Is array index rebalanced after using unset in PHP?

Question

I am trying to remove duplicates from a set of tokens using unset (not considering array_unique for now), however I am getting a few issues.

$keywords = parseTweet ( $tweet );
$term_freq = array(count($keywords));

for($i = 0; $i < count($keywords); $i++){
    $term_freq[$i] = 1;
    for($j = 0; $j < count($keywords); $j++){
        if (($i != $j) && (strcmp($keywords[$i],$keywords[$j]) == 0)){
            unset ( $keywords [$j] );
            unset ( $term_freq [$j] );          
            $term_freq[$i]++; 
        }
    }
}

print_r ( $keywords );
print_r ( $term_freq );

I am aware of why I am getting an error; while the duplicate $j is removed, the for loop still has to reloop for the rest of the keywords and hence will fail when it encounters the missing $j. Checking the contents of the array, I found out that the index of the array skips the index $j. So it reads; [1], [2], [4], ... etc where $j = [3]

I thought that unset also rebalances the array index, am I doing something wrong or missing something completely? I am new to PHP so please bear with me!

This doesn't makes sense: unset ( $term_freq [$j] ); $term_freq[$j]++; you unset it and want to increase it? — Rizier123
– Rizier123, Commented Jan 17, 2015 at 1:26
Why are you initialzing the first element of $term_freq with count($keywords)? the first iteration of the for loop is going to overwrite this. — Barmar
– Barmar, Commented Jan 17, 2015 at 1:28
If you use foreach instead of for, it will just see the elements that still exist. — Barmar
– Barmar, Commented Jan 17, 2015 at 1:29
@Rizier123 Mistakenly referred to the deleted element rather than the intended one! — b0neval
– b0neval, Commented Jan 17, 2015 at 1:35
@Barmar Unfortunately I made a very short-sighted assumption and wrongly thought that I actually initiated the size of the array and not the first element. — b0neval
– b0neval, Commented Jan 17, 2015 at 1:36

Sammitch · Accepted Answer · 2015-01-17 01:58:20Z

2

Check if the index is set or not.
You're making needless, repetitive comparisons. Basically n² comparisons when at most n²/2 are required to compare every value in an array to every other value.

So:

$c = count($keywords)
for($i = 0; $i < $c; $i++){
    $term_freq[$i] = 1;
    for($j = $i+1; $j < $c; $j++){ // magic is $j = $i+1
        if( ! isset($keywords[$j]) { continue; } // skip unset indices
        else if ( strcmp($keywords[$i],$keywords[$j]) == 0 ){
            unset ( $keywords [$j] );
            unset ( $term_freq [$j] );          
            $term_freq[$i]++; 
        }
    }
}

Basically you know you've already checked everything prior to $i, so you can start your inner loop at $i+1 instead of zero.

Also, you only need to count $keywords once, not n² times.

edited Jan 17, 2015 at 1:58

answered Jan 17, 2015 at 1:46

Sammitch

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3 Comments

Sammitch Over a year ago

@cHao corrected. Re-considering it, this method will still scale quite terribly. It would be better to sort the array [with an efficient algo] and then walk over it to count/eliminate duplicate neighbours.

cHao Over a year ago

Might be better to just use array_count_values or array_unique, if i'm understanding the goal of the original code. But that's on them. :)

b0neval Over a year ago

I ended up using 'array_count_values' instead, thanks a lot. I really need to get used to all these little predefined functions that php already has available. I imagine that these kind of predefined functions are fully optimized and it's no use making your own, right?

Barmar · Accepted Answer · 2015-01-17 01:40:03Z

2

Use foreach instead of for.

foreach ($keywords as $i => $kw1){
    $term_freq[$i] = 1;
    foreach ($keywords as $j => $kw2){
        if (($i != $j) && ($kw1 == $kw2){
            unset ( $keywords [$j] );
            unset ( $term_freq [$j] );          
            $term_freq[$i]++; 
        }
    }
}

answered Jan 17, 2015 at 1:40

Barmar

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