how to compile the blue part?

prob2(): By using a pointer to pointer **A and the function malloc()
allocate the memory for the 4x4 matrix A[][]. By using a pointer *b
and the function malloc() allocate the memory for the 4-dimensional vector
b[]. Read the components of A and b from the given input file matrix.dat.
The last line of the input file contains the components of b. The rows of
matrix A are the first four lines of the input file. Print the components
of A[][] row by row, and the components of b[]. Using the function gauss()
given in the lecture notes week9.txt, solve the system of linear algebraic
equations A[][]*x[]=b[]. Print the components of the solution vector x[].
Calculate and print the length of this vector. Free the allocated memory.
..

the code that i constructed is as followed starting with the gauss

Code:

---

double **A, *b;
      int row;
        s=gauss(4,A,b);
        printf("x[]={%7.3f", s);

        for(row=0; row<4; row++) free(A[row]);
        printf("\nThe length of x[] is");
        free(A);
        free(b);
        fclose(input);
}
        return;
}


double gauss(int n, double **A, double *b)
        {
        double one = 1.0, zero=0.0;
        double i, j;
        double s;
        int nm, row, col, Arow;
        nm = n-1;
        if(n==1)
        {
        b[0] /= A[0][0]; A[0][0] = one;
        return;
        }
        for(row=0; row<nm; row++)
        {
        i = A[row][row]; A[row][row] = one;
        for(col=row+1; col<n; col++) A[row][col] /=i;
        b[row] /=i;
        for(Arow=row+1; Arow<n; Arow++)
        {
        j=A[Arow][row]; A[Arow][row]=zero;
        for(col=row+1; col<n; col++) A[Arow][col] -= j * A[row][col];
        b[Arow] -= j * b[row];
        }
        }
        b[nm] /= A[nm][nm];
        for(row=nm-1; row>=0; row--)
        {
        for(col=row+1; col<n; col++) b[row] -=A[row][col]*b[col];
        s=b[row];
        }
        return s;

---

Whata do you mean? What's the problem?

I tried to printf x[];
but the result gave me nothing! is there something wrong with "double gauss(int n, double **A, double *b)?

You never allocate any memory for A and b, only free it.

Also the code is a bit fragmented, and you should use a [code] [/code] box.

Meh, added tags, but the indentation is still crap.
SourceForge.net: Indentation - cpwiki

i still dont understand...if you were to write it? how would you structure it? since it uses fscanf/sscanf...im confused

Perhaps if the code was indented half-way decent,
a) you might figure out the answer
b) people might be more inclined to help you.

Code:

---

double gauss(int n, double **A, double *b)
{
  double one = 1.0, zero=0.0;
  double i, j;
  double s;
  int nm, row, col, Arow;

  nm = n-1;

  if(n==1)
  {
    b[0] /= A[0][0];
    A[0][0] = one;
    return;
  }

  for(row=0; row<nm; row++)
  {
    i = A[row][row];
    A[row][row] = one;
    for(col=row+1; col<n; col++)
      A[row][col] /=i;
    b[row] /=i;

    for(Arow=row+1; Arow<n; Arow++)
    {
      j=A[Arow][row];
      A[Arow][row]=zero;
      for(col=row+1; col<n; col++)
        A[Arow][col] -= j * A[row][col];
      b[Arow] -= j * b[row];
    }
  }

  b[nm] /= A[nm][nm];
  for(row=nm-1; row>=0; row--)
  {
    for(col=row+1; col<n; col++)
      b[row] -=A[row][col]*b[col];
    s=b[row];
  }
  return s;
}

---