php pop/push/shift/unshift, which to use for queue

问题:

In PHP there are two ways to use an array as a stack (LIFO)
and two ways to use them as a queue (FIFO).

One could implement a stack with push & pop,
but the same can be done with unshift & shift.

Similarly one could implement a queue with push & shift,
but the same can be done with unshift & pop.

To demonstrate:

echo "stack push & pop approach:\n";
$s = []; array_push($s, 'first'); array_push($s, 'second'); array_push($s, 'third');
echo     array_pop($s) . '-' .    array_pop($s) . '-' .     array_pop($s) .  "\n";
echo "stack unshift & shift approach:\n";
$s = []; array_unshift($s, 'first'); array_unshift($s, 'second'); array_unshift($s, 'third');
echo     array_shift($s) . '-' .     array_shift($s) . '-' .      array_shift($s) .  "\n";
echo "queue push & shift approach:\n";
$q = []; array_push($q, 'first'); array_push($q, 'second'); array_push($q, 'third');
echo     array_shift($q) . '-' .  array_shift($q) . '-' .   array_shift($q) .  "\n";
echo "queue unshift & pop approach:\n";
$q = []; array_unshift($q, 'first'); array_unshift($q, 'second'); array_unshift($q, 'third');
echo     array_pop($q) . '-' .       array_pop($q) . '-' .        array_pop($q) .       "\n";

Which outputs:

stack push & pop approach:
third-second-first
stack unshift & shift approach:
third-second-first
queue push & shift approach:
first-second-third
queue unshift & pop approach:
first-second-third

So which sets of functions to use?!

回答1:

Short answer

For stacks use push & pop (add to end, take from end).

For queues use push & shift (add to end, take from beginning).

Considerations

Documentation

When considering the PHP documentation as of today 2016-12-29 for these functions we find this:

For array_push():
the description mentions "array_push() treats array as a stack"
and the example uses "$stack".

For array_pop():
the description mentions nothing on stacks or queues
and the example uses "$stack".

For array_shift():
the description mentions nothing on stacks or queues (but mentions required reindexing)
and the example uses "$stack".

For array_unshift():
the description mentions nothing on stacks or queues (but mentions required reindexing)
and the example uses "$queue".

So this suggests using push & pop for stacks (based on mention in description)
and suggests using unshift & pop for queues (based on only mention of queue in examples).

This seems a bit thin... A suggestion to improve the documentation has been submitted here: https://bugs.php.net/bug.php?id=73839

Performance

Running this:

echo "\nstack push & pop approach     :  ";
$a=[];$i=0;$t=microtime(true); while($i<100000) { array_push($a, 'dummy'); array_pop($a); ++$i; }; echo (microtime(true) - $t);
echo "\nstack unshift & shift approach:  ";
$a=[];$i=0;$t=microtime(true); while($i<100000) { array_unshift($a, 'dummy'); array_shift($a); ++$i; }; echo (microtime(true) - $t);
echo "\nqueue push & shift approach   :  ";
$a=[];$i=0;$t=microtime(true); while($i<100000) { array_push($a, 'dummy'); array_shift($a); ++$i; }; echo (microtime(true) - $t);
echo "\nqueue unshift & pop approach  :  ";
$a=[];$i=0;$t=microtime(true); while($i<100000) { array_unshift($a, 'dummy'); array_pop($a); ++$i; }; echo (microtime(true) - $t);

Returns this:

stack push & pop approach     :  0.011210918426514
stack unshift & shift approach:  0.015399217605591
queue push & shift approach   :  0.011627912521362
queue unshift & pop approach  :  0.015273094177246

For stacks this suggests to use push & pop: natural terminology, matching the mention in the documentation and also performing better (which makes sense considering the reindexing with both unshift & shift).

For queues this suggests to use push & shift, despite the mention in the documentation.