[R] SVM - calculating values problem

[PELE Benoît (Acoss)]

Finally, I did not find the detail of the “R_svmpredict” function that may be a C program but applying the function named “svmpred” in the svminternals.pdf document I solved my problem.

Thank you and best regards, Benoit.

De : Sarah Goslee <sarah.goslee using gmail.com>
Envoyé : mercredi 28 août 2019 12:50
À : PELE Benoît (Acoss) <benoit.pele using acoss.fr>
Cc : R-help using r-project.org
Objet : Re: [R] SVM - calculating values problem

You could download the package code from CRAN and look yourself at what the predict function is doing.

Sarah

On Wed, Aug 28, 2019 at 6:35 AM PELE Benoît (Acoss) <benoit.pele using acoss.fr<mailto:benoit.pele using acoss.fr>> wrote:
Hello everybody,

That is the first time that I am working on a SVM modeling and I would like to calculate by myself the result values from the SVM for each line of my database (named x_appr_svm).

First I tested a linear SVM model using the e1071 package and to calculate the individual results by myself I did the next things :
Retrieving the model coefficients  : coef_svm<-t(svm$coefs) %*% x_appr_svm[svm$index,]
Calculating the values for each line : p2<-x_appr_svm %*% t(coef_svm) - svm$rho
Using the predict function to compare : p1<-attr(predict(object=svm, newdata=x_appr_svm, decision.values=T), "decision.values")
--> p1 and p2 are the same.

Next I tested a polynomial SVM model using the same package and the same method knowing that the model parameters are :
degree=2,  gamma=0.02, coef0=0.01
The calculation of the individual values becomes (I guess) : p2<-(0.02*x_appr_svm %*% t(coef_svm)+0.01)^2-svm$rho
--> p1 and p2 are really different!

Despite of my searching, I do not understand why or where is the problem in my second p2 formula. Do you see the mistake?

Thank you for your help and have a good day, Benoit (France).

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