c++ using "indexing" over a 2D matrix, under this structure

Question

don't know why but i get an error: after this structure i can't index the matrix, so i cant use the "indexing method" over the defined matrix.Can anyone tell me why? or how to fix it?

Header:
const int days=31;
const int exp=6;

struct Array{
int days;
int exp;
int **M;
};

Constuctor:

void constr(Array loc){
//Construct of 31*6 Matrix, were 31 nr. of days and 6 specific types:
//0-HouseKeeping, 1-Food, 2-Transport, 3-Clothing, 4-TelNet, 5-others
loc.days = days;
loc.exp = exp;
loc.M = new int*[loc.days];
for(int i=0; i<loc.days;i++ ){
   loc.M[i] = new int[loc.exp];
   for (int j = 0; j< loc.exp; j++){
       loc.M[i][j] = 0;
   }
}
}

Controller.cpp

 void add(int cant,int tip, Array M){
//Adds to current day the amount to a specific type
currDay();
M[currentDay][tip] += cant; ////////////error
}


void insert(int zi,int tip,int cant, Array M){
//Adds to current day the amount to a specific type
M[zi][tip] = cant; ///////////error
}

void removeDay(int day, Array M){
for(int i = 0; i<6; i++)
    M[day][i] = 0; ///////////error

//zi and tip ~ day type... for easier read.
//i need to manage the expenses of a family in a month  doesn't matter which
ERROR: error: no match for 'operator[]'

UI(where constructor is used):

int main(){
Array M;
constr(M);
printMenu();
return 0;
}

Your example is not minimal (the `constr` function is not used) and it could be reduced into a single file. Could you please reduce it and give the line of the error? — thiton, Mar 28 '12 at 12:14
The `constr` function is allocating a copy of the original object. So this will cause memory leaks at exit and original object won't be allocated. It works in the 3 others function because pointer is copied too (if it has been allocated) — Patrice Bernassola, Mar 28 '12 at 12:26
I need to wash my eyes, is this question really tagged `C++`? — Andre, Mar 28 '12 at 12:31

score 2 · Answer 1 · answered Mar 28 '12 at 12:16

2

You're not accessing the member:

M.M[currentDay][tip]

instead of

M[currentDay][tip]

Or you could define operator [] for your struct:

struct Array{
    int days;
    int exp;
    int **M;
    int*& operator[] (int idx) { return M[idx]; }
};

answered Mar 28 '12 at 12:16

Luchian Grigore

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thanks... sorry for dummy questions just started with c++ :) – Bogdan Maier Mar 28 '12 at 12:18

score 1 · Answer 2 · edited May 23 '17 at 10:08

1

You are trying to call operator[] on the type array. You need to get the pointer member first.

M.M[day][i];

That said: You are not writing C++ but some obscure form of bad C. You might want to have a look at the book list and read one of them before pursuing coding any further.

edited May 23 '17 at 10:08

Community

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1

answered Mar 28 '12 at 12:16

pmr

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Some programmer dude · Answer 3 · 2012-03-28T12:33:05.443

You have at least two problems:

The first is that you are using the actual structure as the array, which wont work. Use e.g. M.M[day][i].
The second is that when you create the array, you pass the structure by value, this means it will be copied to a local variable in the constr function and the data will not be available in the function calling constr. Pass it as a reference instead, i.e. void constr(Array &loc).

The second problem can also be solved by using a constructor in the structure, instead of a separate function:

const int DAYS=31;
const int EXP=6;

struct Array{
    int days;
    int exp;
    int **M;

    // Constructor, called when an instance of structure/class is created
    Array(){
        days = DAYS;
        exp = EXP;
        M = new int*[days];
        for(int i=0; i<days;i++ ){
            M[i] = new int[exp];
            for (int j = 0; j< exp; j++){
                M[i][j] = 0;
            }
        }
    }

    // Destructor, called when structure/class is destroyed
    ~Array(){
        if(M){
            for(int i=0;i<days;i++){
                if(M[i])
                    delete [] M[i];
            }
            delete [] M
        }
    }

    // Copy constructor, called when instance of structure/class is copied
    Array(const Array &array){
        days = array.days;
        exp = array.exp;
        M = new int*[days];
        for(int i=0; i<days;i++ ){
            M[i] = new int[exp];
            for (int j = 0; j< exp; j++){
                M[i][j] = array.M[i][j];
            }
        }
    }
};

Be careful this can cause memory leaks if operator delete[] is not called before var goes out of scope — Patrice Bernassola, Mar 28 '12 at 12:28
@PatriceBernassola Thanks for the remainder, added destructor — Some programmer dude, Mar 28 '12 at 12:33

c++ using "indexing" over a 2D matrix, under this structure

3 Answers3