找到两个在数组中相等的值

Question

 $arr = array(
     1, 1, 2, 3, 4
 );

How to find out the pair from this array ?
Keep in mind that the pair from array could be any number (1,2,4,3,2) or (3,3,1,2,4); I just give an random example above.

if there is a pair in array
   echo "The pair number is 1";

Answer 1

All of my methods will return the desired result as long as there IS a duplicate. It is also assumed because of your sample input, that there is only 1 duplicate in the array. The difference between my methods (and the other answers on this page) will be milliseconds at most for your input size. Because your users will not be able to distinguish between any of the correct methods on this page, I will suggest that the method that you implement should be determined by "readability","simplicity", and/or "brevity". There are many coders who always default to for/foreach/while loops. There are others who always defer to functional iterators. Your choice will probably just come to down to "your coding style".

Input:

$arr=[1,1,2,3,4];

Method #1: array_count_values(), arsort(), key()

$result=array_count_values($arr);
arsort($result);                 // this doesn't return an array, so cannot be nested
echo key($result);
// if no duplicate, this will return the first value from the input array

Explanation: generate new array of value occurrences, sort new array by occurrences from highest to lowest, return the key.

---

Method #2: array_count_values(), array_filter(), key()

echo key(array_filter(array_count_values($arr),function($v){return $v!=1;}));
// if no duplicate, this will return null

Explanation: generate the array of value occurrences, filter out the 1's, return the lone key.

---

Method #3: array_unique(), array_diff_key(), current()

echo current(array_diff_key($arr,array_unique($arr)));
// if no duplicate, this will return false

Explanation: remove duplicates and preserve the keys, find element that went missing, return the lone value.

Further consideration after reading: https://www.exakat.io/avoid-array_unique/ and the accepted answer from array_unique vs array_flip I have a new favorite 2-function one-liner...

Method #4: array_count_values(), array_flip()

echo array_flip(array_count_values($arr))[2];
  // if no duplicate, this will show a notice because it is trying to access a non-existent element
  // you can use a suppressor '@' like this:
  // echo @array_flip(array_count_values($arr))[2];
  // this will return null on no duplicate

Explanation: count the occurrences (which makes keys of the values), swap the keys and values (creating a 2-element array), access the 2 key without a function call. Quick-smart!

If you wanted to implement Method #4, you can write something like this:(demo)

$dupe=@array_flip(array_count_values($arr))[2];
if($dupe!==null){
    echo "The pair number is $dupe";
}else{
    echo "There were no pairs";
}

There will be many ways to achieve your desired result as you can see from all of the answers, but I'll stop here.