为结构赋值时遇到困难

Question

我使用循环和fget来运行这个输入文件。

CSCI112 Programming with C 3
CSCI127 Joy and Beauty of Data 4
CSCI132 Basic Data Structures and Algorithms 4
CSCI338 Computer Science Theory 3
CSCI215 Social and Ethical Issues in CS 3
ARCH112 Introduction to Design 3
COMX112 Interpersonal Skills in the Workplace 1
HSTA101 American History 4
XXXXXXX XXXXXX X

我必须确定CSCI类的数量(以及其他事情，但这些并不重要)。目前，我使用的是一个类结构，它包含一个字符串ID、一个字符串名和一个int信用。

我的问题是，在我的类数组中，所有的ID都被存储为"XXXXXXX“。我不明白为什么for循环没有正确地为类分配值。

如果我运行这个程序，输出是

Class: ID-XXXXXXX, Name-XXXXXX X, Credits-3
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-4
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-4
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-3
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-3
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-3
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-1
Class: ID-XXXXXXX, Name-XXXXXX X, Credits-4
CSCI classes: 0
CSCI class credits: 0

显然，这些类是不正确的，而且CSCI数字也是错误的。我不明白如何正确分配学分，但ID和名字却不是？

这是我的代码：

#include <stdio.h>
#include <string.h>
#include "class.h"

int main(){

  //variable declaration for classes
  char line[100];
  char line_copy[100];
  char ccred;

  class_t classes[100];
  int i = 0;

  //infinite loop
  for(;;){

    //read in input from file
    fgets(line, sizeof(line), stdin);
    strcpy(line_copy, line);

    classes[i].ID = strtok(line, " ");
    classes[i].name = strtok(NULL, "1234");

    //remove trailing space from class name
    classes[i].name[strlen(classes[i].name) - 1] = 0;
    ccred = line_copy[strlen(line_copy) - 2];
    classes[i].credits = ccred - '0';

    //stop reading input if not a valid class
    if(strcmp(classes[i].ID, "XXXXXXX") == 0){
      break;
    }   
    i++;            
  }

  int ii;
  for(ii = 0; ii < i; ii++){
    printf("Class: ID-%s, ", classes[ii].ID);
    printf("Name-%s, ", classes[ii].name);
    printf("Credits-%d\n", classes[ii].credits);
  }

  //find CSCI classes and credits only
  int CSCIclasses = 0;
  int CSCIcredits = 0;
  char toCompare[] = "CSCI";
  int k; 
  for(k = 0; k < i; k++){
    if(strncmp(toCompare, classes[k].ID, 4) == 0){
      printf("CSCI course!");
      CSCIclasses++;
      CSCIcredits += classes[k].credits;
    }
  }

  printf("CSCI classes: %d\n", CSCIclasses);
  printf("CSCI class credits: %d\n", CSCIcredits);


  return 0;
}

我的问题是:如何正确地分配类结构的值并获得适当数量的CSCI类和学分？

谢谢你的帮助!

Answer 1

strtok函数返回指向要标记的缓冲区的指针。这意味着所有结构的所有ID和name指针都指向同一个line数组。

您可以使用malloc和strcpy的动态分配来复制字符串(或者通用但非C标准的strdup函数)。或者对结构中的字符串使用数组(和strcpy)。

Answer 2

虽然您现在了解到，line和您所做的任何复制，或者引用line中某个地址的任何指针，都将指向line中在read循环末尾的最终内容，但是您接下来可能会遇到由于name中的空格而试图标记每一行的问题。

虽然可以使用strtok，在'space'上保存每个前一个指针，然后使用前一个指针作为name的结束指针，但是简单地使用strchr查找第一个空格，使用strrchr来设置指向nul-char的指针(或者使用strlen将指针设置为结束字符)，然后使用备份查找最后一个'space'则容易得多。这样，您知道第一个空格和最后一个空格之间的所有内容都是name，最后一个空格之后的字符是credits。这使得处理起来相当简单。

总之，您可以这样做:为credits使用一个固定数组(您知道这是6-chars )，并为name分配存储空间，并消除line_copy沿行遍历指针，选择所需的内容并不会修改line。

#include <stdio.h>
#include <stdlib.h>     /* for malloc/free */
#include <string.h>
// #include "class.h"

