are you allowed to have a numeric key in a lua table?

Question

The following example is supposed to make a table, that can convert between a number and a string and back again, but fails to run.

Is it because I'm using a numeric key in a dictionary type way? Or is it because lua starts table indices from 1?

Is there a better way to accomplish this?

dyeColor = {
    0="black"    ,  black     = 0,
    1="red"      ,  red       = 1, 
    2="green"    ,  green     = 2,
    3="brown"    ,  brown     = 3,
    4="blue"     ,  blue      = 4,
    5="purple"   ,  purple    = 5,
    6="cyan"     ,  cyan      = 6,
    7="lightGray",  lightGray = 7,
    8="gray"     ,  gray      = 8,
    9="pink"     ,  pink      = 9,
    10="lime"     ,  lime      =10,
    11="yellow"   ,  yellow    =11,
    12="lightBlue",  lightBlue =12,
    13="magenta"  ,  magenta   =13,
    14="orange"   ,  orange    =14,
    15="white"    ,  white     =15}

using this online interpreter (http://repl.it/languages/Lua) it gives the error

[string "stdin"]:2: '}' expected (to close '{' at line 1) near '='attempt to call a nil value

offset one of the downvotes of the question because the OP has engaged and did show some effort. Can't offset the other myself, but might not even if I could because the answer *does* really amount to read and understand the basic syntax of the language itself. — RBerteig, Apr 29 '13 at 22:39

Doug Currie · Accepted Answer · 2013-04-29T21:37:19.177

You need to put the numeric indices in brackets:

dyeColor = {
    [0]="black"     ,  black     = 0,
    [1]="red"       ,  red       = 1, 
    [2]="green"     ,  green     = 2,
    [3]="brown"     ,  brown     = 3,
    [4]="blue"      ,  blue      = 4,
    [5]="purple"    ,  purple    = 5,
    [6]="cyan"      ,  cyan      = 6,
    [7]="lightGray" ,  lightGray = 7,
    [8]="gray"      ,  gray      = 8,
    [9]="pink"      ,  pink      = 9,
    [10]="lime"     ,  lime      =10,
    [11]="yellow"   ,  yellow    =11,
    [12]="lightBlue",  lightBlue =12,
    [13]="magenta"  ,  magenta   =13,
    [14]="orange"   ,  orange    =14,
    [15]="white"    ,  white     =15}

You can save yourself some typing with:

dyeColor = {
    [0]="black"     ,
    [1]="red"       ,
    [2]="green"     ,
    [3]="brown"     ,
    [4]="blue"      ,
    [5]="purple"    ,
    [6]="cyan"      ,
    [7]="lightGray" ,
    [8]="gray"      ,
    [9]="pink"      ,
    [10]="lime"     ,
    [11]="yellow"   ,
    [12]="lightBlue",
    [13]="magenta"  ,
    [14]="orange"   ,
    [15]="white"    }

for i = 0, #dyeColor do dyeColor[dyeColor[i]] = i end

Lua permits Names as fieldspecs in the form Name = exp but not numbers. Numbers must be in brackets. The same is true of field references. You may say

dyeColor.black

but not

dyeColor.0 -- you may say dyeColor[0] of course

I'll accept your answer, if you can offer an explanation as to how your answer is different to the code given in the question (from the interpreters point of view) — Ryan Leach, Apr 29 '13 at 21:27
You can even save some more typing by not defining index except for `[0]`. — hjpotter92, Apr 29 '13 at 22:49
Yes; in this case I view the numbers as documentation of the fact that the mapping is significant, i.e., this is not just an array, so it's nice to see the association. This is a matter of taste, of course. — Doug Currie, Apr 30 '13 at 16:53

score 0 · Answer 2 · answered Apr 30 '13 at 09:19

0

#dyeColor is undefined if it contain [0] so you can

dyeColor = {} do
  local tmp= {
    "black"     ,
    "red"       ,
    "green"     ,
    "brown"     ,
    "blue"      ,
    "purple"    ,
    "cyan"      ,
    "lightGray" ,
    "gray"      ,
    "pink"      ,
    "lime"      ,
    "yellow"    ,
    "lightBlue" ,
    "magenta"   ,
    "orange"    ,
    "white"    
  }
  for i, v in ipairs(tmp) do
    local j = i - 1
    dyeColor[v], dyeColor[j] = j, v
  end
end

answered Apr 30 '13 at 09:19

moteus

2,187
1
13
16

Is length undefined for tables with an item [0]? My reading of both the 5.1 and 5.2 definition leads me to conclude that it is defined, e.g., 5.2: the set of its positive numeric keys is equal to {1..n} for some integer n. I guess it depends on whether you consider 0 positive in this context. In 5.1, it is certainly defined: any integer index n such that t[n] is not nil and t[n+1] is nil -- there is only one such value even when [0] is used. – Doug Currie Apr 30 '13 at 17:01
I'm guessing in this solution that the mapping starts at 1? if so, this answer is useless, as the 0 was needed to represent data that I don't have control over. – Ryan Leach May 01 '13 at 05:34
Oh. Really. #dyeColor is defined if it contain [0]. – moteus May 07 '13 at 11:44

Ray Osgod · Answer 3 · 2013-04-30T13:39:39.120

Improving Doug Curie's solution:

local dyeColor = {
    [0] = "black", -- You must set 0 explicitly
    "red"        , -- Unnamed items get their numeric index 
    "green"      , -- automatically, starting at 1.
    "brown"      ,
    "blue"       ,
    "purple"     ,
    "cyan"       ,
    "lightGray"  ,
    "gray"       ,
    "pink"       ,
    "lime"       ,
    "yellow"     ,
    "lightBlue"  ,
    "magenta"    ,
    "orange"     ,
    "white"
}
for i = 0, #dyeColor do dyeColor[dyeColor[i]] = i end

Even though the length of a table with [0] is undefined in Lua 5.2, it actually returns the index of the last element. The code above also works in Lua 5.1 and LuaJIT.

are you allowed to have a numeric key in a lua table?

3 Answers3

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