in reply to Re^2: Generating Unique numbers from Unique strings
in thread Generating Unique numbers from Unique strings

You are completely right: all the hash complexities are handled by perl. I've mention that to let the author of the original question got an impression, how it works, and, implement his own logic, if the standard hashes aren't enough for him.

BTW, does perl caches the string hash value? I.e. is the following approach: 

my $index = $hash->{$string};
...
my $value1 = $array1->[$index];
my $value2 = $array2->[$index];
...
my $valueN = $arrayN->[$index];

[download]

comparing to the following approach: 

my $value1 = $hash1->{$string};
my $value2 = $hash2->{$string};
...
my $valueN = $hashN->{$string};

[download]

I.e. I know that number of keys is fixed and isn't changed at runtime, so, I sort them, and store their order in hash, and then in future I always use arrays to store/retrieve values associated with string.

Is the first approach faster than the second?

WBR, basiliscos.

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Re^4: Generating Unique numbers from Unique strings
by Marshall (Canon) on Apr 03, 2016 at 19:43 UTC
    You asked: "BTW, does perl caches the string hash value?". No, I don't think so. And there is no need to do so because the C equation that calculates the integer hash value runs like a rocket! I've got C code that uses and benchmarks 2 of the recent Perl hash functions, but the computer that it is on, is dead right now! Bummer. To calculate the hash function is very, very fast. I experimented with different C formulations of the equation and found that gcc even on lowest optimize setting coded them essentially the same. On a modern Intel processor even a int*2 is as fast as a left shift 1. I was amazed, but decades of development have gone into that thing and literally millions of transistors!

    Before writing extra code that you think will make a performance difference, benchmark the code and see where you are at. Start with the most straight forward HLL code that implements a reasonable algorithm. Use the features of the language because they have been highly optimized and are likely to produce good results with a clear algorithm.

    An array access by index will be faster than a hash table lookup, but not by all that much unless you do this a bazillion times. BenchMark your code and see for yourself.

    Update: in my experience the 80/20 or even the 90/10 rule applies. 10% of the code does 90% of the "real work". Forget about optimizing the 90%, you must find the 10% where it really matters.

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      An array access by index will be faster than a hash table lookup, but not by all that much unless you do this a bazillion times.

      Accessing an array is measurably, and for some things substantially, faster than accessing a hash; but the big problems with using an array for the OPs usage are:

      1. Creating an array that has 4 billion slots;
      2. Or truncating the 32-bit value to some more reasonable size and then dealing with collisions.

      Perl's hashes take care of this; and do so very efficiently.

      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". I knew I was on the right track :)
      In the absence of evidence, opinion is indistinguishable from prejudice.
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        I agree completely.
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Re^4: Generating Unique numbers from Unique strings
by choroba (Cardinal) on Apr 03, 2016 at 16:23 UTC
    > Is the first approach faster than the second?

    What prevents you from writing a short script using Benchmark and finding out yourself?

    ($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

    [download]
[reply]
[d/l]

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