双数组，大小作为用户输入

Question

I want to do something like this

int n,m; //or cin>>n>>m;

a[n][m];

//then do whatever with the array

The problem is that Visual Studio gives me errors, while dev c++ doesn't. I want to compile it in VS.

Answer 1

Even if your compiler supports VLA(Variable Length Arrays), you didn't declared a properly:

int a[n][m];
^^^

You should use std::vector which is a standard way

std::vector<std::vector<int> > a(n, std::vector<int>(m));

Answer 2

That depends on the compiler...
Use of std::vector is always recommended for such need.
But if you have to , then you can allocate that memory on the heap like this...

using new (recommended in C++)...

cout << "Enter n & m:";
int n, m;
cin >> n >> m;

int** p = new int*[n];
for (int i = 0 ; i < n; i++) {
    p[i] = new int[m];
}

or using malloc (Do it in C. Not recommended in C++)...

cin >> n >> m;

int** p = (int**) malloc (n * (int*));

for (int i = 0; i < n; i++) {
    p[i] = (int*) malloc(m * (int));
}

for a 2D array of int s.

But remember to delete or free it after your use.

Answer 3

You could use a std::vector<Type>(n*m) , that would probably be the nicest way.

If however you want to stay with the array, I guess that it would compile if you allocate the memory on the Heap instead of the stack by calling new/malloc. But remind yourself of freeing the memory afterwards and please check the user input before you do it to prevent malicious inputs.

Answer 4

array needs constant paras.

instaead, you can use vector< vector <int> >

Answer 5

Variable length arrays are proposed in the upcoming C++14 standard , but not yet in the language.

You can however use a std::vector of std::vector of whatever type you want.

Like

std::vector<std::vector<int>> a(n, std::vector<int>(m));

The above declaration create a vector of vectors of integers, the outer vector of size n and the inner vector of size m .