[英]Reducing dimensionality on training data with PCA in Matlab
這是一個后續問題:
為了對新的10維測試數據進行分類,我是否還必須將訓練數據減少到10維?
我試過了:
X = bsxfun(@minus, trainingData, mean(trainingData,1));
covariancex = (X'*X)./(size(X,1)-1);
[V D] = eigs(covariancex, 10); % reduce to 10 dimension
Xtrain = bsxfun(@minus, trainingData, mean(trainingData,1));
pcatrain = Xtest*V;
但是使用帶有此分類器和10維測試數據會產生非常不可靠的結果嗎? 我做的事情根本就是錯誤嗎?
編輯:
X = bsxfun(@minus, trainingData, mean(trainingData,1));
covariancex = (X'*X)./(size(X,1)-1);
[V D] = eigs(covariancex, 10); % reduce to 10 dimension
Xtrain = bsxfun(@minus, trainingData, mean(trainingData,1));
pcatrain = Xtest*V;
X = bsxfun(@minus, pcatrain, mean(pcatrain,1));
covariancex = (X'*X)./(size(X,1)-1);
[V D] = eigs(covariancex, 10); % reduce to 10 dimension
Xtest = bsxfun(@minus, test, mean(pcatrain,1));
pcatest = Xtest*V;
您必須減少訓練和測試數據,但兩者都以相同的方式。 因此,一旦從訓練數據中獲得PCA的縮減矩陣,就必須使用此矩陣來降低測試數據的維數。 簡而言之,您需要一個適用於訓練和測試元素的恆定變換。
使用你的代碼
% first, 0-mean data
Xtrain = bsxfun(@minus, Xtrain, mean(Xtrain,1));
Xtest = bsxfun(@minus, Xtest, mean(Xtrain,1));
% Compute PCA
covariancex = (Xtrain'*Xtrain)./(size(Xtrain,1)-1);
[V D] = eigs(covariancex, 10); % reduce to 10 dimension
pcatrain = Xtrain*V;
% here you should train your classifier on pcatrain and ytrain (correct labels)
pcatest = Xtest*V;
% here you can test your classifier on pcatest using ytest (compare with correct labels)
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