Facing error in c++ arrays and pointers

Question

I am writing this c++ code to multiply two 2d matrices and return the resultant 2d matrix.

The error is cannot convert ll (*)[2] to ll**

ll** multiply(ll a[2][2],ll b[2][2])
{
    ll ans[2][2];
    ans[0][0]=((a[0][0]*b[0][0])%mod+(a[0][1]*b[1][0])%mod)%mod;
    ans[0][1]=((a[0][0]*b[0][1])%mod+(a[0][1]*b[1][1])%mod)%mod;
    ans[1][0]=((a[1][0]*b[0][0])%mod+(a[1][1]*b[1][0])%mod)%mod;
    ans[1][1]=((a[1][0]*b[0][1])%mod+(a[1][1]*b[1][1])%mod)%mod;
    return ans;
}

Just use `std::array` instead of messing around with pointers and raw arrays? — UnholySheep, Jun 09 '17 at 20:02
Even if this compiled it would be wrong because you are trying to return the address of a local variable. — interjay, Jun 09 '17 at 20:02
related/dupe: https://stackoverflow.com/questions/8617683/return-a-2d-array-from-a-function — NathanOliver, Jun 09 '17 at 20:03

score 2 · Answer 1 · answered Jun 09 '17 at 20:15

You have a fallen into a trap that seems to get a lot of newcomers to the language.

Passing 2D arrays to a function and returning 2D arrays to a function leads to error prone code. Using a strut/class removes those errors.

In your case, since the size of the arrays is fixed, you can easily use a struct

struct MyMatrix
{
   ll data[2][2];
};

then, update multiply to use MyMatrix instead of 2D arrays.

MyMatrix multiply(MyMatrix const& a, MyMatrix const& b)
{
    MyMatrix ans;
    ...
    return ans;
}

You can change the function to an overloaded operator too.

MyMatrix operator*(MyMatrix const& a, MyMatrix const& b)
{
    MyMatrix ans;
    ...
    return ans;
}

and simplify usage to:

MyMatrix a{ fill in the data for a};
MyMatrix b{ fill in the data for b};
MyMatrix c = a*b;

Facing error in c++ arrays and pointers

1 Answers1