ROC curve with Matlab using svmtrain

Hello I am working with a data set containing x_values which I have called SVMdata(a matrix of 17*41) and target values which are the labels for the classification of these data('a' for the first group and 'b'for the second group). I would like to obtain the ROC curve for my data. I have used the following code:

x=SVMdata';
group=SVMdataS1;
groups = ismember(group,'a');
% divide the original data to training and test data set
Q = size(x,1);
>> Q1 = floor(Q*0.80);
>> Q2 = Q-Q1;
ind = randperm(Q);
ind1 = ind(1:Q1);
ind2 = ind(Q1+(1:Q2));
>> x1 = x(ind1,:);
>> t1 = groups(ind1,:);
>> x2 = x(ind2,:);
>> t2 = groups(ind2,:);
options=optimset('maxiter',1000);  
%train with the training set
>> svm = svmtrain(x1, t1, ...
                 'Autoscale',true, 'Showplot',false, 'Method', 'QP', ...
                 'Kernel_Function', 'polynomial', 'polyorder',1,'quadprog_opts',options);
shift = svm.ScaleData.shift;
scale = svm.ScaleData.scaleFactor;
x2 = bsxfun(@plus,x2,shift);
x2 = bsxfun(@times,x2,scale);
sv = svm.SupportVectors;
alphaHat = svm.Alpha;
bias = svm.Bias;
kfun = svm.KernelFunction;
kfunargs = svm.KernelFunctionArgs;
f = kfun(sv,x2,kfunargs{:})'*alphaHat(:) + bias;
f = -f;
[X,Y,T,AUC] = perfcurve(t2,f,1);

When I run this program, I get the following error: Error using perfcurve>membership (line 633) Positive class is not found in the input data.

Error in perfcurve (line 387)
[W,subYnames] = membership(labels(sorted),weights(sorted),...

Your help is greatly appreciated. best

Answer 1

[Ilya]

The error says that 1 is not found in t2. Note that your groups variable is logical, and so are t1 and t2, but 1 is double. Try replacing 1 with true or replacing t2 with double(t2).

[Mohamad]

Dear llya Thank you very much. You have been too helpful. best

[Thi Huong Hoa Trinh]

Thanks so much. You have help me a lot. I also have the same error