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From: axtens on 23 Jan 2008 23:23
G'day everyone,

I have a knapsack packing algorithm in C. I've got it working within
itself okay. I have figured out how to get the data from VB to Fortran
and from Fortran into C. What I can't seem to work out is how to get
the data back from C to Fortran. It's probably something profoundly
simple ...

BTW, the change in argument order (p, w v/v w, p) is deliberate.

The code:
Fortran:
subroutine KnapsackA43( n, p, w, solution, mass, optimal )
!dec$ attributes dllexport, stdcall, alias : "KnapsackA43" ::
KnapsackA43
!dec$ attributes reference :: n, p, w, solution, mass, optimal

INTEGER N
REAL*8 P(N)
REAL*8 W(N)
REAL*8 SOLUTION(N)
REAL*8 MASS
REAL*8 OPTIMAL

interface
subroutine knapsack_A43 [C, Alias:'_knapsack_A43(a)32'] (n, w, p,
solution, mass, optimal)
! dec$ attributes reference :: n, w, p, solution, mass, optimal
INTEGER n
REAL*8 w(n)
REAL*8 p(n)
REAL*8 solution(n)
REAL*8 mass
REAL*8 optimal
end subroutine knapsack_A43
end interface
call knapsack_A43( n, w, p, solution, mass, optimal )
return
end subroutine KnapsackA43

C:
/*
** knaps2.c
*/
/*
** September 26, 1997
** this program implements Algorithm 4.3
** for solving Problem 4.1 (0,1)-Knapsack
** with simple pruning.
*/
/*
** Compile with:
** gcc knaps2.c -o knaps2
**
** Run with:
** knaps2 fname
** or
** knaps2
*/

#include <stdlib.h>

void __stdcall knapsack_A43( int, double*, double*, double*, double,
double );

void Knapsack2( int, double );
double DotProd( int, double*, double* );

int igNodes;
int igNumber;
double * dgProfits;
double * dgWeights;
double * dgSolution;
double dgCapacity;
double dgOptimal;

void __stdcall knapsack_A43( int iNumber, double *dWeights, double
*dProfits, double *dSolution, double dCapacity, double dOptimal )
{

igNodes = 0;
igNumber = iNumber;
dgProfits = dProfits;
dgWeights = dWeights;
dgSolution = dSolution;
dgCapacity = dCapacity;

Knapsack2( 0, 0.0 );

dOptimal = dgOptimal;
//dWeights = dgWeights;
//dProfits = dgProfits;
//dSolution = dgSolution;
return;
}

double DotProd( int n, double* A,double* B)
{
int i;
double ans;
ans=0.0;
for( i=0; i < n;i++ ) {
ans += ( A[ i ] * B[ i ] );
}
return( ans );
}

void Knapsack2( int ell, double curW )
/*
** Algorithm 4.3
*/
{
int i;
igNodes = igNodes + 1;
if ( ell == igNumber ) {
if ( DotProd( igNumber, dgProfits, dgSolution ) > dgOptimal ) {
dgOptimal = DotProd( igNumber, dgProfits, dgSolution );
for( i = 0; i < igNumber; i++ ) {
dgSolution[ i ] = dgSolution[ i ];
}
}
}
else {
if( curW + dgWeights[ ell ] <= dgCapacity ) {
dgSolution[ ell ] = 1.0;
Knapsack2( ell + 1, curW + dgWeights[ ell ] );
}
dgSolution[ ell ] = 0.0;
Knapsack2( ell + 1, curW );
}

}

VB6 (more for completeness than anything else as I don't have any
problems getting the data out of VB into Fortran):
Declare Sub KnapsackA43 Lib "debug\boal0.dll" (n As Integer, p As
Double, w As Double, x As Double, mass As Double, profit As Double)

....

Dim i As Integer
Dim mass As Double
Dim profit As Double
Const mass_limit As Double = 26#
Dim p(4) As Double
Dim w(4) As Double
Dim x(4) As Double
Const n As Integer = 4

p(0) = 24#
p(1) = 13#
p(2) = 23#
p(3) = 15#
p(4) = 16#

w(0) = 12#
w(1) = 7#
w(2) = 11#
w(3) = 8#
w(4) = 9#


Debug.Print " "

Debug.Print " "
Debug.Print "Object", "Profit", "Mass", "Profit Density",
"Solution"
Debug.Print " "
For i = 0 To n
Debug.Print i, p(i), w(i), p(i) / w(i), x(i)
Next

KnapsackA43 n, p(0), w(0), x(0), mass_limit, profit

Debug.Print " "

Debug.Print " "
Debug.Print "Object", "Profit", "Weight", "Profit Density",
"Solution"
Debug.Print " "
For i = 0 To n
Debug.Print i, p(i), w(i), p(i) / w(i), x(i)
Next

Debug.Print " "
Debug.Print " Mass Limit " & mass_limit
Debug.Print " Profit " & profit


Kind regards,
Bruce.
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