Sparse matrix compressed on rows in C++ - c++

Question

I have to implement the CSR matrix data structure in C++ using 3 dynamic arrays (indexing starts at 0) and I've got stuck. So I have to implement 2 functions:
1) modify(int i, int j, TElem e) - modifies the value of (i,j) to e or adds if (if it does not exist) or deletes it if e is null.
2) element(int i, int j) const - returns the value found on (i,j)
I wanted to test my code in the next way:
Matrix m(4,4); m.print(); It will print:
Lines: 0 0 0 0 0
Columns:
Values:
(And this is fine)
Now if I want to modify: m.modify(1,1,5); //The element (1,1) will be set to 5
The output of m.print(); will be:
Lines: 0 1 1 1 1
Columns: 1
Values: 5 (which again is fine)
And now if I want to print m.element(1, 1) it will return 0 and m.element(0, 1) will return 5.
This is my implementation of element(int i, int j) :
int currCol;
for (int pos = this->lines[i]; pos < this->lines[i+1]; pos++) {
currCol = this->columns[pos];
if (currCol == j)
return this->values[pos];
else if (currCol > j)
break;
}
return NULL_TELEM;
The constructor looks like this:
Matrix::Matrix(int nrLines, int nrCols) {
if (nrLines <= 0 || nrCols <= 0)
throw exception();
this->nr_lines = nrLines;
this->nr_columns = nrCols;
this->values = new TElem[100];
this->values_capacity = 1;
this->values_size = 0;
this->lines = new int[nrLines + 1];
this->columns = new TElem[100];
this->columns_capacity = 1;
this->columns_size = 0;
for (int i = 0; i <= nrLines; i++)
this->lines[i] = NULL_TELEM;
}
This is the "modify" method:
TElem Matrix::modify(int i, int j, TElem e) {
if (i < 0 || j < 0 || i >= this->nr_lines || j >= nr_columns)
throw exception();
int pos = this->lines[i];
int currCol = 0;
for (; pos < this->lines[i + 1]; i++) {
currCol = this->columns[pos];
if (currCol >= j)
break;
}
if (currCol != j) {
if (!(e == 0))
add(pos, i, j, e);
}
else if (e == 0)
remove(pos, i);
else
this->values[pos] = e;
return NULL_TELEM;
}
And this is the inserting method:
void Matrix::add(int index, int line, int column, TElem value)
{
this->columns_size++;
this->values_size++;
for (int i = this->columns_size; i >= index + 1; i--) {
this->columns[i] = this->columns[i - 1];
this->values[i] = this->values[i - 1];
}
this->columns[index] = column;
this->values[index] = value;
for (int i = line; i <= this->nr_lines; i++) //changed to i = line + 1;
this->lines[i]++;
}
Can somebody help me, please? I can't figure out why this happens and I really need to finish this implementation these days.
It just can't pass the next test. And if I want to print the elements i have (4,0)=0 (4,1)=0 ... (4,8)=0 and (4,9)=3. Now this looks pretty weird why it happens.
void testModify() {
cout << "Test modify" << endl;
Matrix m(10, 10);
for (int j = 0; j < m.nrColumns(); j++)
m.modify(4, j, 3);
for (int i = 0; i < m.nrLines(); i++)
for (int j = 0; j < m.nrColumns(); j++)
if (i == 4)
assert(m.element(i, j) == 3);
//cout << i << " " << j << ":" << m.element(i, j)<<'\n';
else
assert(m.element(i, j) == NULL_TELEM);
}

Answer 1

When you call modify(1, 1, 5) with an empty matrix (all zeros), that results in a call to add(0, 1, 1, 5). That increments columns_size and values_size (both to 1), the for loop body will not execute, you update columns[0] to 1 and values[0] to 5, then increment all the lines values starting at element lines[1], setting them all to 1 (lines[0] will still be 0). But lines[1] should indicate the element we just added, so it should be 0, since the value is found using columns[0].
The for loop at the end of add should start at element line + 1.