Sub-sorting a table?

[Jasoco]

Is it possible to sort a table by one parameter, then sub-sort it again by another parameter? And possibly again and again if needed?

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table.sort(t, function(a,b) return a>b end)

Doesn't seem to be an easy way that I can see. Maybe I'm missing something?

[ejmr]

I am sorry if I am misunderstanding your question, but wouldn’t a series of

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table.sort()

calls work, with each using a more specific comparison function to sort by the sub-parameters you want?

[bartbes]

Actually, it's not that hard, just use an or in your sort function:

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Lua 5.2.1  Copyright (C) 1994-2012 Lua.org, PUC-Rio
> test = {
>> {1, 1},
>> {1, 2},
>> {2, 1},
>> {2, 2}}
> table.sort(test, function(a, b) return a[1] > b[1] or a[2] > b[2] end)
> for i, v in ipairs(test) do print(v[1], v[2]) end
2	2
2	1
1	2
1	1

[kikito]

Is it possible to sort a table by one parameter, then sub-sort it again by another parameter? And possibly again and again if needed?

I'm not understanding this. Can you give an example? i.e. an "unsorted" table vs the same table sorted the way you want.

[micha]

Is it possible to sort a table by one parameter, then sub-sort it again by another parameter? And possibly again and again if needed?

I'm not understanding this. Can you give an example? i.e. an "unsorted" table vs the same table sorted the way you want.

Jasoco wants to sort a table by a parameter. Then afterwards he wants to sort all entries, that are equal in one entry, with respect to another entry. This is also called Lexicographical order http://en.wikipedia.org/wiki/Lexicographic_order.

[kikito]

Then afterwards he wants to sort all entries, that are equal in one entry, with respect to another entry. This is also called Lexicographical order http://en.wikipedia.org/wiki/Lexicographic_order.

Ow! Then you just write the sorting conditions in your sorting function. You can sort by 1,2 or any other properties.

Say that you have a list of people with ages and names, and you want it sorted by age first, and by name later.

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people = { { name='Bob', age=21 }, { name='Peter', age=18 }, { name='Andrew', age=18 } }

This will sort them by age only

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table.sort(people, function(a,b) return a.age < b.age end) -- Peter, Andrew, Bob

This will sort them by name only (undoing the previous sort):

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table.sort(people, function(a,b) return a.name < b.name end) -- Andrew, Bob, Peter

If you want to sort them out by name only when their age is the same. Well, add that to the sorting function!

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table.sort(people, function(a,b)
  if a.age == b.age then return a.name < b.name end
  return a.age < b.age
end) -- This will give you Andrew, Peter, Bob

[Inny]

You can also use the __lt metamethod to this end. But what everyone else said is how you want to go about this, i.e. define a comparison function that moves on to other properties when the primary properties are equal.

[Robin]

Note that kikito's solution is better than bartbes'.

Why?

Suppose you want to compare {2, 2}, {3, 1}.
bartbes' version:

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table.sort(test, function(a, b) return a[1] > b[1] or a[2] > b[2] end)
-- return 2 > 3 or 2 > 1
-- return false or true
-- return true

kikito's version:

table.sort(test, function(a, b) if a[1] == b[1] then return a[2] > b[2] end return a[1] > b[1] end
-- if 2 == 3 then return 2 > 1 end return 2 > 3
-- if false then return true end return false
-- return false

You only want to look at the second condition if the first condition would compare equal.

[micha]

Lua does short-cut-evaluation. If you evaluate

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a or b

First a is evaluated and if it is true, then b is not evaluated. So in terms of speed, there should not be a large difference.

But I just discovered, that bartbes' and kikito's solution sort differently, as bartbes' solution does not check of equality (and I understand the original post such that equality has to be checked, otherwise the relation is not antisymmetric in the sense that it is possible to have a>b and b>a at the same time. If we try f({2,1},{1,2}) we get true and with f({1,2},{2,1}), too.).

Here is my proposed solution:

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table.sort(test, function(a, b) return a[1] > b[1] or (a[1] == b[1] and a[2] > b[2]) end)

[Robin]

That's... basically what I said.