我需要一种方法来读取C中的浮点数两次。每次以< Ctrl> -d结尾 [英] I need a way to read in floats twice in C. Each time ended by <Ctrl>-d

问题描述

我需要读入多项式的系数（浮点数），并按ctrl-d标记结尾。
然后我需要读入x值并显示f（x）。还要以ctrl-d结尾。

I need to read in the coefficients (as floats) of a polynomial and mark the end by ctrl-d. Then I need to read in x values and display f(x). Also ending that with ctrl-d.

到目前为止，我已经使用 scanf 函数进行了尝试。读取系数的效果很好，但是第一次输入ctrl-d后，scanf将不会读取x值。

So far I have tried it with the scanf function. Reading the coefficients work well but after typing ctrl-d the first time, scanf will not read in x values.

#include <stdio.h>


int main(){
    int count = 0;
    float poly[33];
    while(scanf("%f", &poly[count]) != EOF){   // reading the coeffs
        count++;
    }
    printf("Bitte Stellen zur Auswertung angeben\n");

    float x;
    float res;

    while(scanf("%f", &x) != EOF){   //Here it Fails. Since scanf still sees the EOF from before
        res = 0;
        for(int i = 1; i < count; i++){
            res += poly[i-1] * x + poly[i];
        }
        printf("Wert des Polynoms an der Stelle %f: %f\n", x, res);
    }
}

推荐答案

Re -在第一次循环后打开 stdin 可能会起作用

Re-opening stdin may work after the first loop

freopen(NULL, "rb", stdin);

或考虑 @ Jonathan Leffler 的想法， clearerr（stdin）。

不使用 Ctrl d 结束输入（关闭 stdin ），使用 Enter ？

How about instead of using Ctrld to end input (which closes stdin), use Enter?

创建一个函数以读取 float 的 line 。

Create a function to read a line of float.

#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>

int read_rest_of_line(FILE *stream) {
  int ch;
  do {
    ch = fgetc(stream);
  } while (ch != '\n' && ch != EOF);
  return ch;
}

// Read a line of input of float`s.  Return count
int read_line_of_floats(float *x, int n) {
  bool char_found = false;
  int count;
  for (count = 0; count < n; count++) {
    // Consume leading white-space looking for \n - do not let "%f" do it
    int ch;
    while (isspace((ch = getchar()))) {
      char_found = true;
      if (ch == '\n') {
        return count;
      }
    }
    if (ch == EOF) {
      return (count || char_found) ? count : EOF;
    }
    ungetc(ch, stdin);
    if (scanf("%f", &x[count]) != 1) {
      read_rest_of_line(stdin);
      return count;
    }
  }
  read_rest_of_line(stdin);
  return count;
}

以上仍然需要一些有关边缘情况的工作： n == 0 ，当发生罕见的输入错误时， size_t ，处理非数字输入等。

然后需要 float 输入时使用它。

Above still needs some work concerning edges cases: n==0, when a rare input error occurs, size_t, handling of non-numeric input, etc.
Then use it whenever float input is needed.

#define FN 33
int main(void) {
  float poly[FN];
  int count = read_line_of_floats(poly, FN);

  // Please specify positions for evaluation
  printf("Bitte Stellen zur Auswertung angeben\n");

  float x;
  float res;

  while (read_line_of_floats(&x, 1) == 1) {
    res = 0;
    for (int i = 1; i < count; i++) {
      res += poly[i - 1] * x + poly[i];
    }
    // Value of the polynomial at the location
    printf("Wert des Polynoms an der Stelle %f: %f\n", x, res);
  }
}

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