array_flip to perform key-value exchange first, then use the isset method to judge, and compare 1000 times in an array of 100,000 elements
<?php
$arr = array();
// 创建10万个元素的数组
for($i=0; $i<100000; $i++){
$arr[] = $i;
}
// 键值互换
$arr = array_flip($arr);
// 记录开始时间
$starttime = getMicrotime();
// 随机创建1000个数字使用isset比较
for($j=0; $j<1000; $j++){
$str = mt_rand(1,99999);
isset($arr[$str]);
}
// 记录结束时间
$endtime = getMicrotime();
echo 'run time:'.(float)(($endtime-$starttime)*1000).'ms<br>';
/**
* 获取microtime
* @return float
*/
function getMicrotime(){
list($usec, $sec) = explode(' ', microtime());
return (float)$usec + (float)$sec;
}
?>
run time:1.2781620025635ms
Use array_flip and isset to determine whether an element exists. Compare 1000 times in an array of 100,000 elements. The running time takes about 1.2 milliseconds
Therefore, for comparison of large arrays, using array_flip and isset methods will be much more efficient than in_array.
Quickly remove duplicates
1. Use array_unique method to remove duplicates
To deduplicate array elements, we generally use the array_unique method. This method can deduplicate elements in the array.
<?php $arr = array(1,1,2,3,3,3,4,4,5,6,6,7,8,8,9,9,9); $arr = array_unique($arr); $arr = array_values($arr); print_r($arr); ?>
Output:
Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 [5] => 6 [6] => 7 [7] => 8 [8] => 9 )
After deduplication, the key values will be out of order. You can use array_values to reorder the key values.
2. Use array_unique method to remove duplicates for efficiency
<?php
$arr = array();
// 创建100000个随机元素的数组
for($i=0; $i<100000; $i++){
$arr[] = mt_rand(1,99);
}
// 记录开始时间
$starttime = getMicrotime();
// 去重
$arr = array_unique($arr);
// 记录结束时间
$endtime = getMicrotime();
$arr = array_values($arr);
echo 'unique count:'.count($arr).'<br>';
echo 'run time:'.(float)(($endtime-$starttime)*1000).'ms<br>';
echo 'use memory:'.getUseMemory();
/**
* 获取使用内存
* @return float
*/
function getUseMemory(){
$use_memory = round(memory_get_usage(true)/1024,2).'kb';
return $use_memory;
}
/**
* 获取microtime
* @return float
*/
function getMicrotime(){
list($usec, $sec) = explode(' ', microtime());
return (float)$usec + (float)$sec;
}
?>
unique count:99 run time:653.39303016663ms use memory:5120kb
Use the array_unique method to remove duplicates. The running time takes about 650ms and the memory usage is about 5m
3. Faster array deduplication method
PHP has a key-value exchange method array_flip. We can use this method to remove duplicates. Because of key-value exchange, the original duplicate values will become the same key.
Then perform a key-value exchange again, and exchange the keys and values back to complete the deduplication.
<?php
$arr = array();
// 创建100000个随机元素的数组
for($i=0; $i<100000; $i++){
$arr[] = mt_rand(1,99);
}
// 记录开始时间
$starttime = getMicrotime();
// 使用键值互换去重
$arr = array_flip($arr);
$arr = array_flip($arr);
// 记录结束时间
$endtime = getMicrotime();
$arr = array_values($arr);
echo 'unique count:'.count($arr).'<br>';
echo 'run time:'.(float)(($endtime-$starttime)*1000).'ms<br>';
echo 'use memory:'.getUseMemory();
/**
* 获取使用内存
* @return float
*/
function getUseMemory(){
$use_memory = round(memory_get_usage(true)/1024,2).'kb';
return $use_memory;
}
/**
* 获取microtime
* @return float
*/
function getMicrotime(){
list($usec, $sec) = explode(' ', microtime());
return (float)$usec + (float)$sec;
}
?>
unique count:99 run time:12.840032577515ms use memory:768kb
Use the array_flip method to remove duplicates. The running time takes about 18ms and the memory usage is about 2m
Therefore, using the array_flip method to remove duplicates reduces the running time by 98% compared to using the array_unique method, and the memory usage is reduced by 4/5;