laziness, impatience, and hubris PerlMonks

### Re^2: list of four digit lock combinations without repeated digits

 on Jun 21, 2018 at 01:23 UTC ( #1217086=note: print w/replies, xml ) Need Help??

> it generates these kinds of patterns very efficiently

I tried Algorithm::Combinatorics for a similar task. Both of these examples generate an array of strings n characters in length of all combinations of letters (e.g. 4 = 'aaaa'..'zzzz'). Am I doing something stupid with the module, or perl?

Perl string iteration:
real 0m0.129s
```
time perl -wle '
\$n = shift; die "need a number" unless \$n and \$n =~ /^\d+\$/;
\$a = "a" x \$n; \$c = 0; \$e = 26**\$n;
while () { push @x, \$a; \$a++ and \$c++; last if \$c == \$e }
print scalar @x' 4
Using Algorithm::Combinatorics:
real 0m0.858s
```
time perl -MAlgorithm::Combinatorics=:all -wle '
\$n = shift; die "need a number" unless \$n and \$n =~ /^\d+\$/;
@_ = ("a".."z"); \$i = variations_with_repetition(\@_,\$n);
while (\$c =  \$i->next){
for  (@\$c) { \$x .= \$_ }
push @x, \$x; undef \$x }
print scalar @x' 4
Be careful with the parameter because this can make a very big array.
4 is only ~2MB but 5 is 70MB and the 6 character array is around 2000MB.

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Re^3: list of four digit lock combinations without repeated digits
by BrowserUk (Patriarch) on Jun 21, 2018 at 04:34 UTC
Am I doing something stupid with the module, or perl?
1. This: for(@\$c){ \$x .= \$_ } is a really slow way to build a string from an array.
2. Package variables are slower than lexicals.
3. Postfix loops generally run more quickly than those with scoped bodies.

Try this and see how it compares on your system:

```perl -MAlgorithm::Combinatorics=:all -wle'my \$i=variations_with_repeti
+tion(["a".."z"],\$ARGV[0]); my @x; push @x, qq[@\$_] while \$_=\$i->next;
+ print scalar @x' 4

In general, those algorithms that require more selection than variations_with_repetition() -- almost any of the other algorithms -- is where A;:C shines over a pure perl implementation.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
In the absence of evidence, opinion is indistinguishable from prejudice. Suck that fhit
Very interesting info, thank you. I often wonder about such efficiency when writing perl but it doesn't usually matter too much. I do write postfix loops because they're pretty and one-line friendly (I ❤ print\$_ for@_) but not when they're nested because I tend to forget about them. If they're really much faster I could use comments to remind myself. Anyway your version is slightly faster (~0.750) but the data has whitespace and when using map to remove that it gets even slower than mine. What's going on?

```time perl -MAlgorithm::Combinatorics=:all -wle'
my \$i=variations_with_repetition(["a".."z"],\$ARGV[0]);
my @x; push @x, map {s/ //gr} qq[@\$_] while \$_=\$i->next;
print scalar @x' 4
real 0m1.050s

Profile:
```perl -d:NYTProf -MAlgorithm::Combinatorics=:all -wle'
my \$i=variations_with_repetition(["a".."z"],\$ARGV[0]);
my @x; push @x, map {s/ //gr} qq[@\$_] while \$_=\$i->next;
print scalar @x' 4; nytprofhtml --open

STOP REINVENTING WHEELS, START BUILDING SPACE ROCKETS!CPAN 🐫
when using map to remove that it gets even slower than mine. What's going on?

You are starting the regex engine 210 times.

If you don't want the spaces, don't put them in to start with. Rather than "@\$_" use join'',@\$_ or pack 'A*', @\$_, or set \$"=undef.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
In the absence of evidence, opinion is indistinguishable from prejudice. Suck that fhit

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