accessing a variable from another method

Hey,
I have trouble accessing a matrix from another method. I have a matrix allocated like this in one method

Code:

DTM **dtmpixel = new DTM*[ny_dtm];
for (int k=0; k<ny_dtm; k++)
{
	dtmpixel[k] = new DTM[nx_dtm];
}

How can I access it from another method?
Regards

You would pass the pointer to that second function.

That sounds good =) How would I go about doing that?
Cheers

Well, besides passing the pointer (of type DTM**), you would also need to pass the dimensions of the matrix (i.e., ny_dtm and nx_dtm) in order for that function to correctly access the elements of the matrix.

By the way, are you aware of containers like std::vector that would help you avoid having to manually manage memory?

Thanks for your reply, I am aware of vectors, but I wanted to try the "normal way" first. Now it seems to work except for I dont know how to pass nx_dtm and ny_dtm. My function who creates the DTM** now returns it to main and the next method can "take it". But I can only return one value, right?

In C++, vectors are the "normal way".

You can only return 1 value from a function, but you can pass pointer or reference parameters to a function and modify their values, thereby "returning" more than 1 value.

Should I from say function a call function b with the pointer, and additional, dimension data as arguments instead of returning the data as I did now? And how could I create the same matrix in first post with vectors? I tried it but... it didnt work out =/
Thanks!

Originally Posted by larne

Should I from say function a call function b with the pointer, and additional, dimension data as arguments instead of returning the data as I did now?

That would be one option. An even better option, moving towards creating a full blown matrix class, would be to define:

Code:

struct Matrix
{
    DTM** dtmpixel;
    size_t ny_dtm;
    size_t nx_dtm;
};

Then you can pass a Matrix pointer to the function that initialises the Matrix object (and later on this initialisation function would become a constructor when you decide to write a proper class).

Originally Posted by larne

And how could I create the same matrix in first post with vectors?

Code:

std::vector<std::vector<DTM> > dtmpixel(ny_dtm, std::vector<DTM>(nx_dtm));

Basically, you create a vector of ny_dtm number of vectors and initialise each of them with nx_dtm number of DTM objects.

Perfect, thanks!

there was no problem just changing the allocation to vector style and using the same code except for passing the vector back to main. Im not sure what´s going wrong here (but I did have some beers doing it). I tried putting receiving datatype as vector<DTM>. Is "vector" enough? No my compilation error is

Code:

error: conversion from `std::vector<std::vector<RADAR, std::allocator<RADAR> >, std::allocator<std::vector<RADAR, std::allocator<RADAR> > > >' to non-scalar type `std::vector<RADAR, std::allocator<RADAR> >' requested|

The vector I am passing on is vector<RADAR> containing instances of DTM. Or if if this is confusing how do I pass a vector to another function?
Any idea?

What is your code?

to make it short,
header:

Code:

// headers

// Constants
//#define M_PI  3.1415926535897932384626433832795

// classes
class DTM
{
...

};

class RADAR
{

	vector<DTM> radarpixel;

	public:
	void adddata(DTM pt)
	{
		radarpixel.push_back(pt);
	}
	int getsize()
	{
		return radarpixel.size();
	}
	DTM getdata(int n)
	{
		return radarpixel[n];
	}

};

// funcions
vector<RADAR> a();
void b(vector<RADAR>);

main:

Code:

#include "header.h"

int main()
{

	vector<RADAR> radar = a();

	b(radar);

	return 0;
}

function a.

Code:

vector<RADAR> a()
{
	...
    vector<vector<DTM> > dtmpixel(ny_dtm, vector<DTM>(nx_dtm));

    vector<vector<RADAR> > radarpixel(ny_radar, vector<RADAR>(nx_radar));
	
    radarpixel[tmplin][tmpcol+midcol].adddata(dtmpixel[y][x]);

    return radarpixel;
}

function b:

Code:

#include "header.h"

void b(vector<RADAR> radarpixel)
{
...
}

Well, you declared radarpixel to be a vector<vector<RADAR> >, but then you return it from a(), whose return type is vector<RADAR>.

By the way, unless you really want a copy, it would usually be better to take the arguments by (const) reference.

ok, being a rookie and all, how could I return a copy correctly? ...and I dont really understand (const) reference. Pass the pointer? In this case, how?
Thanks for your help

Originally Posted by larne

ok, being a rookie and all, how could I return a copy correctly?

You're returning a copy correctly, but copying could be expensive if certain compiler applied optimisations are not applicable.

Originally Posted by larne

and I dont really understand (const) reference. Pass the pointer? In this case, how?

I mean:

Code:

void b(vector<RADAR>& radarpixel)
{
    // ...
}

or:

Code:

void b(const vector<RADAR>& radarpixel)
{
    // ...
}

If you don't understand the above syntax then you need to read a good introductory book on C++.