Array#inject to create a hash versus Hash[*array.collect{}.flatten] -- Speed, segfault

Discussion in 'Ruby' started by Anthony Martinez, Jun 11, 2007.

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    So, I was tinkering with ways to build a hash out of transforming an
    array, knowing the standard/idiomatic

    id_list =3D [:test1, :test2, :test3]
    id_list.inject({}) { |a,e| a[e]=3De.object_id ; a }

    I also decided to try something like this:

    Hash[ *id_list.collect { |e| [e,e.object_id]}.flatten]

    and further (attempt to) optimize it via
    Hash[ *id_list.collect { |e| [e,e.object_id]}.flatten!]
    and=20
    Hash[ *id_list.collect! { |e| [e,e.object_id]}.flatten!]

    Running this via Benchmark#bmbm gives pretty interesting, and to me,
    unexpected, results (on a 3.2 GHz P4, 1GB of RAM, FC5 with ruby 1.8.4)

    require 'benchmark'
    id_list =3D (1..1_000_000).to_a
    Benchmark::bmbm do |x|
    x.report("inject") { id_list.inject({}) { |a,e| a[e] =3D e.objec=
    t_id ; a} }
    x.report("non-bang") { Hash[ *id_list.collect { |e| [e,e.object_id]}.f=
    latten] }
    x.report("bang") { Hash[ *id_list.collect { |e| [e,e.object_id]}.f=
    latten!] }
    x.report("two-bang") { Hash[ *id_list.collect! { |e| [e,e.object_id]}.f=
    latten!] }
    end

    Rehearsal --------------------------------------------
    inject 16.083333 0.033333 16.116667 ( 9.670747)
    non-bang 1657.050000 1.800000 1658.850000 (995.425642)
    bang 1593.716667 0.016667 1593.733333 (956.334565)
    two-bang 1604.816667 1.350000 1606.166667 (963.803356)
    -------------------------------- total: 4874.866667sec

    user system total real
    inject 5.183333 0.000000 5.183333 ( 3.102379)
    non-bang zsh: segmentation fault ruby

    Ow?

    Also, I just thought of a similar way to accomplish the same thing:

    x.report("zip") { Hash[ *id_list.zip(id_list.collect {|e| e.object_id}=
    )] }

    Array#collect! won't work right with this, of course, but it seems to
    have equally-bad performance. Is Array#inject just optimized for this,
    or something?

    -- Pi


    --=20
    panic ("No CPUs found. System halted.\n");
    2.4.3 linux/arch/parisc/kernel/setup.c

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    Anthony Martinez, Jun 11, 2007
    #1
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  2. The reason why you are seeing this (performance as well as timing) is
    most likely caused by the different approach. When you use #inject you
    just create one copy of the Array (the Hash). When you use #collect you
    create at least one additional copy of the large array plus a ton of two
    element arrays. That's way less efficient considering memory usage and
    GC. You'll probably see much different results if your input array is
    much shorter (try with 10 or 100 elements).

    Kind regards

    robert
     
    Robert Klemme, Jun 11, 2007
    #2
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  3. That segfault shouldn't be happening... I guess you've found a bug. I
    modified your benchmark slightly (reduced id_list to 100_000 elements,
    since I don't want to wait more than an hour for it to finish) and I
    didn't get a segfault, but the results are definitely strange:

    Rehearsal --------------------------------------------
    inject 0.320000 0.350000 0.670000 ( 0.682610)
    non-bang 7.700000 0.590000 8.290000 ( 8.522170)
    bang 7.150000 0.230000 7.380000 ( 7.392471)
    two-bang 6.500000 0.220000 6.720000 ( 6.766079)
    ---------------------------------- total: 23.060000sec

    user system total real
    inject 0.700000 0.310000 1.010000 ( 1.019340)
    non-bang 131.260000 1.440000 132.700000 (132.823762)
    bang 134.610000 1.280000 135.890000 (136.323224)
    two-bang 137.260000 0.750000 138.010000 (138.027025)

    Why is the rehearsal so much faster? Especially for the various
    collect implementations?
    (I'm running Debian Etch, Ruby 1.8.5.)



    I expect that the fastest way to do what you want is like this:

    id_list = [:test1, :test2, :test3]
    result={}
    id_list.each { |e| result[e]=e.object_id }

    Anyway, that's what I usually do.

    I have several times wanted a method on Enumerable that helps you
    transform it into a hash. Something like:

    module Enumerable
    def map_to_hash
    result={}
    each { |e|
    k,v=*yield(e)
    result[k]=v
    }
    return result
    end
    end

    id_list.map_to_hash {|x| [x,x.object_id]}

    I don't know what the performance of this might be.... It's creating a
    2-element hash on each loop now, but maybe that has minimal cost.
     
    Caleb Clausen, Jun 11, 2007
    #3
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    Ah. This does make sense. Now, I throw in the #zip variant!
    Well, the general trend is that any of the non-idiomatic solutions are
    pretty horrible, although the two-bang gets vaguely better than 0 or 1 on
    bigger arrays:

    1000 elements:
    Rehearsal --------------------------------------------
    inject 0.010000 0.000000 0.010000 ( 0.007391)
    non-bang 0.000000 0.000000 0.000000 ( 0.006602)
    bang 0.000000 0.000000 0.000000 ( 0.008503)
    two-bang 0.010000 0.000000 0.010000 ( 0.010136)
    zip 0.790000 0.220000 1.010000 ( 1.202124)
    ----------------------------------- total: 1.030000sec

    user system total real
    inject 0.010000 0.010000 0.020000 ( 0.012621)
    non-bang 0.020000 0.000000 0.020000 ( 0.022407)
    bang 0.020000 0.000000 0.020000 ( 0.019274)
    two-bang 0.020000 0.000000 0.020000 ( 0.027521)
    zip 2.240000 0.670000 2.910000 ( 5.804656)


    10000 elements:
    Rehearsal --------------------------------------------
    inject 0.050000 0.010000 0.060000 ( 0.079325)
    non-bang 0.140000 0.010000 0.150000 ( 0.265995)
    bang 0.150000 0.010000 0.160000 ( 0.184713)
    two-bang 0.140000 0.020000 0.160000 ( 0.202579)
    zip 77.510000 24.270000 101.780000 (134.493519)
    --------------------------------- total: 102.310000sec

    user system total real
    inject 0.110000 0.030000 0.140000 ( 0.146650)
    non-bang 0.450000 0.020000 0.470000 ( 0.526816)
    bang 0.470000 0.020000 0.490000 ( 0.583837)
    two-bang 0.440000 0.040000 0.480000 ( 0.520914)
    zip 222.910000 69.370000 292.280000 (389.541023)


    Extra-ouch! This is what I get for trying to be clever.
    --=20
    Too many errors on one line (make fewer)
    -- Apple MPW C compiler

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    Anthony Martinez, Jun 11, 2007
    #4
  5. :)

    I'd say figures for the 1000 elements array are not too meaningful. You
    should probably wrap those tests in a repetition. And if you are really
    trying to get the fastest solution I'd also try

    h = {}
    id_list.each {|e| h[e] = e.object_id }

    This is usually the fastest in my experience (unless of course the
    timing is dominated by what you do in the block).

    Kind regards

    robert
     
    Robert Klemme, Jun 11, 2007
    #5
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