如何创建一个程序，该程序将读取c中用逗号分隔的无限用户输入字符串？

Question

I have this code that needs to compute for the rational roots of a polynomial

int n,co;
char coef1[10],coef2[10],coef3[10],coef4[10];

printf("Enter the highest degree of the input polynomial:");
scanf("%d",&n);

co=n+1;

printf("Enter %d integer coefficients starting from the 0th degree\n", co);

printf("Separate each input by a comma:");

This part was only used as a test if user input would be read correctly when the degree entered is 2

scanf("%10[^,],%10[^,],%s",coef1,coef2,coef3,coef4);

printf("%s %s %s",coef1,coef2,coef3);

My problem is how to print %10[^,] the same number of times as n that the user inputs (which should be possible for infinite input)and to add %s to the end. Also,even if I do that, I would need a way to declare coef1,coef2 etc. the same number of times as co.

Answer 1

It's not quite clear what you want. I assume that you want to read in a set of coefficients and keep track of how many coefficients were entered.

scanf isn't suited to this approach, because you already have to specify how many parameters you want to read in the format file. scanf also doesn't recognise new lines, so that a line-based format cannot be read without a baroque format syntax. fgets reads whole lines. strtok separates strings at certain separators. You could use a combination of these.

Next, you should determine how you want to store your coefficients. Using individual entities like coef1 , coef2 and so on is not very useful. Use an array.

Putting ths into practice (albeit for floting-point coefficnents), you get:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define N 10

int coeff_read(char *line, double  coeff[], int max)
{
    int n = 0;
    char *token = strtok(line, ",");

    while (token) {
        char *tail;
        double x = strtod(token, &tail);

        if (tail == token) return -1;

        if (n < max) coeff[n] = x;
        n++;

        token = strtok(NULL, ",");
    }

    return n;
}

void coeff_print(const double *coeff, int n)
{
    for (int i = 0; i < n; i++) {
        if (i) printf(", ");
        printf("%g", coeff[i]);
    }
    puts("");
}

int main()
{
    char line[200];
    double coeff[N];
    int ncoeff;

    while (fgets(line, sizeof(line), stdin)) {
        ncoeff = coeff_read(line, coeff, N);

        if (ncoeff < 0) {
            puts("Invalid input.");
            continue;
        }

        if (ncoeff > 0) {
            if (ncoeff > N) ncoeff = N;
            coeff_print(coeff, ncoeff);
        }
    }

    return 0;
}

Things to note:

• The array has a fixed number of entries, N . Only ncoeff of these are valid. This is a usual approach in C, but it means that you input will have at most N coefficients.
• If you really want "infinite" input, you must cater for big arrays. Dynamic allocation can provide arrays of variable size. That's an advanced topic. (The function coeff_read returns the actual number of coefficients, which may be more than the array size. That's not useful in this example, but you could use this information to scan the string with max == 0 , allocate as appropriate and then scan again with the allocated data.)
• strtok considers runs of consecutive commas like so ,,, as one single separator. If, for example, you want empty entries to mean zero, you'll have to use another method.
• There's another limitation here: The maximum line length. If you are going for infinite or at least very long input, you cannot be sure that the line length is sufficient. You will need another solution.
• The library function strtod provides more information in its return value, in the tail pointer and in the library's error code, errno . You could make use of it in order to find out whether the number is out of range or has extra cahacter after it. For the sake of simlicity (and laziness), I haven't.
• You want integers instead of floating-point numbers. The library function strtol is the integer equivalent to strtod .

Answer 2

Do not recommend infinite input as that is a security hole. Instead allow many.

To use scanf("%lf", &data) within a line, need to first look for '\\n' with other code.

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

int GetLineOfDoubles(FILE *stream, double *dest, size_t length) {
  int i = 0;
  int ch;

  while (1) {
    while (isspace(ch = fgetc(stream)) && ch != '\n')
      ;
    if (ch == '\n' || ch == EOF) break;

    if (i > 0) {
      if (ch != ',') return 0; // Missing comma

      while (isspace(ch = fgetc(stream)) && ch != '\n')
        ;
      if (ch == '\n' || ch == EOF) return 0;  // Missing data
    }

    ungetc(ch, stdin);

    double data;
    if (scanf("%lf", &data) != 1) return 0; // Bad data
    if (i == length) return 0; // Too much data
    dest[i++] = data;
  }

  if (i > 0) return i;
  return ch == EOF ? EOF : 0;
}