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Not sure where to post this but I'm having issues listing all the permutations of elements in a table in Pico 8 using this code:

output = {}

function permutation(a, n)
    if n == 0 then
         add(output,a)
    else
        for i = 1, n do
            a[i], a[n] = a[n], a[i]                     
            permutation(a, n - 1)
            a[i], a[n] = a[n], a[i]
        end
    end
end
cls()
permutation({"d","z","2"},3,1)

for i=1,#output do
    for j=1,#output[i] do
        print(output[i][j],j*8,i*10)
    end 
end

It just seems to add the table with the the elements in the initial order to the output table, but the output table does end up with the correct number of element tables. that is, trying to find the permutations of a 3 element array yields and output table with length 6.

P#67435 2019-09-09 16:18 ( Edited 2019-09-09 16:49)

1

You need to add a copy of a to output, not a itself.

P#67448 2019-09-09 21:20

Wow thanks Saffith, I KNEW it was some kind of pass by reference issue.

But my initial attempt to implement a table copy method didn't work and I fell down a rabbit hole of incorrect assumptions about my code....

EDIT for posterities sake here's a working change:

output = {}

function copy_t(tbl)
    local t = {}
    for key, value in pairs(tbl) do
      t[key] = value
    end
    return t
end

function permutation(a, n)
    if n == 0 then

         add(output,copy_t(a))
    else
        for i = 1, n do
            a[i], a[n] = a[n], a[i]                     
            permutation(a, n - 1)
            a[i], a[n] = a[n], a[i]
        end
    end
end
cls()
permutation({"d","z","2"},3,1)

for i=1,#output do
    for j=1,#output[i] do
        print(output[i][j],j*8,i*10)
    end 
end
P#67449 2019-09-09 21:29 ( Edited 2019-09-09 21:33)

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