Which is faster? Comparison or assignment?

Solution 1:

This isn't just premature optimization, this is micro-optimization, which is an irrelevant distraction.

Assuming your array is of boolean type then your comparison is unnecessary, which is the only relevant observation.

Solution 2:

Well, since you say you're sure that this matters you should just write a test program and measure to find the difference.

Comparison can be faster if this code is executed on multiple variables allocated at scattered addresses in memory. With comparison you will only read data from memory to the processor cache, and if you don't change the variable value when the cache decides to to flush the line it will see that the line was not changed and there's no need to write it back to the memory. This can speed up execution.

Solution 3:

Edit: I wrote a script in PHP. I just noticed that there was a glaring error in it meaning the best-case runtime was being calculated incorrectly (scary that nobody else noticed!)

Best case just beats outright assignment but worst case is a lot worse than plain assignment. Assignment is likely fastest in terms of real-world data.

Output:

• assignment in 0.0119960308075 seconds
• worst case comparison in 0.0188510417938 seconds
• best case comparison in 0.0116770267487 seconds

Code:

<?php
$arr = array();

$mtime = explode(" ", microtime());
$starttime = $mtime[1] + $mtime[0];

reset_arr($arr);

for ($i=0;$i<10000;$i++)
    $arr[i] = true;


$mtime = explode(" ", microtime());
$firsttime = $mtime[1] + $mtime[0];
$totaltime = ($firsttime - $starttime);
echo "assignment in ".$totaltime." seconds<br />"; 

reset_arr($arr);

for ($i=0;$i<10000;$i++)
    if ($arr[i])
        $arr[i] = true;

$mtime = explode(" ", microtime());
$secondtime = $mtime[1] + $mtime[0];
$totaltime = ($secondtime - $firsttime);
echo "worst case comparison in ".$totaltime." seconds<br />"; 

reset_arr($arr);

for ($i=0;$i<10000;$i++)
    if (!$arr[i])
        $arr[i] = false;

$mtime = explode(" ", microtime());
$thirdtime = $mtime[1] + $mtime[0];
$totaltime = ($thirdtime - $secondtime);
echo "best case comparison in ".$totaltime." seconds<br />"; 

function reset_arr($arr) {
    for ($i=0;$i<10000;$i++)
        $arr[$i] = false;
}

Solution 4:

I believe if comparison and assignment statements are both atomic(ie one processor instruction) and the loop executes n times, then in the worst-case comparing then assigning would require n+1(comparing on every iteration plus setting the assignement) executions whereas constantly asssigning the bool would require n executions. Therefore the second one is more efficient.