php: check if an array has duplicates
Solution 1
You can do:
function has_dupes($array) {
$dupe_array = array();
foreach ($array as $val) {
if (++$dupe_array[$val] > 1) {
return true;
}
}
return false;
}
Solution 2
I know you are not after array_unique()
. However, you will not find a magical obvious function nor will writing one be faster than making use of the native functions.
I propose:
function array_has_dupes($array) {
// streamline per @Felix
return count($array) !== count(array_unique($array));
}
Adjust the second parameter of array_unique()
to meet your comparison needs.
Solution 3
Performance-Optimized Solution
If you care about performance and micro-optimizations, check this one-liner:
function no_dupes(array $input_array) {
return count($input_array) === count(array_flip($input_array));
}
Description:
Function compares number of array elements in $input_array
with array_flip'ed elements. Values become keys and guess what - keys must be unique in associative arrays so not unique values are lost and final number of elements is lower than original.
Warning:
As noted in the manual, array keys can be only type of int
or string
so this is what you must have in original array values to compare, otherwise PHP will start casting with unexpected results. See https://3v4l.org/7bRXI for an example of this fringe-case failure mode.
Proof for an array with 10 million records:
- The top-voted solution by Jason McCreary: 14.187316179276s 🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌
- The accepted solution by Mike Sherov: 2.0736091136932s 🐌🐌
- This answer's solution: 0.14155888557434s 🐌/10
Test case:
<?php
$elements = array_merge(range(1,10000000),[1]);
$time = microtime(true);
accepted_solution($elements);
echo 'Accepted solution: ', (microtime(true) - $time), 's', PHP_EOL;
$time = microtime(true);
most_voted_solution($elements);
echo 'Most voted solution: ', (microtime(true) - $time), 's', PHP_EOL;
$time = microtime(true);
this_answer_solution($elements);
echo 'This answer solution: ', (microtime(true) - $time), 's', PHP_EOL;
function accepted_solution($array){
$dupe_array = array();
foreach($array as $val){
// sorry, but I had to add below line to remove millions of notices
if(!isset($dupe_array[$val])){$dupe_array[$val]=0;}
if(++$dupe_array[$val] > 1){
return true;
}
}
return false;
}
function most_voted_solution($array) {
return count($array) !== count(array_unique($array));
}
function this_answer_solution(array $input_array) {
return count($input_array) === count(array_flip($input_array));
}
Notice that accepted solution might be faster in certain condition when not unique values are near the beginning of huge array.
Solution 4
$hasDuplicates = count($array) > count(array_unique($array));
Will be true
if duplicates, or false
if no duplicates.
Solution 5
$duplicate = false;
if(count(array) != count(array_unique(array))){
$duplicate = true;
}
Related videos on Youtube
Comments
-
Mala about 2 years
I'm sure this is an extremely obvious question, and that there's a function that does exactly this, but I can't seem to find it. In PHP, I'd like to know if my array has duplicates in it, as efficiently as possible. I don't want to remove them like
array_unique
does, and I don't particularly want to runarray_unique
and compare it to the original array to see if they're the same, as this seems very inefficient. As far as performance is concerned, the "expected condition" is that the array has no duplicates.I'd just like to be able to do something like
if (no_dupes($array)) // this deals with arrays without duplicates else // this deals with arrays with duplicates
Is there any obvious function I'm not thinking of?
How to detect duplicate values in PHP array?
has the right title, and is a very similar question, however if you actually read the question, he's looking for array_count_values.-
Admin almost 14 yearsDo you just want to know if there are any duplicates or the quantity and value of said duplicates etc?
-
Mala almost 14 yearsI only need to know if there are any duplicates. Returning a boolean is perfect.
-
Felix Kling almost 14 yearsHonestly I think
if(count($array) == count(array_unique($array)))
is the best you can get. You have to traverse the array this way or another and I think the built-in are optimized for that.array_flip
could be considered too. -
Mike Sherov almost 14 years@Felix, you can do better than that. That does three loops, one to create the unique array, one to count it, and one to count the original.
-
Felix Kling almost 14 years@Mike Sherov: Are you sure? I couldn't find anything about it, but I had hoped that PHP arrays have some internal property that keeps track of the length. Do you have an information about this? I would be very interested.
-
Mike Sherov almost 14 years@Felix, I was always taught that count was an expensive operation in PHP, and that it required looping through. Maybe that's wrong.
-
Mike Sherov almost 14 years@Felix, have a look at these: maettig.com/code/php/php-performance-benchmarks.php josephscott.org/archives/2010/01/php-count-performance mikegerwitz.com/2010/03/28/php-performance-array-iteration I'm not really sure where that leaves us. Yes, doing count() multiples times is slow, but it may well be faster than my answer.
-
-
Mala almost 14 yearsthanks for the suggestion. My thought at finding a better algorithm is simply that, technically speaking, once you've finished running whatever the builtin
array_unique
does, you should be able to know if there were any dupes. Thus, anything that does at least as much work asarray_unique
does more work than necessary. Although yeah, if such a function doesn't exist, I don't particularly feel like writing it. -
Jason McCreary almost 14 yearsIf you only care about if it has dupes, then that's what I would do. If you care about more than just if it has dupes, then you're right, the above may do more work than it needs. Anything you write is going to be O(n^2). Even if you bail out early. As you said, it's not common that you have dupes. So is it worth your time to make something magical?
