php數組函數array_key_exists()小結

array_key_exists()函數判斷某個數組中是否存在指定的key，如果key存在，則返回true，否則返回flase

array_key_exists(key,array);

key：必需。規定鍵名
array：必需。規定輸入的數組

<?php
$a = array('a'=>'Dog','b'=>'Cat');
if(array_key_exists('a',$a)){
  echo 'Key exists!';
} else{
  echo 'Key does not exist!';
}
?>

輸出：Key exists!

array_key_exists為什麼比in_array快？

array_key_exists 和 in_array 查詢的東西都不一樣吧
array_key_exists 判斷是否有鍵值
array_key_exists(a,arr)->if(isset(arr[a]))就是true

而in_array 需要去遍歷值 遍歷到了才跳出循環

追問：
是不是數組的索引有單獨的存儲單元，而且優化過，array_key_exists的時間復雜度是o(1), 而in_array是o(n) ??

追答：
重復雜度來說是這樣

array_key_exists  是判斷某個鍵有沒有值

in_array  要遍歷一次 獲取是否相同 不知道建的情況下必須遍歷

PHP中isset與array_key_exists的區別

1.對於數組值的判斷不同，對於值為null或''或false,isset返回false，array_key_exists返回true;

2. 執行效率不同，isset是內建運算符，array_key_exists是php內置函數，isset要快一些。請參考：PHP 函數實現原理及性能分析

3.當用isset訪問一個不存在索引數組值時，不會引起一個E_NOTICE的php錯誤消息;

4.array_key_exists 會調用get_defined_vars判斷數組變量是否存在，isset不用;

　　測試代碼：

<?php
function
microtime_float()
{
list($usec, $sec) = explode(" ", microtime());
return ((float)$usec + (float)$sec);
}
$test_arr['aa']='dd';
$test_arr['bb']='';
$test_arr['cc']=NULL;
$test_arr['dd']=false;
$test_arr= array('aa'=>'dd','bb'=>'','cc'=>null,'dd'=>false);
echo "isset aa is ";var_dump(isset($test_arr['aa']));echo "n";
echo "isset bb is ";var_dump(isset($test_arr['bb']));echo "n";
echo "isset cc is ";var_dump(isset($test_arr['cc']));echo "n";
echo "isset dd is ";var_dump(isset($test_arr['cc']));echo "n";
echo "isset none is ";var_dump(isset($test_arr['none']));echo "n";
echo "key_exist aa is ";var_dump(array_key_exists('aa',$test_arr));echo "n";
echo "key_exist bb is ";var_dump(array_key_exists('bb',$test_arr));echo "n";
echo "key_exist cc is ";var_dump(array_key_exists('cc',$test_arr));echo "n";
echo "key_exist dd is ";var_dump(array_key_exists('dd',$test_arr));echo "n";
echo "key_exist none is ";var_dump(array_key_exists('none',$test_arr));echo "n";
$time_start = microtime_float();
for($i=0;$i<100;$i++){
isset($test_arr['aa']);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "isset 100 is $timen";
for($i=0;$i<10000;$i++){
isset($test_arr['aa']);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "isset 10000 is $timen";
for($i=0;$i<1000000;$i++){
isset($test_arr['aa']);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "isset 1000000 is $timen";
//++++++++++++++++++++++++++++++
$time_start = microtime_float();
for($i=0;$i<100;$i++){
array_key_exists('aa',$test_arr);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "array_key_exists 100 is $timen";
for($i=0;$i<10000;$i++){
array_key_exists('aa',$test_arr);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "array_key_exists 10000 is $timen";
for($i=0;$i<1000000;$i++){
array_key_exists('aa',$test_arr);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "array_key_exists 1000000 is $timen";