CSDN老博文迁移-牛顿插值法的C语言实现

  #include <stdio.h>

  #define N 100
  double diff_quot(double *x, double *y, int start, int end);
  double compute_y(double *x, double *y, double *t, int len, double xx);
  int main(void) {
    double x[N], y[N], t[N], xx, yy, mul = 1.0;
    int i, j, n, start, len, ch;
    char chh;

    len = 0; //len 保存了数组x和y的长度
    start = 0; //start是每一次循环之后添加元素的起始位置

    while (1) {
      printf("输入要添加的点数: ");
      scanf("%d", &n);
      len += n;

      //输入数组x和y
      printf("输入x: ");
      for (i = start; i < start + n; i++) {
        scanf("%lf", &x[i]);
      }
      printf("输入y: ");
      for (i = start; i < start + n; i++) {
        scanf("%lf", &y[i]);
      }
      start += n; //不断跟新start的位置

      //开始求各阶差商了
      for (i = 0; i < len; i++) {
        t[i] = diff_quot(x, y, 0, i);
      }
      printf("\n");
      //打印各阶差商
      for (i = 0; i < len; i++) {
        printf("%12.5lf ", t[i]);

      }
      printf("\n");

      //计算插入点的函数值
    again:
      printf("输入要计算的x值: ");
      scanf("%lf", &xx);

      yy = compute_y(x, y, t, len, xx);
      printf("\nxx对应的值是 %lf: \n", yy);

      printf("继续求值吗1继续2停止？\n");
      scanf("%d", &ch);
      if (ch == 1) {
        goto again;
      }

      //小技巧来了，输入一个数字来判断是否继续输入
      printf("继续添加点吗? 1 for yes and 2 for no\n");
      scanf("%d", &ch);
      if (ch == 1) {
        continue;
      } else if (ch == 2) {
        break;
      }

    }
    system("pause");
    return 0;
  }

  double diff_quot(double *x, double *y, int start, int end) {
    if (start == end) {
      return y[start];
    } else {
      return ( (diff_quot(x, y, start, end-1) - diff_quot(x, y, start+1, end)) / (x[start] - x[end]) );
    }
  }

  double compute_y(double *x, double *y, double *t, int len, double xx) {
    double ret = t[0];
    double mul = 1.0;
    int i;
    for (i = 1; i < len; i++) {
      mul *= (xx - x[i-1]);
      ret += t[i] * mul;
    }
    return ret;
  }