計算給我NaN

Question

我試圖使用梯度下降實現多項邏輯迴歸，但是我的成本函數開始爲權重指定NaN值。有人可以告訴我做錯了什麼嗎？

function [ cost ] = costFunctionMultiNominal(inputX,resultY,weights) 
%UNTITLED8 Calculates the cost for gradient descent,assumes inputX has one 
%additional feature for constant and Weights is a classes X features matrix 

[rows,cols] = size(inputX); 
numOfClasses = size(weights,1); 
summation = 0; 
for i=1:rows 
    classLevelSummation = 0; 
    for j=1:numOfClasses 
     if resultY(i)==j 
      denominatorSum = 0; 
      for l=1:numOfClasses 
       denominatorSum = denominatorSum + exp((inputX(i,:)*weights(l,:)')-4444); 
      end 
      **classLevelSummation = classLevelSummation + log(exp(inputX(i,:)*weights(j,:)'-4444)/denominatorSum);** 
     end 
    end 
    summation = summation + classLevelSummation; 
end 
cost = summation/(-rows); 
end 

這裏是權重更新功能：

function [ Weights ] = 
    getWeightsUsingGradientDescentMultiNominal(trainingX,resultY,iterMax,Alpha,weight0,lambda) 

%Returns updated weights through gradient descent,weight0 are the intial randomized weights 
% Detailed explanation goes here 

rows = size(trainingX,1); 
cols = size(trainingX,2)+1; 
Weights = weight0; 
numOfClasses = size(Weights,1); 
%Adding one's to the input data for the constant terms 
a = ones(rows,1); 
X = [a trainingX]; 
%Each column corresponds to one weight, updating weights column wise: 
%Also plot cst function simultaneously 
tempCost = 0; 
display(costFunctionMultiNominal(X,resultY,Weights)); 
plot(1,costFunctionMultiNominal(X,resultY,Weights),'r'); 
hold on; 
for n=1:iterMax 
    %Have to do this for all classes, i.e rows in weigths 
    for j = 1:numOfClasses 
     %First Calculating the Sigma over rows for all X 
     summation = zeros(1,cols); 
     for i=1:rows 
      p = -1 * calculatePofJMultiNominal(X(i,:),Weights,j); 
      if resultY(i) == j 
       p = 1 + p; 
      end 
      summation = summation + X(i,:)*p; 

     end 
     Weights(j,:) = Weights(j,:) - (Alpha)*(summation/(-rows) + lambda*Weights(j,:)); 
    end 
    cost = costFunctionMultiNominal(X,resultY,Weights); 
    display(cost); 
    costDiff = tempCost - cost; 
    if i~=0 && abs(costDiff)/cost <= 0.0001 
     display('Breaking because of cost very less!'); 
     break; 
    end 
    tempCost = cost; 

    plot(i,cost,'r'); 
end 
hold off; 
end 

據我瞭解，在NaN是因爲在指數方面的大量到來。我嘗試從指數（-4444）減少大量數據，但無濟於事。

我試圖DBSTOP如果楠，告訴我它停在成本函數的線（在上面的代碼中粗體）：

classLevelSummation = classLevelSummation + log(exp(inputX(i,:)*weights(j,:)'-4444)/denominatorSum); 

classLevelSummation變成NaN，即使刪除了大量的恆定值-4444

Answer 1

log(exp(blah)/denominator)沒有必要取冪然後取日誌 - 這兩個操作相互撤銷，可能是因爲調用超出了浮點範圍。如果您還記得log(A/B)等於log(A) - log(B)，您可以在不使用exp的情況下重新編寫此代碼。

但是，exp中的一個簡單溢出可能會給你一個Inf而不是NaN。您應該檢查denominatorSum的值，因爲它也來自指數項。