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  1. /* 
  2.  * File:   main.cpp
  3.  * Author: piter cf16 eu
  4.  *
  5.  * Created on March 7, 2014, 12:01 AM
  6.  * http://stackoverflow.com/questions/22237322/solve-ax-b-a-lower-triangular-matrix-in-c/22237724?noredirect=1#comment33770886_22237724
  7.  */
  8.  
  9. #include <iostream>
  10. #include <algorithm>
  11. #include <iterator>
  12.  
  13. /**
  14.  * solve linear equation system by forward
  15.  * substitution on lower triangular matrix
  16.  * @param n matrix rank (dimension of solution space)
  17.  * @param a lower triangular matrix
  18.  * @param b right side
  19.  * @param x solution
  20.  * http://k...content-available-to-author-only...u.tr/~pehlivan/numerical_analysis/chap02/BackSubstitution.pdf
  21.  */
  22. void solve( int n, float a[4][4], float b[], float x[]) {
  23.   float s;
  24.  
  25.   for( int i = 0; i < n; i++) {
  26.         s = 0;
  27.         for( int j = 0; j < i; j++) {
  28.             s = s + a[ i][ j] * x[ j];
  29.         }
  30.         x[ i] = ( b[ i] - s) / a[ i][ i];
  31.    }
  32. }
  33.  
  34. /*
  35.  * 
  36.  */
  37. int main(int argc, char** argv) {
  38.  
  39.     int n = 4;
  40.     float a[4][4] = { 3, 0, 0, 0, -1, 1, 0, 0, 3, -2, -1, 0, 1, -2, 6, 2};
  41.     float b[4] = { 5, 6, 4, 2};
  42.     float res[4];
  43.     solve( n, a, b, res);
  44.     std::copy( res, res + 4, std::ostream_iterator<float>( std::cout, ","));
  45.     return 0;
  46. }
Success #stdin #stdout 0s 3340KB
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Standard input is empty
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1.66667,7.66667,-14.3333,50.8333,
https://ideone.com/3p3N04
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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