Graphs - return a pointer to the adjacency matrix

Question

I am implementing a work of Graph Theory that deals with the game Flood-It which is a problem in flooding in Graphs.

I’m right at the beginning and implemented the graph structure yet. The structure of it is:

typedef struct {
    int V; 
    int A; 
    int **adj; 
} Grafo;

The third field must be a pointer to the adjacency matrix of the graph. The function that creates the matrix is the following:

static int ** MATRIZADJint(int linhas,const int colunas, int valor){
    int **m = malloc(linhas * sizeof(int *)); // aloca linhas
    for (vertice i = 0; i < colunas; ++i)
        m[i] = malloc(colunas * sizeof(int)); // aloca colunas
    for (vertice i = 0; i < linhas; ++i)
        for (vertice j = 0; j < colunas; ++j)
            m[i][j] = valor; // preenche a matriz de adjacencias inicialmente com 0's
    return m;
}

Unfortunately Visual Studio is not accepting the implementation, presenting the following error message:

malloc - a value of type "void *" cannot be used to initialize an entity of type "int **" = - a value of type "void *" cannot be used to initialize an entity of type "int **"

Does anyone know what it can be?

Answer 1

Just cast a cast for the right type on malloc:

static int ** MATRIZADJint(int linhas, const int colunas, int valor) {
    int **m = (int **) malloc(linhas * sizeof(int *)); // aloca linhas
    for (int i = 0; i < colunas; ++i) {
        m[i] = (int *) malloc(colunas * sizeof(int)); // aloca colunas
    }
    for (int i = 0; i < linhas; ++i) {
        for (int j = 0; j < colunas; ++j) {
            m[i][j] = valor; // preenche a matriz de adjacencias inicialmente com 0's
        }
    }
    return m;
}

This is because there are a few cases where C++ is not compatible with C and this is one of them. The function malloc has a kind of return void *. The C accepts that a void * is assigned to any pointer without problems, but C++ requires the cast.

I also don’t understand why you declared the variables of the ties for with vertice instead of int.

In addition, I strongly recommend the use of { and } in the bonds for to avoid certain kinds of problems and unpleasant surprises. But that would already be subject to another question.