Re^2: next unless condition

Over the last 20 years, I have seen this construct written in the very perlish manner of :

$cell || next;
[download]

which is the same as:

$cell or next;
[download]

The explanation that I have "heard" in the past is that this performed faster than other functionally equivalent alternatives. I have always been skeptical of this POV and too lazy to actually test it

I personally like the way next unless $cell reads for loop control and often use this construct. In general, I am not a big fan of unless and general use If ( not $cell ) with the exception of loop control. My rationale behind this is that I think it is easier to read and comprehend for someone who comes behind me to maintain any code that I have created. Please note the use of belief here. I do not purport that to be a fact.

lbe

Comment on Re^2: next unless condition Select or Download Code

Replies are listed 'Best First'.
Re^3: next unless condition by ikegami (Patriarch) on Apr 01, 2016 at 15:51 UTC
The explanation that I have "heard" in the past is that this performed faster than other functionally equivalent alternatives. That's not the case. `$ perl -MO=Concise,-exec -e'f() unless $x' 1 <0> enter 2 <;> nextstate(main 1 -e:1) v:{ 3 <#> gvsv[x] s 4 <\|> or(other->5) vK/1 5 <0> pushmark s 6 <#> gv[f] s/EARLYCV 7 <1> entersub[t3] vKS/TARG,1 8 <@> leave[1 ref] vKP/REFC -e syntax OK $ perl -MO=Concise,-exec -e'$x or f()' 1 <0> enter 2 <;> nextstate(main 1 -e:1) v:{ 3 <#> gvsv[x] s 4 <\|> or(other->5) vK/1 5 <0> pushmark s 6 <#> gv[f] s/EARLYCV 7 <1> entersub[t4] vKS/TARG,1 8 <@> leave[1 ref] vKP/REFC -e syntax OK` [download]	[reply] [d/l]
Re^3: next unless condition by dsheroh (Monsignor) on Apr 01, 2016 at 07:51 UTC
There is a difference in operator precedence between `$cell \|\| next` and `$cell or next`, so they're not quite the same. While they're effectively identical in this case, even the minor modification of adding an assignment results in `$x = $cell \|\| next` and `$x = $cell or next` behaving differently. The efficiency explanation you've heard is accurate when dealing with extremely-close-to-the-metal languages and non-optimizing compilers, since testing whether a memory location already holds the value 0 is one of the fastest tests you can do at the machine level. But optimizing compilers are smart enough to figure out for themselves that `if (not $cell) ...` is equivalent to `$cell \|\| ...` while still allowing you to write it the more readable way, and Perl is so far from the metal that these kinds of concerns are completely inapplicable anyhow.	[reply] [d/l] [select]
Re^4: next unless condition by Marshall (Canon) on Apr 01, 2016 at 14:45 UTC
I agree that you should write the clearest HLL code possible. At the local college I work with ASM students and sometimes we play "beat the compiler". With the right example, even a beginning student can do this vs the highest optimization level. This example would not be a good one because `if (not $cell) ...` and `$cell \|\| ...` will code every similarly even with a "dumb" compiler. They are essentially the same. Load from memory, ALU operation to set flags, jump on flags that were just set. The ALU operation could be a `or eax,eax`(nothing but set flags) or `neg eax` (negate and set flags). Different kind of jump instruction required but timing is the same. I wouldn't worry about this at all. A bigger mistake in source coding is to assume that shorter source code maybe with a bunch of fancy maps, joins, and other functions is faster than longer code that is more straightforward. You can write one line in HLL that takes a lot more work at the low level that something else that takes many HLL lines.	[reply] [d/l] [select]
Re^5: next unless condition by RonW (Parson) on Apr 04, 2016 at 19:13 UTC
Load from memory, ALU operation to set flags, jump on flags that were just set. FWIW, in some CPUs, the load instructions will also set/clear the "zero" and "negative" flags. You are right. As a rule, today's optimizing compilers are very good at producing (near) best machine code. With Perl, I'd say just write the most understandable and maintainable code you can.	[reply]
Re^6: next unless condition by BrowserUk (Patriarch) on Apr 04, 2016 at 20:29 UTC
Re^7: next unless condition by RonW (Parson) on Apr 04, 2016 at 23:45 UTC
Re^6: next unless condition by Marshall (Canon) on Apr 04, 2016 at 20:35 UTC
Re^7: next unless condition by RonW (Parson) on Apr 04, 2016 at 23:55 UTC
Re^3: next unless condition by haukex (Archbishop) on Apr 01, 2016 at 07:44 UTC
Hi lbe, Yes, `$cell \|\| next;` is yet another one to add to the list of ways to do it :-) (Although it's probably easier to get bitten by precedence issues on that one so personally, I'd avoid it.) `if (...) {...}` is definitely what I'd use if the condition or the statement becomes more complex. For example, I've often written code something like `if ($x && $y) { warn "skipping b/c foo"; next }` Regards, -- Hauke D	[reply] [d/l] [select]


Keep It Simple, Stupid
	PerlMonks