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  1. #include <stdio.h>
  2. #define N 3
  3.  
  4. int main(void) {
  5.     int i, j, k;
  6.     double a[N][N] = {
  7.         { 0,-1,9 },
  8.         { 1,3,7 },
  9.         { 3,7,5 }
  10.     };
  11.     double b[N] = { 22,5,-9 }, s;
  12.  
  13.     // 行列と定数項の出力
  14.     for (i = 0; i < N; i++) {
  15.         for (j = 0; j < N; j++) {
  16.             printf("%5.2lf ", a[i][j]);
  17.         }
  18.         printf(" | %5.2lf\n", b[i]);
  19.     }
  20.     printf("-------------------------------\n");
  21.  
  22.     // 前進消去
  23.     for (k = 0; k < N - 1; k++) {
  24.         for (i = k + 1; i < N; i++) {
  25.             double factor = a[i][k] / a[k][k];
  26.             for (j = k; j < N; j++) {
  27.                 a[i][j] -= factor * a[k][j];
  28.             }
  29.             b[i] -= factor * b[k];
  30.         }
  31.     }
  32.  
  33.     // 後退代入
  34.     double x[N];
  35.     for (i = N - 1; i >= 0; i--) {
  36.         s = b[i];
  37.         for (j = i + 1; j < N; j++) {
  38.             s -= a[i][j] * x[j];
  39.         }
  40.         x[i] = s / a[i][i];
  41.     }
  42.  
  43.     // 結果出力
  44.     for (k = 0; k < N; k++) {
  45.         printf("x[%d] = %5.2lf\n", k, x[k]);
  46.     }
  47.  
  48.     return 0;
  49. }
Success #stdin #stdout 0s 5308KB
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stdin
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Standard input is empty
stdout
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 0.00 -1.00  9.00  | 22.00
 1.00  3.00  7.00  |  5.00
 3.00  7.00  5.00  | -9.00
-------------------------------
x[0] =  -nan
x[1] =  -nan
x[2] =  -nan
https://ideone.com/8RapKZ
language:
C (gcc 8.3)
created:
1 day ago
visibility:
public

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