How to solve "Undeclared identifier error" in "if" block?

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I am creating vector in building mex file. I need to creat a vector<vector<int>> which size is according to the input variable. The creating or declaring part is in a if block. I need to use this variable outside the if. Then it has an error "undeclared identifier". I found I should pre-define the variable. But I don't know the size in this way. Do I need to define a global variable? Or any other suggestion? My code is below.

void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const mxArray* prhs[])
int i;
if ( i == 0) // if block
        mwSize Num = mxGetNumberOfElements(prhs[i]);
        vector<vector<int>> V0(Num);
cout << V0.size()<<"\n"; // error

This is a simplified example code. I really need the if block. I think the problem is from the variable domain. But I have not figure out a good way to solve it.

You intuition is correct. The lifetime of V0 ends at the closing curly bracket of the enclosing if. The object is destroyed and the name isn't visible in the outer scope.

You could solve the problem by moving it outside the if.

void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const mxArray* prhs[])
    vector<vector<int>> V0;
    int i; // hey, this is uninitialized!
    if (i == 0) // if block
        mwSize Num = mxGetNumberOfElements(prhs[i]);
    cout << V0.size() << "\n"; // the vector is visible here

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The vector V0 is declared in your if block, and it only exists within that scope. You are accessing it outside of that scope.

Also, you left the parens off the call to size().

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vector< vector<int>> V0(Num); This is declared inside the if, you are trying to use it outside of if. That's the problem.

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