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Thread: Simplex algorithm java

  1. #1
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    Default Simplex algorithm java

    Hey guys! I am trying to write a Simplex algorithm program that reads input from a file. It runs but every time gives a no solution output ('No').
    I have spent several days working on this program and still didn't find a bug.
    import java.util.*;
    import java.io.*;
    import java.math.*;
     
    public class first {
        public static void main (String[]args) throws IOException {
    	/* Reading input from a file*/
    	File inputFile = new File("input.txt");
    	Scanner in = new Scanner(inputFile);
     
    	// Reading the number of free variables
    	int n = in.nextInt();
    	// Reading the number of equations
    	int m = in.nextInt(); 
    	// Defining indexes
    	int i=0, j=0, k=0, l=0; 
    	double f = 0;
     
    //Creating a simplex tableau
     
    	double[][] S = new double[m+1][n+m+1];
    	// Helping variable
    	double x=0;
    	// A very big number 
    	int M=10000; 
     
    	// Filling up the coefficients of function z  
            for (i = 0; i < n; i++) 
                S[m][i] = in.nextDouble();
     
    	// Filling up the coefficients of constraint equations
            for (i = 0; i < m; i++) 
                for (j = 0; j < n; j++)       
                    S[i][j] = in.nextDouble();
     
    	// Filling up the RHS
            for (i = 0; i < m; i++) {
                S[i][n+m] = in.nextDouble();  
    	    // if RHS is negative, change the sign
    	    if (S[i][n+m]<0) {
    		S[i][n+m]=-S[i][n+m];
    		for (j = 0; j < n; j++)
    		    S[i][j]=-S[i][j];
    	    }
    	    x+=S[i][n+m];
     
    	    //Adding basis variables to the simplex table
                S[m][n+i]=0; 
                S[i][n+i]=1;
    	} 
    	S[m][m+n] = x*M;
            x=0;
           // Changing the last row/column of the tableau
           for (j = 0; j < n; j++)
            {
                for (i = 0; i < m; i++)
                    x+=S[i][j];
                S[m][j]=x*M - S[m][j];
                x=0;
            } 
     
    // Now the tableu is filled up
     
    	// Creating an array for the basis indexes
    	int[] num = new int[n];
    	for(i=0; i<n;i++)
    	    num[i]=-1;
    	int ni=0; // number of variables in the basis
     
            File outputFile = new File("output.txt");
            BufferedWriter bw= new BufferedWriter(new FileWriter(outputFile, false));
     
       //Test to see check what is in the tableau 
    	for(i=0;i<=m;i++){
    	    for(j=0;j<=m+n;j++){
    		System.out.println(S[i][j] + " ");
    	    }
    	}
     
     
    //--------------------------------------------------------------------//-----------------------ALGORITHM-------------------------------------//--------------------------------------------------------------------
    	x=S[m][0];
    	for (j = 1; j < n; j++)
    	    if (S[m][j]<x) { 
    		x=S[m][j];
    		k=j;
    	    }
    	//Writing k in the array of basis variables
    	num[ni]=k; 
    	ni++;
     
    	while (S[m][k]>0) {
    	    x=10000;
    	    for (i = 0; i < m; i++)
    		if (S[i][k] > 0)  // For the positive values we count (S[i][n+m]/S[i][k])
    		    if ((S[i][n+m]/S[i][k]) < x) { //Find the minimum among them
    			x = S[i][n+m]/S[i][k];
    			l=i;
    		    }
    	    //If all elements of the column are <= 0, there are no solutions
    	   if (((x-10000)<1.e-009)||((x-10000)>-(1.e-009))) {
    	     bw.write ("No");
    	   	bw.flush();
    	   	bw.close();
    	   	return;
    	   }	
     
    	    // We found k and l indexes, so now we can perform the Gauss-Jordan method
    	    x=S[l][k];
    	    for (j = 0; j <= m+n; j++)
    		S[l][j]/=x;
     
    	    for (i=0; i<=m; i++) {
    		x=S[i][k];
    		if(i!=l) 
    		    for (j = 0; j <= m+n; j++)
    			S[i][j]=S[i][j]-(S[l][j]*x);
    	    }
     
                // Finding the largest maximum element in the last row
                x=S[m][0];
                for (j = 1; j < n; j++)
                    if (S[m][j]>x) {
                        x=S[m][j];
                        k=j;
                    }
                num[ni]=k; // Record the index in the array of indexes of basis variables
                ni++;
    	    // After the first iteration we return to Step 1
    	}
        /*
    	for(i=0;i<=m;i++){
                for(j=0;j<=m+n;j++){
     
    		String str3 = new String();
    		str3 = Double.toString(S[i][j]);
    		bw.write(str3);
    		bw.write(" ");
    	    }
    	    bw.write("\n");
    	}
    	bw.write("\n"); */
     
    	// At this point, we got a solution. Need to check it
    	for (j = n; j < n+m-1; j++)
    	{if ((S[m][j]<1.e-009)&&(S[m][j]>-(1.e-009))) 
    	    	if ((S[m][m+n]<1.e-009)&&(S[m][m+n]>-(1.e-009))) { 
    	       	bw.write ("Unbounded");
    		bw.flush();
    		bw.close();
    		return;
    		}
    		else {
    		    bw.write ("No");
    		    bw.flush();
    		    bw.close();
    		    return;
    		}
    	}
    	// If the solution is not unbounded, then the solution is found.
    	bw.write("Yes\n");
    	double f1;
    	f1 = S[m][n+m];
    	new BigDecimal(f1).setScale(6,  BigDecimal.ROUND_HALF_EVEN);
    	String str1 = new String();
    	str1 = Double.toString(f1);
    	str1 = String.format("%8.6f", f1).replace(',','.');
    	    bw.write(str1.replaceAll(" ","") + "\n");
    	//bw.write(Double.toString(S[m][n+m])+"\n");
    	for(i=0;i<n;i++)
    	{
    	    if ((S[m][i]<1.e-009)&&(S[m][i]>-(1.e-009))) {
    		for (j=0; j<m;j++)
    		    if ((S[j][i]-1<1.e-009)&&(S[j][i]-1>-(1.e-009))) {
    		        f = S[j][n+m];
    			new BigDecimal(f).setScale(6,  BigDecimal.ROUND_HALF_EVEN);
    			String str = new String();
    			str = Double.toString(f);
    			str = String.format("%8.6f", f).replace(',','.');
    			bw.write(str.replaceAll(" ",""));
    			//bw.write(Double.toString(S[j][n+m]));
    			bw.write(" ");
    		    } 
    	    } else 
    		bw.write("0.000000 ");
    	}
    	bw.flush();
    	bw.close();
    	return;
        }
    }


  2. #2
    Super Moderator copeg's Avatar
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    Default Re: Simplex algorithm java

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  3. #3
    Super Moderator Norm's Avatar
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    Default Re: Simplex algorithm java

    still didn't find a bug.
    How do you know there is a bug?
    If you don't understand my answer, don't ignore it, ask a question.

  4. #4
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    Default Re: Simplex algorithm java

    Quote Originally Posted by Norm View Post
    How do you know there is a bug?
    I have tested it with several inputs and only got either "No" or "Unbounded" outputs. I used the textbook examples which definitely had solutions

  5. #5
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    Default Re: Simplex algorithm java

    Then try debugging the code to see where it is going wrong.
    I use printlns to print out the values of variables as they are used. If you have an interactive debugger try that.
    If you understand the algorithm, the debug output should show you where the program is going wrong.
    If you don't understand my answer, don't ignore it, ask a question.

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