Overwriting mat file variable values

[Saif]

Dear all,

i've just started using this library. I'm working on a problem, where i need to update values of a variable in an existing mat file. I tried the following approach to mke it work, may be anyone knows a better way to do it:

The approach i took involves the following step:

1. Open input.mat file
   matoutput = Mat_Open(" input.mat", MAT_ACC_RDWR);

2. Read input.mat file
   matoutvar1 = Mat_VarReadNextInfo(matoutput);
   matoutvar_val = Mat_VarRead(matoutput, matoutvar1->name);

3. Store the variables and other attributes of the input.mat file in a temporary variable, e.g., matvar_t * variable_temp

variable_temp.name = matoutvar_val->name;
variable_temp.class_type = matoutvar_val->class_type;
variable_temp.data_type = matoutvar_val->data_type;
variable_temp.rank = matoutvar_val->rank;
variable_temp.dims = matoutvar_val->dims;

1. Close input.mat file
   Mat_Close(matoutput);

2. Open input.mat file
   matoutput = Mat_Open("input.mat", MAT_ACC_RDWR);

3. Delete the enteries of input.mat file one by one by using the variable names from the variable_temp
   matoutvar1 = Mat_VarReadNextInfo(matoutput);
   Mat_VarDelete(matoutput, matoutvar1->name)

4. Close input.mat file, at this point, input.mat contains nothing.
   Mat_Close(matoutput);

5. Get the new variable values that needed to be assigned to the variable names in variable_temp, e..g,
   double out2[1];
   out2[0] = 5.0;

6. Open input.mat
   matoutput = Mat_Open("input.mat", MAT_ACC_RDWR);

7. Using Mat_VarCreate and Mat_VarWrite save new values corresponding to the variable_temp.name
   matvar1 = Mat_VarCreate(variable_temp.name, variable_temp.class_type, variable_temp.data_type, variable_temp.rank, variable_temp.dims, &out2[0], 0);
   Mat_VarWrite(matoutput, matvar1, MAT_COMPRESSION_NONE);
   Mat_VarFree(matvar1);

8. Close input.mat file
   Mat_Close(matoutput);

Though, the above method works and gives the expected results, however, is there any other method or function in the matio library where i can directly change the variable values for an existing mat file

Any help would be highly appreciated.

PS: I'm not appending new variable or chaning the structure of elements or data types. To be specific, I need to only update the numerical value of a variable
a = 5 in input.mat file to be given a new value, e.g.,
a = 10

[tbeu]

Indeed, this is far too complicated: This is what I propose:

1. mat = Mat_Open(fileName, MAT_ACC_RDWR);
2. matvar = Mat_VarRead(mat, varName);
3. check matvar->data_type and matvar->class_type if it meets your expectations
4. cast matvar->data to the type you expect, e.g., double pointer
5. manipulate matvar->data as you like, but make sure to respect dimensions matvar->dims and to not leak memory
6. Mat_VarDelete(mat, varName); // delete the old variable from file
7. Mat_VarWrite(mat, matvar, MAT_COMPRESSION_NONE); // write the manipulated var to file
8. Mat_VarFree(matvar);
9. Mat_Close(mat);

[Saif]

Thank you for your answer, but it's not working for me and can't figure it out where is the problem :(

int main(int argc, char* argv[]) {

    double out2[3];
    out2[0] = 500.0;
    out2[1] = 100.0;
    out2[2] = 100.0;
    mat_t *mat;
    matvar_t *matoutvar1;
    matvar_t *matvar_val;

    mat = Mat_Open("output.mat", MAT_ACC_RDWR);

    size_t dims[1];
    Mat_GetDir(mat, dims);
    cout << dims[0] << endl;    
    string *variable_temp = new string[dims[0]];
    const char* p_c_str;
    int j = 0;
    // Here I'm reading the variable names in the matoutput file
    // and assinging them to a string array
    while ((matoutvar1 = Mat_VarReadNextInfo(mat)) != NULL) {   
        variable_temp[j] = matoutvar1->name;
        cout << "variable_temp[j]: " << variable_temp[j] << " count j: " << j << endl;      
        j = j + 1;      
    }
    Mat_Close(mat);

        // Here I'm overwriting the values of the variable name

    for (int i = 0; i < dims[0]; i++) {
        mat = Mat_Open(outputFile.c_str(), MAT_ACC_RDWR);
        p_c_str = variable_temp[i].c_str();
        cout << "p_c_str " << p_c_str << endl;
        // Reads the variable with the given name from a MAT file
        matvar_val = Mat_VarRead(mat, p_c_str);
        cout << "matvar_val->data" << matvar_val->data << endl;
        double *newData = static_cast<double*>(matvar_val->data);
        cout << "matvar_val->data" << newData << endl;
        newData = &out2[i];

        Mat_VarDelete(mat, matvar_val->name);
        Mat_VarWrite(mat, matvar_val, MAT_COMPRESSION_NONE);
        Mat_VarFree(matvar_val);
        Mat_Close(mat);
    }

    getchar();

    return 0;
}

[tbeu]

It works if you get the assignment right:

newData[0] = out2[i];

If you want to assign the data at once, provided dimensions do not change, you need to free the old data and set the mem_conserve flag for the new data:

free(matvar_val->data);
matvar_val->data = &out2[i];
matvar_val->mem_conserve = 1;

[tbeu]

And, there is no reason to open and close the MAT file repeatedly in the loop. It is enough to open once and close once.

[Saif]

Thanks for your quick reply. I tried both approaches and i can get the results which i need :)