Design a function to write the data in a matrix into files

nanchuangyeyu · #1 12-Jul-2009, 03:23

Hi,
My coding objective is quite simple: design a function to write the data in a matrix into files. And here is my code:

CPP / C++ / C Code:

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int matwrite(float* mp,int row_sz,int col_sz,string mat_file);

int main()
{
	// create a simple binary file first for later matrix operation.
	float myMat[3][3] = {1,2,3,4,5,6,7,8,9};
	string file_name = "mat.bin";
	matwrite(myMat,3,3,file_name);	
}
int matwrite(float* mp,int row_sz,int col_sz,string file_name)
{
    ofstream mat_file(file_name.c_str(),ios::binary);
	for (int row=0; row<row_sz; row++)
	{
		for (int col=0; col<col_sz; col++)
		{
			mat_file.write(reinterpret_cast<char*>(&mp[row][col]),sizeof(float));
		}
	}
	cout<<"The matrix has been successfully written to file."<<endl;
	return 0;
}

Errors occured as following during compilation:

Code:

e:\code zone\practice code\matrix_class\matrix\matrix_class\mat_demo.cpp(14) : error C2664: 'matwrite' : cannot convert parameter 1 from 'float [3][3]' to 'float *'
        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

e:\code zone\practice code\matrix_class\matrix\matrix_class\mat_demo.cpp(23) : error C2109: subscript requires array or pointer type

Any one can tell me what's the matter and tell me how to fix it? Thank u in advance.

davekw7x · #2 12-Jul-2009, 09:48

Quote:

Originally Posted by nanchuangyeyu

Hi,
My coding objective is quite simple: design a function to write the data in a matrix into files.

Here's the thing: In order for the compiler to generate code in the function to acess elements of a 2-dimensional declared array, it hast to know the size of the second dimension. The following will work:

CPP / C++ / C Code:

int matwrite(float mp[][3],int row_sz,int col_sz,string mat_file);
.
.
.
	matwrite(myMat,3,3,file_name);	
.
.
.

See Footnote.

Note that you don't have to tell it the first dimension. (It doesn't hurt if you do it, but it doesn't help either.) With that function definition, the routine will work no matter what the size of the first dimension is.

In general for multi-dimensional arrays, if you want to pass a pointer to a function, you must give the array name and the sizes of all dimensions except the first. The compiler simply must have this information in order to generate code to access the array elements. Period. Full stop.

In order to have a function that will work on different sizes of matrices (something other than "x" by three), you will have to work with pointers and memory allocation to get your "arrays," not with declared arrays.

Now, since this is C++ (not C), you might consider using vectors instead of 1-dimensional arrays and vectors of vectors instead of 2-dimensional arrays. The user notation can be made the same (using mat[i][j] to access the elements of the "matrix") bit for many people it's actually easier to implement.

I think using pointer arrays is a valuable thing to learn, but I think that using vectors and vectors of vectors is also important. Chacun à son goût!

Regards,

Dave
Footnote: In C and C++ there really are no such things as multi-dimensonal arrays. A 2-D array is actually an array of 1-D arrays. Note that for all arrays in C and C++ the name of the array is a pointer to the first element in the array.

In your example, with myMat[3][3], the following is true:

myMat[0] is a pointer to a 3-element array.
The elements are myMat[0][0], myMat[0][1], myMat[0][2]

myMat[1] is a pointer to a 3-element array.
The elements are myMat[1][0], myMat[1][1], myMat[1][2]

myMat[2] is a pointer to a 3-element array.
The elements are myMat[2][0], myMat[2][1], myMat[2][2]

The following is completely, absolutely, exactly equivalent for the function definition:

CPP / C++ / C Code:

int matwrite(float (* mp)[3],int row_sz,int col_sz,string mat_file);

This says that mp is a pointer to an array of three floats, and the results are exactly, absolutely, completely equivalent to the other definition.

The actual preferred notation for initializing your array would be:

CPP / C++ / C Code:

    float myMat[3][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };

This emphasizes that myMat is an array of three arrays. Each of the three is enclosed in braces.

The elements are stored in sequential memory locations whether you put the inner braces or not, so they aren't absolutely required (some compilers give warnings, but the code works without them), but writing it like this emphasizes the "array of arrays" principle.

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Design a function to write the data in a matrix into files

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