## 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]);
V0.resize(Num);
}
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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