#define MAXID   8       /* maximum ID, characters & structs   */
#define MAXC  128       /* if you need constants, define them */
#define MAXS MAXC       /*     (don't use magic numbers)      */

typedef struct {
    char ID[MAXID],     /* fixed 8 chars for 6 + 1 needed for ID */
        *name,          /* you must allocate storage for name */
        credits;
} class_t;

int main(){

    char line[MAXC] = "",       /* variable declaration for classes */
        *toCompare = "CSCI";    /* CSCI classes and credits only */
    int CSCIclasses = 0,    
        CSCIcredits = 0,
        i, n = 0;    
    size_t maxlen = 0;
    class_t classes[MAXS] = {{ .ID = "" }};

    for (; n < MAXS; ) {    /* loop continually for input */

        char *p = line,             /* pointer to line */
            *ep = NULL;             /* end pointer for line */

        if (!fgets (line, sizeof(line), stdin))  /* read/validate line */
            break;

        ep = strchr (line, ' ');   /* set end pointer to 1st space */

        if (ep) {                       /* validate 1st space found */
            size_t len = ep - p;        /* get length of ID */
            if (len > MAXID - 1) {      /* validate it fits + nul-char */
                fprintf (stderr, "error: ID exceeds MAXID.\n");
                return 1;
            }
            strncpy (classes[n].ID, p, len);    /* copy ID */
            classes[n].ID[len] = 0;     /* nul-terminate ID */
        }
        else
            continue;                   /* get next line */

        p = ep + 1;                     /* set pointer to start of name */
        ep = strrchr (line, 0);         /* set end pointer to nul-character */

        while (ep > p && *ep != ' ')    /* backup to last space */
            ep--;

        if (*ep == ' ') {           /* if space, parse remainder o fline */
            size_t len = ep - p;    /* len of name */
            if (!(classes[n].name = malloc (len + 1))) { /* allocate */
                perror ("malloc-classes[].name");
                break;
            }
            ep++;                   /* move end pointer to credits */
            strncpy (classes[n].name, p, len);  /* copy to name */
            classes[n].name[len] = 0;       /* nul-termiante name */
            if ('0' <= *ep && *ep <= '9')   /* is credits digit? */
                classes[n++].credits = *ep - '0'; /* assign credits */
            else
                classes[n++].credits = 0;   /* set credits zero */
            if (len > maxlen)
                maxlen = len;
        }
    }

    for (i = 0; i < n; i++)     /* output class information */
        printf ("Class: ID-%s, Name-%s, %-*s Credits-%d\n", classes[i].ID, 
                classes[i].name, (int)(maxlen - strlen(classes[i].name) + 1), 
                " ", classes[i].credits);
    putchar ('\n');             /* tidy up with newline */

    for (i = 0; i < n; i++) {   /* output CSCI courses, free memory */
        if (strncmp (toCompare, classes[i].ID, 4) == 0) {
            printf("CSCI course! - %s\n", classes[i].name);
            CSCIclasses++;
            CSCIcredits += classes[i].credits;
        }
        free (classes[i].name); /* free 'name' - no longer needed */
    }

    printf ("CSCI classes: %d\n", CSCIclasses);     /* output CSCI summary */
    printf ("CSCI class credits: %d\n", CSCIcredits);

    return 0;
}

(注意：常量的定义。如果您需要一个(或多个) #define，或者使用enum来完成它。不要在代码中撒满像100这样的神奇数字。在顶部定义它们，因此，如果您的需求发生变化，您可以在一个简单的地方进行更改，其余的将自动处理)

示例使用/输出

$ ./bin/classes <dat/classes.txt
Class: ID-CSCI112, Name-Programming with C,                      Credits-3
Class: ID-CSCI127, Name-Joy and Beauty of Data,                  Credits-4
Class: ID-CSCI132, Name-Basic Data Structures and Algorithms,    Credits-4
Class: ID-CSCI338, Name-Computer Science Theory,                 Credits-3
Class: ID-CSCI215, Name-Social and Ethical Issues in CS,         Credits-3
Class: ID-ARCH112, Name-Introduction to Design,                  Credits-3
Class: ID-COMX112, Name-Interpersonal Skills in the Workplace,   Credits-1
Class: ID-HSTA101, Name-American History,                        Credits-4
Class: ID-XXXXXXX, Name-XXXXXX,                                  Credits-0

CSCI course! - Programming with C
CSCI course! - Joy and Beauty of Data
CSCI course! - Basic Data Structures and Algorithms
CSCI course! - Computer Science Theory
CSCI course! - Social and Ethical Issues in CS
CSCI classes: 5
CSCI class credits: 17

仔细看一看，如果你还有什么问题，请告诉我。