How to access elements by using pointer instead of array - c++

Question

I've this piece of code which is working by now(at least):
int** constructSparseMatrix(int totalRow, int totalCol, int totalEl) {
int** arr = new int*[totalEl];
//int x = 0;
for(int i = 0; i < totalEl; i++) {
arr[i] = new int[totalCol];
for (int k = 0; k < totalCol; k++) {
if(k == totalCol - 1) {
arr[i][totalCol - 1] = rand () % 101;
} else {
arr[i][k] = rand () % totalRow + 1;
}
}
}
return arr;
}
and I'm trying to access the element inside by using pointer instead of array and here is my try:
int** constructSparseMatrix(int *totalRow, int *totalCol, int totalEl) {
int** arr = new int*[totalEl];
//int x = 0;
for(int i = 0; i < totalEl; i++) {
arr[i] = &totalCol [0];
for (int k = 0; k < *totalCol; k++) {
if(k == *totalCol - 1) {
arr[i][*totalCol - 1] = rand () % 101;
} else {
arr[i][k] = rand () % *totalRow + 1;
}
}
}
return arr;
}
But when I initialize the same way as the working function:
int** arr = constructSparseMatrix(5,3,totalEl);
then I receive this error:
argument of type "int" is incompatible with parameter of type "int *"
First of all, is my conversion from array to pointer version coded properly? If it's correct, how can I initialize it to avoid above error?

Answer 1

First of all it is obvious that this statement in the function
arr[i] = &totalCol [0];
is wrong. It does not make sense.
You declared the function
int** constructSparseMatrix(int *totalRow, int *totalCol, int totalEl);
as having the first and second parameters as pointers but are trying to pass integer literals 5 and 3 to it in the call
int** arr = constructSparseMatrix(5,3,totalEl);
You could write for example
int totalRow = 5;
int totalCol = 3;
//...
int** arr = constructSparseMatrix( &totalRow, &totalCol, totalEl );
But I do not see a sense to declare these parameters like pointers because they are in fact constants within the function that is they are not changed.
And the variable names confuse the reader. For example I would expect that totalRow is used within the function in statement
int** arr = new int*[totalRow];
instead of totalEl
If you want to use pointers within the function then the function can look like
int** constructSparseMatrix( int totalRow, int totalCol, int totalEl )
{
int** arr = new int*[totalEl];
for ( int **p = arr; p < arr + totalEl; ++p )
{
*p = new int[totalCol];
for ( int *q = *p; q < *p + totalCol; ++q )
{
if ( q == *p + totalCol - 1 )
{
*q = rand () % 101;
}
else
{
*q = rand () % totalRow + 1;
}
}
}
return arr;
}

Answer 2

I think you are passing constants 5 and 3 as arguments which causes this error.
you can't pass constant value as it is by reference.Try passing 5 and 3 by storing them in a variable.