-
Mike Sherov almost 14 yearsMagical? Sure it's a microoptimization, but it's not "magic" to write your own function, and I'm not sure it's that a better solution is that much harder to write than this.
-
Jason McCreary almost 14 years@Mike, never said anything of the sort. Just questioning if it was worth the developer's time in this particular instance based on the specification of the original question. No worries.
-
Mike Sherov almost 14 years@Jason, I wasn't trying to come off harsh. I was just trying to convey that it doesn't always hurt to seek your own solution when you already know the easy way.
-
Jason McCreary almost 14 yearsI like it! Just keep in mind that even with an early
return
this is an O(n) function. In addition to the overhead offoreach
and tracking$dupe_array
, I'd love to see some benchmarking. I'd guess for array's with no duplicates, utilizing native functions is faster. Definitely better than O(n^2) though. Nice. -
Jason McCreary almost 14 years@Mike. Agreed. And ultimately it's how we all get better. I was just devil's advocate, in this case, saying that there's nothing wrong with the easy way.
-
Mike Sherov almost 14 years@Jason, yup. I too have been bitten before by premature optimization.
-
Artefacto almost 14 yearsHas a little problem: only works correctly if the values are strings or numbers.
-
FabianoLothor over 11 years#BadCode - The best ways to do this check with functions of PHP itself.
-
EleventyOne over 10 yearsThis code gave me an
undefined offset
error in PHP. Instead, I did:foreach ( $a as $v ) { if ( array_key_exists($v,$dupe) { return true; } else { $dupe[$v] = true; }
-
Alan Turing over 9 years@JasonMcCreary I think you meant to say O(n log n) as the simple solution is just sort the array which is O(n log n) and then run through it once to find duplicates.
-
cartbeforehorse over 9 yearsNot sure why you need
array_keys()
in your answer.array_flip()
already condenses your array if the values are the same. Also,!=
is a sufficient compararor, since the types are inherently the same out ofcount()
(you're the one that mentioned benchmarking). Thereforereturn count(array_flip($arr)) != count($arr);
should be sufficient. -
Gino Pane over 9 yearsI came here only to find exactly this answer:)
-
aksu over 9 yearsYou should give us some clue what the second parameter does.
-
Nikunj Madhogaria over 8 yearsHow does this even work? Since
$dupe_array
has not been defined with any values,$dupe_array[$val]
should return an undefined index! -
Tgr over 8 yearsElegant, but
array_unique
is somewhat slow. If you know the array to only contain integers and strings, you can replace it witharray_flip
for much faster results. -
Salam.MSaif over 6 yearswhat does ++dupe_array[$val] means? and isn't it suppose to be ++dupe_arrays[$key] because simply $val is not $key?
-
Milania over 6 yearsThe idea behind
++dupe_array[$val]
is to count how often each array value appears. But since the values are not initialized, this does not work without notices (in PHP 7.1). However, if you want to keep the counting behaviour, you can fix the code by addingif (!isset($dupe_array[$val])) {$dupe_array[$val] = 0}
before the currentif
-block. -
J. Murray over 4 yearsWhere is the explanation of what this source code will accomplish?
-
Erdal G. over 3 yearsCan you explain why is this faster? Also this returns the contrary. So to have a fair comparison you should test with :
function most_voted_solution($array) { return count($array) === count(array_unique($array)); }
-
s3m3n over 3 years@ErdalG. this is faster because
array_flip
is native PHP function written in C and flip is pretty simple operation. After flipping not unique values are removed as they could create array key conflict. -
mickmackusa over 3 yearsThe techniques in this answer have the same vulnerabilities as @s3m3n's function. 3v4l.org/3FlBJ This is an "apples-vs-oranges" comparison, so I'll argue that any benchmark comparisons are inappropriate because the function do not offer identical behavior.
-
mickmackusa over 3 yearsI find variable variables to generally unattractive solution. This technique may fail in certain scenarios. 3v4l.org/kGLWT Moreso, from PHP7.4 and up.
-
mickmackusa over 3 yearsSome data types will cause this technique to fail, so this is not a reliable/robust solution. 3v4l.org/FSr7P
-
mickmackusa over 3 yearsDespite being a one-liner, this technique appears to be doing more processing than other posted answers. To check if an array is empty without calling
count()
, just do a falsey check using!
: 3v4l.org/O4g3F -
mickmackusa over 3 yearsThis is pretty much a retread of @JasonMcCreary's answer. stackoverflow.com/a/3145647/2943403
-
mickmackusa over 3 yearsThis is pretty much a retread of @JasonMcCreary's answer. stackoverflow.com/a/3145647/2943403
-
mickmackusa over 3 yearsThis is pretty much a retread of @JasonMcCreary's answer. stackoverflow.com/a/3145647/2943403 I have voted to delete this post.
-
Prasad Patel about 3 yearsBut It is throwing duplicate values error even when array has empty values. I have posted answer stackoverflow.com/questions/3145607/… below