Comparing arrays: foreach or for?

[cheesecaker]

Okay well, let's say you have a script that evaluates exam questions. Arrays are used for both the answer key and the test taker's provided answers. Repeated comparison is required, so obviously, a loop fits the bill.

I have the following (VERY basic) for loop set up. I'm supplying only a snippet, because the rest doesn't really matter. Two arrays have been filled, one with the answer key, one with the user's answers, with eight array elements (questions) on each array. Each array's starting index is [1].

for ($i = 1; $i < 8 ; $i++)
{
    if ($answers[$i] == $answerkey[$i])
    {
        $numcorrect++;
    }
}
 
echo $numcorrect;

This correctly outputs the number of correct answers. Now I'm wondering whether I should change this to a foreach loop for some reason, since they're more commonly used with arrays.

[pbmods]

Heya, cheesecaker.

The reason why I am very against using for loops as a method of array traversal in general is because many programmers are tempted to do this:

for($i=0; $i<sizeof($array); $i++)
 

This is in fact the most inefficient way to traverse an array:

• sizeof() gets executed at the start of every single iteration, which is a tremendous waste,
• Any PHP guru can tell you that ++$i is faster than $i++, and
• for doesn't handle non-numeric indexes without modification, whereas foreach handles them automatically.

Incidentally, a better way to write the above example would be this:

for( $i = 0; isset($array[$i]); ++$i )
 

Note the use of isset(), which does the same thing but is exponentially faster than using count() or sizeof() at every iteration.

However, in your case, if the size of the array is actually constant, your code will work very nicely (with the exception of you'll want to change $i++ to ++$i). I wouldn't worry about changing it to a foreach.

If you find that there is a large number of questions (without testing, I couldn't tell you exactly how many), you may actually find foreach to be more efficient, as then you only have to look up one symbol instead of two:

for( $i = 0; isset($array[$i]); ++$i )
{
    echo $array[$i];
}
 

In the above loop, PHP evaluates isset($array[$i]) and echo $array[$i], which means it has to resolve $array and $i, and then find the matching index in $array. That's a lot of symbols to look up.

However, when you use a foreach loop:

foreach( $array as $element )
{
    echo $element;
}
 

PHP internally creates the variable $element and assigns it the value of each element per iteration, so PHP only has to lookup $element once. It's much more efficient.

For small sets of questions and answers, I wouldn't worry about it. For large sets (probably on the scale of >= 50), consider using foreach instead.

[cheesecaker]

Thanks so much for your detailed post.

I plan to implement a dynamic answer key, as well, so a set number of questions is not going to be the case in the final script.

So could you show me how exactly I would replace the current for loop I have with an efficient foreach loop? Remember, I have to compare each answerkey[] array element with its corresponding answers[] array element.

I'm getting confused because foreach only allows you to bring one array in with the $array as $key => $value syntax. Here I need to compare two arrays. Or would I nest a couple of foreach's? For lets me just bring in as many arrays as I want without any special syntax. Anyway, yeah, I'm quite confused as to this.