| Topic: |
DEVELOP > c-Plus-Plus |
| User: |
"Yudan Yi \OSU" |
| Date: |
11 Jun 2005 08:55:21 AM |
| Object: |
question about 2d matrix and pointer in the function definition |
Hi
I define a function, such as
void matrix_multi(double **a, double **b, double **c, int n, int m, int q);
then when I called this function, I must first declare
double **a, double **b, double **c;
My question: is there any way to call the function when I declare
double a[5][10], b[10][5],c[5][5];
matrix_multi(a, b, c, 5, 10, 5); // => will give error message, what should
I do?
Thanks
Yudan
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| User: "James Daughtry" |
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| Title: Re: question about 2d matrix and pointer in the function definition |
11 Jun 2005 09:40:28 AM |
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A two dimensional array doesn't evaluate to a pointer to a pointer, it
evaluates to a pointer to an array of size N, so
double a[5][10];
would require one of two function parameter declarations:
void foo(double arg[5][10]);
or
void foo(double (*arg)[10]);
If you need the second dimension to be variant, then you're SOL unless
you allocate memory to a pointer to a pointer and simulate a two
dimensional array. Alternatively, you could use a container such as
std::vector:
#include <vector>
void foo(const std::vector<std::vector<double> >& arg);
std::vector<std::vector<double> > a(5, std::vector<double>(10));
foo(a);
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| User: "Rolf Magnus" |
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| Title: Re: question about 2d matrix and pointer in the function definition |
11 Jun 2005 03:45:45 PM |
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James Daughtry wrote:
A two dimensional array doesn't evaluate to a pointer to a pointer, it
evaluates to a pointer to an array of size N, so
double a[5][10];
would require one of two function parameter declarations:
void foo(double arg[5][10]);
or
void foo(double (*arg)[10]);
A third version would be:
void foo(double (&arg)[5][10]);
But that would make both dimensions fixed.
If you need the second dimension to be variant, then you're SOL unless
you allocate memory to a pointer to a pointer and simulate a two
dimensional array.
Or make a one dimensional array and do the index calculation yourself.
Alternatively, you could use a container such as std::vector:
#include <vector>
void foo(const std::vector<std::vector<double> >& arg);
std::vector<std::vector<double> > a(5, std::vector<double>(10));
foo(a);
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