Lua - Create a nested table using for loop

Question

I'm a very new to lua so am happy to read material if it will help with tables.

I've decoded a json object and would like to build a table properly using its data, rather than writing 64 lines of the below:

a = {}
  a[decode.var1[1].aId] = {decode.var2[1].bId, decode.var3[1].cId}
  a[decode.var1[2].aId] = {decode.var2[2].bId, decode.var3[2].cId}
  a[decode.var1[3].aId] = {decode.var2[3].bId, decode.var3[3].cId}
  ...etc

Because the numbers are consecutive 1-64, i presume i should be able to build it using a for loop. Unfortunately despite going through table building ideas I cannot seem to find a way to do it, or find anything on creating nested tables using a loop.

Any help or direction would be appreciated.

Personage · Accepted Answer · 2020-02-24 04:51:48Z

1

Lua for-loops are, at least in my opinion, pretty easy to understand:

for i = 1, 10 do
   print(i)
end

This loop inclusively prints the positive integers 1 through 10.

Lua for-loops also take an optional third argument--which defaults to 1--that indicates the step of the loop:

for i = 1, 10, 2 do
   print(i)
end

This loop prints the numbers 1 through 10 but skips every other number, that is, it has a step of 2; therefore, it will print 1 3 5 7 9.

In the case of your example, if I understand it correctly, it seems that you know the minimum and maximum bounds of your for loops, which are 1 and 64, respectively. You could write a loop to decode the values and put them in a table like so:

local a = {}

for i = 1, 64 do
    a[decodevar.var1[i].aId] = {decode.var2[i].bId, decode.var3[i].cId}
end

answered Feb 24, 2020 at 4:51

Personage

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1 Comment

Rob Stevens Over a year ago

perfect - thanks very much. it looks simple when you put it down. i did try this method but it failed - then i realised that the is possible for there to be less than 64 entires so i need to find a way to count through the JSON. But thank you!

Seniru Pasan · Accepted Answer · 2020-02-25 11:13:21Z

0

What you can do is generating a new table with all the contents from the decoded JSON with a for loop.

For example,

function jsonParse(jsonObj)
    local tbl = {}
    for i = 1, 64 do
        a[decodevar.var1[i].aId] = {decode.var2[i].bId, decode.var3[i].cId}
    end
    return tbl
end

To deal with nested cases, you can recursively call that method as follows

function jsonParse(jsonObj)
    local tbl = {}
    for i = 1, 64 do
        a[decodevar.var1[i].aId] = {decode.var2[i].bId, decode.var3[i].cId}
        if type(decode.var2[i].bId) == "table" then
             a[decodevar.var1[i].aid[0] = jsonParse(decode.var2[i].bId)
        end
    end
end

By the way, I can't understand why are you trying to create a table using a table that have done the job you want already. I assume they are just random and you may have to edit the code with the structure of the decodevar variable you have

answered Feb 25, 2020 at 11:13

Seniru Pasan

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1 Comment

Rob Stevens Over a year ago

Cheers!! the data changes and i need to keep a record of it. so the function to create the tables run several times.

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