Attempt to index a nil value

Question

I'm writring a lua script to realize Decision Tree.But it always throws exception in a function when creating the tree.I can't find where it is wrong even if "assert".And sorry for my poor English:P.

DTree.lua:

-- Decision Tree (ID3)
-- by Darksun2010

function calcShannonEnt(dataSet)
    local labelScore={};
    for _,v in pairs(dataSet) do
        local curLabel=v[#v];
        if (labelScore[curLabel]) then
            labelScore[curLabel]=labelScore[curLabel]+1;
        else
            labelScore[curLabel]=1;
        end
    end
    local result=0.0;
    for _,v in pairs(labelScore) do
        local prob=v/#dataSet;
        result=result-prob*math.log(prob,2);
    end
    return result;
end

function splitDataSet(dataSet,axis,value)
    local reDataSet={};
    for _,v in pairs(dataSet) do 
        if v[axis]==value then
            local reSet=table.pack(table.unpack(v,1,axis-1),
                                   table.unpack(v,axis+1,#v));
            if (reSet[1]==nil) then
                table.remove(reSet,1);
            end
            if (reSet[#reSet]==nil) then
                table.remove(reSet);
            end
            table.insert(reDataSet,reSet);
        end
    end
    return reDataSet;
end

function chooseBest(dataSet)
    local numFeaures=#dataSet[1]-1;
    local baseEnt=calcShannonEnt(dataSet);
    local maxGain,bestFeature=0,-1;
    for i=1,numFeaures do
        local featSet={};
        local newGain=0;
        for _,v in pairs(dataSet) do
            featSet[v[i]]=1;
        end
        for k,_ in pairs(featSet) do
            local subDataSet=splitDataSet(dataSet,i,k);
            local prob=#subDataSet/#dataSet;
            newGain=newGain+prob*calcShannonEnt(subDataSet);
        end
        local infoGain=baseEnt-newGain;
        if (infoGain>maxGain) then
            maxGain=infoGain;
            bestFeature=i;
        end
    end
    return bestFeature;
end

function majorityCnt(classList)
    local classCount={};
    for _,v in pairs(classList) do
        if (classCount[v]) then
            classCount[v]=classCount[v]+1;
        else
            classCount[v]=1;
        end
    end
    local maxN,maxK=0,nil;
    for k,v in pairs(classCount) do
        if (v>maxN) then
            maxN,maxK=v,k;
        end
    end
    return maxK;
end

function makeDTree(dataSet,labels)
    local classList={};
    for _,v in pairs(dataSet) do
        table.insert(classList,v[#v]);
    end
    local endFlag=true;
    for _v in pairs(classList) do
        if (not endFlag) then
            break;
        end
        endFlag=endFlag and (v==classList[1]);
    end
    if (#dataSet[1]==1) then
        return majorityCnt(classList);
    end
    local bestFeat=chooseBest(dataSet);
    local bestFeatLabel=labels[bestFeat];
    local myTree,featSet={},{};
    for _,v in pairs(dataSet) do 
        featSet[v[bestFeat]]=1;
    end
    for k,_ in pairs(featSet) do
        local subLabels=labels;
        local subTree=makeDTree(splitDataSet(dataSet,bestFeat,k),subLabels);
        myTree[bestFeatLabel][k]=subTree;
    end
    return myTree;
end

I call it as this:

>labels={'no surfacing','flippers'}

>dataSet={{1,1,'yes'},{1,1,'yes'},{1,0,'no'},{0,1,'no'},{0,1,'no'}}

>makeDTree(dataSet,labels)

DTree.lua:106: attempt to index a nil value (field '?')

stack traceback:

DTree.lua:106: in function 'makeDTree'

DTree.lua:105: in function 'makeDTree'

(...tail calls...)

[C]: in ?

Answer 1

Line 106 is myTree[bestFeatLabel][k]=subTree; but myTree is empty and so myTree[bestFeatLabel] is nil.