small table with index

[Jay Ess 6]

I want to build an array with data as in local tb ={1=a,2=b,3=c} and so on.

 

one question,

1 - I notice that i cant add a number as the index as above? unless i am doing something wrong?

 

[jgab 2,642]

54 minutes ago, Jay Ess said:

I want to build an array with data as in local tb ={1=a,2=b,3=c} and so on.

 

one question,

1 - I notice that i cant add a number as the index as above? unless i am doing something wrong?

 

 

tb ={[1]=a,[2]=b,[3]=c}

 

The reason is  that number indexes are computed, and then you  need to put  it in [...] that computes the index.

tb = {["prefix"..index] =  42} is another example of a computed prefix.

 

tb = {["foo"] =  42} is the same as tb={foo=42}

However, using the string directly as  a key as in the last  example only works if the string is a valid key name (similar  to variable name), so no  spaces or language specific letters.

 

 

[Jay Ess 6]

i notice is i index them as [1] etc then if i jump a number it does not add it to the table, correct? how can i get around that?

[jgab 2,642]

18 minutes ago, Jay Ess said:

i notice is i index them as [1] etc then if i jump a number it does not add it to the table, correct? how can i get around that?

I  don't understand -  can you give  example?

[Jay Ess 6]

local tb ={[1]=a,[2]=b,[3]=c,[23]=d}

when i do a print of k,v it only shows me 1-3 and not 23

i assume that is because the index has to be in order? so how would i get around that? other then keeping them in order?

[jgab 2,642]

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use pairs

[Jay Ess 6]

thank you @jgab

i have 56 values in my array, how do i fetch back value 23 please?

Edited January 24, 2022 by Jay Ess

[jgab 2,642]

a=tb[23]
or

print(tb[23])

[Jay Ess 6]

thank you and to get the index if i know the value?

[petergebruers 1,268]

6 hours ago, Jay Ess said:

i assume that is because the index has to be in order? so how would i get around that? other then keeping them in order?

If you want order, then it is "ipairs" but what you say is correct, but not 100% accurate:

 

Lua 5.3 spec for ipairs (t)

 

Returns three values (an iterator function, the table t, and 0) so that the construction

     for i,v in ipairs(t) do body end
will iterate over the key–value pairs (1,t[1]), (2,t[2]), ..., up to the first nil value.

 

So if it wants item n but n is not a key ... iteration stops.

 

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If your keys are integers, but have "holes", or your keys are any other data type eg a date, you need 2 tables. One is holding the data. The other is a sorted table of the keys. You then do "ipairs" over the sorted table to retrieve the data.

[jgab 2,642]

22 minutes ago, Jay Ess said:

thank you and to get the index if i know the value?

You have to search. Be aware they keys have to be unique - but values don't so you could have

tb[2]=88

tb[77]=88

if you looked for the 88 value, should you return 2 or 77?

If you have unique values too it's a common model to keep a "reverse lookup table" that maps value to keys.

 

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[Jay Ess 6]

your code gives me lua:241: attempt to index a nil value (global 't') copied and pasted exatly as you wrote it.

 

 

also your function is within another function is that why i am getting the error?

[jgab 2,642]

Sorry, the example assigns variable 'a' and then uses variable 't'

Change 

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to

 

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[Jay Ess 6]

still not happy - lua:241: attempt to index a nil value (global 't')

 

function create2wayTable()

   local r, tab,rev = {},{},{}

   function r.add(key, value) tab[key],rev[value]=value,key end

   function r.remove(key) local v = tab[key]; tab[key]=nil; if v~=nil then rev[v]=nil end end

   function r.lookupValue(key) return tab[key] end

   function r.lookupKey(value) return rev[value] end

   return res

end

 

t = create2wayTable()

t.add(1,"First")

print(t.lookupValue("First"))

print(t.lookupKey(1))

 

 

if i undertsnad your table correctly, this is what it would look like manually

tb={{index},{key},{value}}

 

Edited January 24, 2022 by Jay Ess

[jgab 2,642]

Sorry again,

It should return r at the end of the function

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[Jay Ess 6]

@jgab as usual you are amazing and a HUGE help thank you

[Jay Ess 6]

Does it make a difference for run speed if I have a static table and populate the table instead of run add the lines to the table?

 

Using your example how would I write this table without the add functions?

[jgab 2,642]

The add function is to make sure that both tables are updated when adding and element.

There is no relevant change in speed - we are talking about microseconds.

However, we can expose the tables so that we can loop over them

 

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Edited January 26, 2022 by jgab

[Jay Ess 6]

if i put the funtion create2waytable in a another sheet in the qa, why does it give me an error?

[jgab 2,642]

What error do you get?

[Jay Ess 6]

when i use this code for k,k in pairs(t.tab) do print(k,v) end

 

and my add is like this

local pTb = create2wayTable()

    local pIx = 0

    pTb.add(pIx,"Bereshit")

    pIx = pIx + 1

    pTb.add(pIx,"Noach")

 

it only prints me out the value but not the key?

 

re the error i have 5 sheets in the qa in one of the sheets i get this error

attempt to call a nil value (global 'create2wayTable')

 

however on another sheet it does work!

Edited January 26, 2022 by Jay Ess