SVM in openCV throws "cv::Exception at memory location" - c++

Question

I'm trying to create a simple OCR application with SVM, openCV, C++ and Visual Studio 2008 (mfc app).
My training samples are binary images of machine-printed digits (0-9). I want to use DAGSVM for this multi-class problem. So I need to create 45 SVMs, each of which is the SVM of 2 class (SVM(0,1), SVM(0,2)... SVM(8,9)).
Here's how things are going:
SVM's parameters:
CvSVMParams params;
params.svm_type = CvSVM::C_SVC;
params.kernel_type = CvSVM::LINEAR;
params.term_crit = cvTermCriteria(CV_TERMCRIT_ITER, 100, 1e-6);
Data of training images of class i are stored in matrix trainData[i] (each row is the pixels of a 28x28 image, which means the matrix has 784 cols).
When training each SVM, I create 2 matrix called curTrainData & curTrainLabel.
for (int i = 0; i < 9; i++)
for (int j = i+1; j < 10; j++)
{
curTrainData.create(trainData[i].rows + trainData[j].rows, 784, CV_32FC1);
curTrainLabel.create(curTrainData.rows, 1, CV_32FC1);
// merge 2 matrix: trainData[i] & trainData[j]
for (int k = 0; k < trainData[i].rows; k++)
{
curTrainLabel.at<float>(k, 0) = 1.0; // class of digit i
for (int l = 0; l < 784; l++)
curTrainData.at<float>(k,l) = trainData[i].at<float>(k,l);
}
for (int k = 0; k < trainData[j].rows; k++)
{
curTrainLabel.at<float>(k + trainData[i].rows, 0) = -1.0; // class of digit j
for (int l = 0; l < 784; l++)
curTrainData.at<float>(k + trainData[i].rows,l) = trainData[j].at<float>(k,l);
}
svms[i][j].train(curTrainData, curTrainLabel, Mat(), Mat(), params);
}
I got error at the call svms[i][j].train.... The full error is:
Unhandled exception at 0x75b5d36f in svm.exe: Microsoft C++ exception: cv::Exception at memory location 0x0022af8c..
To tell the truth I don't fully understand SVM implemented in openCV and I can't find any example of them working with objects in images.
I'm really grateful if someone can tell me what is (are) wrong :(
Update 09/03:
I had mistaken. The error comes from:
str.Format(_T("Results\trained_%d_%d.xml"), i, j);
svms[i][j].save(CT2A(str));
str is a CString variable.
It remains even if I change to:
svms[i][j].save("Results\trained.xml");
I've created the folder Results and others files are written well into it (files for methods fopen(), imwrite()...). I don't know why I can't add the folder when it comes to this save method of svm.

Answer 1

If you use backslash "\", you have to put "\\" instead (or you can use a frontslash "/").