Passing a matrix from main function to user defined function.

[kjones2k1]

Hello,

I am in an entry level C++ course. We have a homework problem that requires us to prompt the user to enter five sets of three numbers (type double) and store the numbers in a 5 x 3 array called Data[5][3]. Then, pass the matrix Data[5 ][3 ] from main ( ) to a function float mean (const int Data [ ][ ], int, int ) that receives the array Data [5 ][3 ] and its dimensions as arguments to find the row and column mean of matrix Data and prints the results in function main ( ). Here is a copy of my syntax:

#include <iostream>
#include <iomanip>
using namespace std;



float mean(const double, int, int);



int main()
{
const int row=2, col=3;
double data[row][col];
float avgrows[5];



//Entering values for the matrix
cout<<"Enter two sets of three number."<<endl;


for(int i=0; i<row; i++)
{
for(int j=0; j<col; j++)
{
cout<<"Enter row "<<i+1<<": ";
cin>>data[0]>>data[1]<<data[2];
}
}


//Stores row means into an array
for(i=0; i<row; i++ )
{
avgrows=mean(data, row, col);
}



//Print array
for(int x=0; x<5; x++)
{
cout<<avgrows[x]<<" ";
}


return 0;
}


float mean (const double data, int rows, int columns)
{
float average, sum=0;
for(int i=0; i<rows; i++)
{
for(int j=0; j<columns; j++)
{
sum+=data[j];
}
average=sum/columns;
return average;
}
}

Once I compile this cpp file, I get 4 errors, they are:

--------------------Configuration: matrix passing - Win32 Debug--------------------
Compiling...
matrix passing.cpp
1.) error C2676: binary '<<' : 'class std::basic_istream<char,struct std::char_traits<char> >' does not define this operator or a conversion to a type acceptable to the predefined operator


2.)error C2664: 'mean' : cannot convert parameter 1 from 'double [2][3]' to 'const double' There is no context in which this conversion is possible


3.) error C2109: subscript requires array or pointer type


4.) error C2109: subscript requires array or pointer type
Error executing cl.exe.


matrix passing.exe - 4 error(s), 0 warning(s)

I can't figure what the error mean, nor can I figure out what else to do. I've been racking my brain the last couple of days. If some one could shine some light on this matter, I would greatly appreciate it. I've also attached a copy of my cpp file.

Thank you.

[Narue]

float mean(const double data[][3], int rows, int columns);

Yes, you must specify all but the first dimension, though if you want to include the size of the first dimension as well, that's okay too.

[craju45]

float mean(const double, int, int);
this is ur declaration, here the first argument is only double and not a array or pointer.
but in the function you are using this as if it is array.
sum+=data[j];
here u are meaing that data is array, but in ur declation u have given as just double.

just chage to
float mean( **double, int, int);

[Narue]

>just chage to
>float mean( **double, int, int);
Don't answer questions if you don't know the answer yourself. The OP is passing a two dimensional array, which is *not* the same type as a pointer to a pointer. This:

double data[][3]

is equivalent to this:

double (*data)[3]

which is completely incompatible with this:

double **data

[kjones2k1]

So, what you are saying is i should change

float mean(const double data, int rows, int columns)

to

float mean(const double (*data)[3], int rows, int columns)

I'm still a bit confused.

[Narue]

What I'm saying is that until you know the difference, you should simply copy your array declaration:

double data[5][3];

Is copied into the function declaration:

double mean ( double data[5][3], int rows, int columns );

This will always work, and it saves you the confusion of "clever" people trying to convince you that an array is a pointer.

[kjones2k1]

Thanks for the help. I am now able to pass the matrix, but I'm still having problems calculating the mean of each row and the mean of each column.

here is my mean function:

float mean (const double data[2][3], int rows, int columns)
{
    float average, sum=0;
    for(int i=0; i<rows; i++)
    {
        for(int j=0; j<columns; j++)
        {
            sum+=data[i][j];

        }
        average = sum /rows;
        return average;
    }
}