Zero out a section of a multidimensional array

Question

I have a multidimensional array (a representation of a matrix). I need to zero out, not the entire array, but a section of it. What's the best method of doing this? I tried using memset, but it just gives me a typecast error.

Example

_matrix[row][column] = memset(
                        _matrix[row][column], 
                        0, 
                        sizeof(_matrix[row][column])
                    );

Declaration

float** _matrix = new float*[NUM_ROWS][NUM_COL];

What is the section you want zeroed out? A row? column? Rows and columns? — Marlon, Dec 27 '11 at 22:56
@Marlon what do you think? I do have a good book on C/C++ - and I'm taking a class this quarter on it :P. — zeboidlund, Dec 27 '11 at 22:59
there is a host of problems with your snippet. C++ has very large learning curve - and you have to understand memory layout and indexing. you really should be asking more basic questions or read similar posts. — Anycorn, Dec 27 '11 at 23:01
@Holland : He said a _good_ book. ;-] Which book are you reading? — ildjarn, Dec 27 '11 at 23:02
cf: http://stackoverflow.com/questions/5633609/c-matrix-class-suggestions, http://stackoverflow.com/questions/2076624/c-matrix-class — Anycorn, Dec 27 '11 at 23:03
The OpenGL red book might be a good book on OpenGL, but it's not a good book to learn C++ from. — Fred Foo, Dec 27 '11 at 23:06
Would you have any suggestions? I'm looking for something which I can literally open up for a few minutes and scan through, rather than read from cover to cover. I'm passed the basics of programming - I just came from C#. — zeboidlund, Dec 27 '11 at 23:09
@Holland : See this thread: [The Definitive C++ Book Guide and List](http://stackoverflow.com/q/388242/636019) — ildjarn, Dec 27 '11 at 23:10

score 2 · Answer 1 · answered Dec 27 '11 at 22:55

2

The first parameter to memset() is the address of the location to zero. So:

memset(&_matrix[row][column], ...)

However, in your case the following would be far more straightforward:

_matrix[row][column] = 0.0;

answered Dec 27 '11 at 22:55

Greg Hewgill

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score 0 · Answer 2 · answered Dec 27 '11 at 23:07

This is tagged as C++ yet you're not really doing any C++. Why not just do that? Avoid all the lower level memset and direct array nonsense.

I certainly think you should take some of the previous advice and really learn the underlying C, memory access, etc., but in C++ this could simply be avoided.

A quick line of code to get you on your way:

typedef std::vector<std::vector<int> > Array2D;

Zero out a section of a multidimensional array

2 Answers2