Why the 2-D array prints 1 where it should not? - c++

Question

My code prints 1 at every position except the first column. I want it to print 1 at certain specific places (r, c) where r is the row and c is the column. Also, can anybody tell me how do I initialize the 2-D array with 0 at every index?
I have tried to reset the values of r and c to zero but nothing changes.
int main()
{
int T;
cin>>T;
while(T--)
{
vector <long long int>R;
vector <long long int>C;
long long int N, M, K, r, c, i, j, k; // N -> rows, M -> columns
cin>>N>>M>>K;
long long int arr[N][M];
cout<<K<<endl;
for(i=0; i<N; i++)
{
for(j=0; j<M; j++)
{
arr[i][j] = 0;
}
}
for(k=0; k<K; k++)
{
cin>>r>>c;
arr[r][c] = 2;
r = 0; c = 0;
}
for(i=0; i<N; i++)
{
for(j=0; j<M; j++)
{
cout<<arr[i][j]<<" ";
}
cout<<endl;
}
}
return 0;
}
Input:
1 4 4 9 1 4 2 1 2 2 2 3 3 1 3 3 4 1 4 2 4 3

Answer 1

Some remarks:
Firstly, C-Style Array sizes must be known at compile time for standard C++, so you can't read them from cin (this is a GCC extension).
So if you need an array with a size known only at runtime, I suggest std::vector.
Additionally, in C++ it is common practice to define the loop variable inside the scope of the for-loop, like so:
for (int i = 0; i < N ++i)
This keeps the surrounding scope cleaner. An array or struct can be initialized to zero in C++ with the following uniform initialization syntax:
SomeStruct x{}; // All members are default initialized (e. g. for built-in types: 0)
SomeType myArray[N][M]{}; // All elements are default initialized
A comment like this
// N -> rows, M -> columns
is really just saying you should be naming your variables rows/cols and not N/M. Make your code self-documenting ;)
I would not recommend using std::endl when you want a line-break, as it also flushes the buffer, which when writing to files could
have serious performance implications (for console output it's no problem). It is better practice to use '\n' for line-breaks and use std::flush explicitly if you need it.
I've omitted the outer while loop and made Rows/Columns constant for simplicity. With std::vector you should use Rows*Cols as size in a
1D array and then map a position (i,j) to the 1D index with the formula:
arr2D[i][j] = arr1D[i*Cols + j];
If you choose to let the user specify rows/cols and use
std::vector<int> vec(rows*cols);
As your array.
int main()
{
constexpr int rows{3};
constexpr int cols{3};
int numUserValues{};
cin >> numUserValues;
int arr[rows][cols]{};
cout << numUserValues << '\n';
for(int k=0; k<numUserValues; k++)
{
int r{};
int c{};
if (cin>>r>>c && r>=0 && r<rows && c>=0 && c<cols) {
arr[r][c] = 2;
}
}
for(int i=0; i<N; i++)
{
for(int j=0; j<M; j++)
{
cout<<arr[i][j]<<" ";
}
cout<<'\n';
}
return 0;
}
Your program is very user-unfriendly, since you have no prompts at all (e.g. cout << "Enter number of rows: ";, but this is my test-run, as you can see all elements are zero except for the ones I specified:
$ ./test
5
5
0 0
1 1
2 2
0 1
0 2
2 2 2
0 2 0
0 0 2
Your question is hard to answer, because you say "I want to print 1 at position ...", while your code says arr[r][c] = 2; ;)
"My code prints 1 at every position except first column", would mean for a 5x5:
0 1 1 1 1
0 1 1 1 1
0 1 1 1 1
0 1 1 1 1
0 1 1 1 1
Which is simply not the case and thus false information.

Answer 2

I want it to print 1 at certain specific places (r, c) where r is the row and c is the column
You are doing this:
cin>>r>>c;
arr[r][c] = 2;
when the input is "1 4", which invokes Undefined Behavior (UB), because you go out of range, since the 2D array you have is of 4x4 dimensions, thus you would need to do something like this instead:
arr[r - 1][c - 1] = 1;
since arrays are zero indexed. I changed 2 to 1, since this is what you asked for.
I have tried to reset the values of r and c to zero but nothing changes.
This has no actual effect, since r and c are going to be overwritten upon next input.