Which is more efficient? 1 loop with with 2 equations or 2 loops with one equation?

Question

For example, you have the loop:

$i=0;
while($i<5) {
    $x[$i] = $a[$i] + $b[$i];
    $y[$i] = $x[$i] / $c[$i];
    $i++;
}

Does this calculate faster than:

$i=0;
while($i<5) {
    $x[$i] = $a[$i] + $b[$i];
    $i++;
}

$i=0;
while($i<5) {
    $y[$i] = $x[$i] / $c[$i];
    $i++;
}

Or are they the same? I have absolutely no idea how exactly code is compiled or executes.

Thanks for the answers. I'm very new to programming so wasn't aware that it was possible to test the efficiency of code.

Once you will be able to tell yourself what `performance` really means, you shall be able to answer this yourself. Or to do some testes and measures... — Marcin Orlowski, Aug 10 '17 at 09:53
If you want to see how it is compiled, take a look at http://derickrethans.nl/projects.html#vld, or provide these scripts to https://3v4l.org. — localheinz, Aug 10 '17 at 10:07

score 1 · Answer 1 · answered Aug 10 '17 at 09:55

You can measure the time spent by your self and figure out the answer...

example, borrowed from Tracking the script execution time in PHP

$rustart = getrusage();

// script start
$i=0;
while($i<5) {
    $x[$i] = $a[$i] + $b[$i];
    $i++;
}


// Script end
function rutime($ru, $rus, $index) {
    return ($ru["ru_$index.tv_sec"]*1000 + intval($ru["ru_$index.tv_usec"]/1000))
     -  ($rus["ru_$index.tv_sec"]*1000 + intval($rus["ru_$index.tv_usec"]/1000));
}

$ru = getrusage();
echo "This process used " . rutime($ru, $rustart, "utime") .
    " ms for its computations\n";
echo "It spent " . rutime($ru, $rustart, "stime") .
    " ms in system calls\n";

score 0 · Answer 2 · answered Aug 10 '17 at 09:57

The second piece of code will definitly take more time. You do exactly the same operations in both pieces of code, the only two differences being that you double the loop itself and the incrementations on $i.

This code :

while ($i < 5) {
    ++$i;
}

will consume the same amount of time wherever you put it. If you write that code twice, it will take twice the time.

That's why putting both assignments in the same loop will give you the better performance.

score 0 · Answer 3 · answered Aug 10 '17 at 10:13

There is a very simple way to find out, just feed the code to http://3v4l.org and take a look at the output!

Example 1

Performance: https://3v4l.org/0eUfI/perf#output
VLD Opcodes: https://3v4l.org/0eUfI/vld#output

Example 1

Performance: https://3v4l.org/LioV9/perf#output
VLD Opcodes: https://3v4l.org/LioV9/vld#output

Note Generally speaking, however, it depends on what you are doing in the loops, and what you want to optimize for:

performance of the code
readability of the code

If you want to measure the performance yourself, take a look at benchmarking tools, such as

http://www.phpbench.com

or profilers, such as

or application performance services, such as

Which is more efficient? 1 loop with with 2 equations or 2 loops with one equation?

3 Answers3

Example 1

Example 1