List all `i` for which `data[i_Integer]` is defined

Question

I am reading a file with Get[] that contains a semi-colon separated sequence of sub-scripted definitions like this:

data[1] = {stuff};
data[5] = {otherStuff};
data[99] = {yetMoreStuff};

What is the cleanest way to programatically decide for what values of i is data[i] defined? A list of the indices would be nice, e.g. {1, 5, 99}.

A hacky way would be to loop through the range of possible values to see which ones don't have head "data" (e.g. Select[data/@Range[1,1000],(Not[MatchQ[#,_data]])?]), but this is unattractive since it isn't general (e.g. it won't find data[dog] = "Max"; if we remove the integer subscript requirement) and assumes that one can choose an upper bound. It would also be slow and waste memory.

Answer 1

I'm still not at my Mathematica machine but it occurs to me that DownValues[data] will probably return a list of rules that you might be happy to manipulate to get out the is. Perhaps something like this;

Cases[DownValues[data],RuleDelayed[HoldPattern[data[i_Integer]],rhs_]:>i]

Answer 2

data[1] = {stuff};
data[5] = {otherStuff};
data[99] = {yetMoreStuff};

Cases[
 DownValues @ data,
 _[_@_@x_Integer, _] :> x
]

{1, 5, 99}