lua table.sort not behaving as expected

Question

I have a program that evaluates sets of images and assigns them a given value, now I would like to sort the output of this program, to do this I have the following code:

 function SelectTop(params,images,count)
    local values={}  
    for k,v in pairs(images) do
        local noError,res=pcall(evaluate,params,v)

    if noError then 
        values[v]=res
    else

    values[v] = 9999999999999999999999999999999999999999999999999999999999
    end
  end
function compare(a,b)
  return a[2] < b[2]
end

  table.sort(values,compare)
  print(values)
end

where we can reasonably assume the output of evaluate to be akin to math.random(7000) (the actual code is far more complex and involves neural networks). Now I would expect the output to be sorted but instead I get something like this:

 {
  table: 0x40299d30 : 4512.3590053809
  table: 0x40299580 : 4029.3450116073
  table: 0x40298dd0 : 6003.9508240314
  table: 0x40297de0 : 6959.9145312802
  table: 0x40297630 : 4265.2784117677
  table: 0x40296e40 : 3850.0829011681
  table: 0x40296690 : 4007.2308907069
  table: 0x40296ec0 : 3840.5216952082
  table: 0x4029a770 : 5059.1475464564
  table: 0x40299fc0 : 6058.9603651599
  table: 0x40299810 : 1e+58
  table: 0x40299060 : 1e+58
  table: 0x402988b0 : 5887.729117754
  table: 0x402978c0 : 3675.7295252455
  table: 0x40296920 : 1e+58
  table: 0x4029aa00 : 5624.6042279879
  table: 0x40295bf8 : 1391.8185365923
  table: 0x40296458 : 4276.09869066
  table: 0x40299aa0 : 1e+58
  table: 0x402992f0 : 6334.3641972965
  table: 0x40298300 : 2660.5004512843
  table: 0x40298b40 : 6200.373787482
  table: 0x40296148 : 6178.926312832
  table: 0x40298380 : 1559.5307868896
  table: 0x40295968 : 1e+58
  table: 0x40296bb0 : 6708.7545218628
  table: 0x4029b550 : 1484.2931717456
  table: 0x40298400 : 1638.1286256175
  table: 0x40298070 : 3762.7368939272
  table: 0x402963d8 : 1500.002116023
  table: 0x4029ac90 : 2486.2695974502
  table: 0x40295e88 : 1e+58
  table: 0x40297b50 : 4806.6468870717
  table: 0x4029a4e0 : 4328.0636461426
  table: 0x402973a0 : 4757.4343171052
  table: 0x4029a250 : 3998.8649821268
}

So why does table.sort not work here? I would assume that some sort of sorting would happen here?

Anybody know what I'm doing wrong?

So if we want a full example we can do something like this:

function evaluate (a,b)
    return math.random(7000)
end
SelectTop(nil,{ {a, b, c}, {d, e, f}, {g, e, f}, {f, e, f} },0)

output:

{ table: 0x41c2af18 : 5560
  table: 0x41c2afa8 : 4131
  table: 0x41c2af60 : 4892
  table: 0x41c2aff0 : 5273
}

table.sort is used to sort an array like table, i.e, the index must be integers 1, 2, ... to n, while values isn't one such table. Sorting a hash like table makes little sense. — Yu Hao
– Yu Hao, Commented May 2, 2017 at 10:11

Vlad · Accepted Answer · 2017-05-02 10:12:11Z

4

table.sort works on arrays, not on dictionaries.

You'll need to replace values[v]=res with something like values[#values+1]= {v, res} and adjust compare accordingly.

Right now table.sort will see empty array - there's no items idx 1/2/3/..., because you're indexing results with image itself.

answered May 2, 2017 at 10:12

Vlad

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1 Comment

Thijser Over a year ago

Thanks that's indeed the issue!

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