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-   -   naive gaussian elimination program not working (http://www.go4expert.com/forums/naive-gaussian-elimination-program-t7696/)

 Shae 4Dec2007 07:16

naive gaussian elimination program not working

Hey all,

I'm trying to write this exceedingly simple-looking naive Gaussian elimination program, but it isn't working. I got the algorithm right out of the math book. It even looks a lot like other working examples I've seen.

It appears that it gives me the the correct values in the upper-right triangle, but it doesn't give me the zeros in the lower-left. I've even tried a couple of other algorithms and still had trouble. Oh, and I know that the line for operation on the vector B is missing. Just trying to get one step at a time.

Can anyone give me any idea what I'm doing wrong?

thanks!

Code:

```/* Naive Gaussian Elimination */ /* includes */ #include<math.h> #include<stdlib.h> #include<stdio.h> /* ********************* */ /* global variables */ // I know, global variables are bad :/ // two-dimensional array for the default 'A' array // zeros make array indexes match up w/matrix ones double A[8][8] = { { 0, 0, 0, 0, 0, 0, 0 },                 { 0, 0.1897, 0.3784, 0.6449, 0.7271, 0.4449, 0.1730, 0.0118 },                 { 0, 0.1934, 0.8600, 0.8180, 0.3093, 0.6946, 0.9797, 0.8939 },                 { 0, 0.6822, 0.8537, 0.6602, 0.8385, 0.6213, 0.2714, 0.1991 },                 { 0, 0.3028, 0.5936, 0.3420, 0.5681, 0.7948, 0.2523, 0.2987 },                 { 0, 0.5417, 0.4966, 0.2897, 0.3704, 0.9568, 0.8757, 0.6614 },                 { 0, 0.1509, 0.8998, 0.3412, 0.7027, 0.5226, 0.7373, 0.2844 },                 { 0, 0.6979, 0.8216, 0.5341, 0.5466, 0.8801, 0.1365, 0.4692 } }; /* another known problem for debugging */ // zeros make array indexes match up w/matrix ones /* double A[8][8] = { { 0, 0, 0, 0, 0, 0, 0 },                 { 0, 6, -2, 2, 4, 0, 0, 0 },                 { 0, 12, -8, 6, 10, 0, 0, 0 },                 { 0, 3, -13, 9, 3, 0, 0, 0 },                 { 0, -6, 4, 1, -18, 0, 0, 0},                 { 0, 0, 0, 0, 0, 0, 0 },                 { 0, 0, 0, 0, 0, 0, 0  },                 { 0, 0, 0, 0, 0, 0, 0  } }; */ // one-dimensional array for default vector 'b' double b[8] = { 0, 0.5000, -0.3000, -0.2000, 0.7000, -0.9000, 0.3000, -0.1000 }; /* ********************* */ /* Main */ int main(){ /* variables */ char overwrite; /* prototypes */ void printArray(); void printVector(); void performGaussianElimination(); /* Say hello */ printf("\n\n\nThis program will solve a system of equations with Naive Gaussian Elimination. \n\n"); printf("The default matrix used will be: "); printArray(); printf("\n\nThe default vector used will be: "); printVector(); performGaussianElimination(); printf("\nThe matrix after forward elimination is:"); printArray(); return 0; } /* ********************* */ /* Print Array Function (working correctly) */ void printArray(){ int i,j; printf("\n");   // loop through rows   for(i=1; i <=7; i++){                 // output column values         for(j=1; j<=7; j++) {         printf("%lf  ", A[i][j]);         } // end inner 'for'         printf("\n");   } // end outer 'for' return; } /* ********************* */ /* Print Vector Function */ void printVector(){ int j; printf("\n");         // output column values         for(j=1; j<=7-1; j++) {         printf("%lf  ", b[j]);         } // end inner 'for'         printf("\n"); return; } /* ********************* */ /* Perform Gaussian Elimination Function */ void performGaussianElimination(){ int i,j,k,m; double xmult; // forward elimination  for(k=1; k<7; k++)  {   for(i=k+1; i<=7; i++){     if(A[k][k]!=0){     xmult = A[i][k]/A[k][k];     }     A[i][k] = xmult;     for(j=k+1; j<=7; j++){       A[i][j] = A[i][j] - xmult * A[k][j];     }   }  } // do backward substitution, not done yet return; }```

 shabbir 4Dec2007 09:26

Re: naive gaussian elimination program not working

You have submitted the above thread as an article and I have moved to forum for discussion. Remember to click on New Thread button rather than Submit Article button.

 Shae 4Dec2007 21:27

Re: naive gaussian elimination program not working

Oops. Sorry.

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