Pondering the differences in PHP array traversal (implementation of array_diff)

Function array_diff ($ array_1, $ array_2 ){
$ Diff = array ();

Foreach ($ array_1 as $ k => $ V1 ){
$ Flag = false;
Foreach ($ array_2 as $ V2 ){
If ($ flag = ($ V1 = $ V2 )){
Break;
}
}

If (! $ Flag ){
$ Diff [$ K] = $ V1;
}
}

Return $ diff;
} Although the implementation is acceptable, the efficiency of this function is terrible. So I thought about it again and optimized it.AlgorithmThe second function looks like this:

Function array_diff ($ array_1, $ array_2 ){
Foreach ($ array_1 as $ key => $ item ){
If (in_array ($ item, $ array_2, true )){
Unset ($ array_1 [$ key]);
}
}

Return $ array_1;
} Well, this is almost comparable to the speed of the original array_diff function. But is there any better way to optimize it? From chinaunixArticle(Sorry, I cheated.) I found that PHP could write like this:

Function array_diff ($ array_1, $ array_2 ){
$ Array_2 = array_flip ($ array_2 );
Foreach ($ array_1 as $ key => $ item ){
If (isset ($ array_2 [$ item]) {
Unset ($ array_1 [$ key]);
}
}

Return $ array_1;
} This function is very efficient, even faster than the original array_diff function. The reason is as follows:

Because keys are organized by hash, the search is fast;
The value is only stored by the key organization, and there is no index in itself. Every search is traversed. Summary
This is a tip of the PHP language, but in terms of traversing and comparing the values of the array, if you need to compare the value with the key reversal, it is indeed much more efficient than the normal value pair.

For example, function 2 needs to call the in_array function to determine whether it is in the function cyclically. function 3 only checks whether the array exists with the key. With the array keys and values in different organization indexes, the efficiency is much higher than expected.

AppendixCodeCopy codeThe Code is as follows: <? PHP
Function microtime_float (){
List ($ USEC, $ Sec) = explode ("", microtime ());
Return (float) $ USEC + (float) $ Sec );
}

Function array_diff2 ($ array_1, $ array_2 ){
$ Diff = array ();

Foreach ($ array_1 as $ k => $ V1 ){
$ Flag = false;
Foreach ($ array_2 as $ V2 ){
If ($ flag = ($ V1 = $ V2 )){
Break;
}
}

If (! $ Flag ){
$ Diff [$ K] = $ V1;
}
}

Return $ diff;
}

Function array_diff3 ($ array_1, $ array_2 ){
Foreach ($ array_1 as $ key => $ item ){
If (in_array ($ item, $ array_2, true )){
Unset ($ array_1 [$ key]);
}
}

Return $ array_1;
}

Function array_diff4 ($ array_1, $ array_2 ){
$ Array_2 = array_flip ($ array_2 );
Foreach ($ array_1 as $ key => $ item ){
If (isset ($ array_2 [$ item]) {
Unset ($ array_1 [$ key]);
}
}

Return $ array_1;
}

//////////////////////////////

For ($ I = 0, $ ary_1 = array (); $ I <5000; $ I ++ ){
$ Ary_1 [] = rand (100,999 );
}

For ($ I = 0, $ ary_2 = array (); $ I <5000; $ I ++ ){
$ Ary_2 [] = rand (100,999 );
}

Header ("Content-Type: text/plain; charset = UTF-8 ");

$ Time_start = microtime_float ();
Array_diff ($ ary_1, $ ary_2 );
Echo "function array_diff run". (microtime_float ()-$ time_start). "seconds \ n ";

$ Time_start = microtime_float ();
Array_diff2 ($ ary_1, $ ary_2 );
Echo "function array_diff2 run". (microtime_float ()-$ time_start). "seconds \ n ";

$ Time_start = microtime_float ();
Array_diff3 ($ ary_1, $ ary_2 );
Echo "function array_diff3 run". (microtime_float ()-$ time_start). "seconds \ n ";

$ Time_start = microtime_float ();
Array_diff4 ($ ary_1, $ ary_2 );
Echo "function array_diff4 run". (microtime_float ()-$ time_start). "seconds \ n ";
?>