使用qsort和结构对列表进行排序

Question

我是C初学者，使用结构体。我试图创建一个对日期列表进行排序的程序。用户首先输入日期的数量，然后输入日期本身，即月份，日期和年份。然后使用qsort我想按时间顺序排序（首先是年份，然后是月份然后是一天）。我试着先排序一年，但是我只输出了“0”。

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

typedef struct { 
    char* month; 
    int day; 
    int year; 
} date; 

int sort(const void* a, const void* b) 
{ 

    date* date1 = (date*)a; 
    date* date2 = (date*)b; 

    if (date2->year != date1->year) { 
     int year2 = date2->year; 
     int year1 = date2->year; 
     if (year1 < 14) { 
      year1 = year1 + 100; 
     } 
     if (year2 < 14) { 
      year2 = year2 + 100; 
     } 
     int yearcompare = year2 - year1; 
     return -yearcompare; 
    } 
} 

output(date* ar, int i, int n) 
{ 

    for (i = 0; i < n; i++) { 
     //printf("Enter the date (month day year) i n the following format: text number number"); 
     // printf("%s ", ar[i].month); 
     //printf("%d ", ar[i].day); 
     printf("%d\n", ar[i].year); 
    } 
} 

int main() 
{ 
    int n; 
    int i; 
    int MIN_SIZE = 0; 
    int MAX_SIZE = 1000; 

    while (1) { 
     printf("Enter number of dates you want to enter (between 1 and 10000):\n"); 
     scanf("%d", &n); 

     if (n < MIN_SIZE) { 

      printf("You have entered a number lower than 0\n"); 
     } 

     if (n > MAX_SIZE) { 

      printf("You have entered a number higher than 1000\n"); 
     } 

     else { 

      break; 
     } 
    } 

    date* ar = malloc(sizeof(int) * n); 
    //ALLOCATE MEMORY 

    printf("Enter the date (month day year) in the following format: text, number(between 1 and 31), number(between 00 and 12): \n"); 
    for (i = 0; i < n; i++) { 
     scanf("%s", ar[i].month); 
     scanf("%d", &ar[i].day); 
     scanf("%d", &ar[i].year); 
    } 

    qsort(ar, n, sizeof(date), sort); 

    output(ar, i, n); 
} 

Answer 1

看起来好像需要相当多的帮助将拼图的所有部分放在一起。首先，在的date中，您包含char *month。这是一个指针，当您分配ar时将会未初始化，这意味着您需要单独分配ar[i].month。你可以自由地这样做（在这种情况下你可以有效地使用strdup），但是为什么？如果您正在接受当月的字符串输入，则最大长度为10个字符（September + nul-byte）。只需使用静态声明的month或10或更多字符并避免month上的动态分配。例如，您可以通过单个#define指令声明用于代码的有用常量，也可以使用全局enum来完成相同的操作，例如，

/* constants for max chars, max day, max year, max size */ 
enum { MAXC = 12, MAX_DAY = 31, MAX_YEAR = 2017, MAX_SIZE = 1000 }; 

typedef struct { 
    char month[MAXC]; /* either make static or allocate separately */ 
    unsigned day; 
    unsigned year; 
} date; 

下列车残骸你会遇到在混合字符和在scanf数字输入不清空每次被调用时输入缓冲器（例如stdin）。这意味着，如果用户输入的内容不是'n'的有效小数（例如，如果他不小心碰到'q'而不是'1'），则"q\n"将保留在输入缓冲区中，该输入缓冲区将作为您在下面的ar[0].month的输入。为了防止发生这种情况，您需要手动清空输入缓冲区（或者使用fgets，然后使用sscanf来解析用户输入 - 使用scanf进行用户输入有许多缺陷）。

尽管如此，您可以很容易地清空stdin。你可以用int c; while ((c = getchar()) != '\n' && c != EOF) {}做在线或创建一个简短的功能，如果你会反复调用它来减少输入，如：

/* empty character remaining in stdin */ 
void empty_stdin() 
{ 
    int c; 
    while ((c = getchar()) != '\n' && c != EOF) {} 
} 

当你把输入（是否与scanf家庭的功能或fgets（或任何其他方式），总是验证用户输入。对于所有你知道一只猫可以踩着键盘上。此外，经常检查EOF它通过按Ctrl + d或CTRL + Z指示用户取消了输入（在windoze上）例如：

 while (1) {  /* obtain valid 'n', compare with using fgets below */ 

     int rtn; /* varaible to save return of scanf -- always validate */ 

     printf ("Enter number of dates to be entered (between 1 & 1000): "); 
     if ((rtn = scanf ("%d", &n)) != 1) { /* if conversion failed */ 
      if (rtn == EOF) { /* test for user cancelation of input */ 
       fprintf (stderr, "note: user canceled input, exiting.\n"); 
       return 0; 
      }     /* otherwise simply an invalid input */ 
      fprintf (stderr, "error: invalid input.\n"); 
      goto tryagain; 
     } 

     if (n < 0) {   /* invalid input < 0 */ 
      fprintf (stderr, "error: invalid input (n < 0).\n"); 
      goto tryagain; 
     } 

     if (n > MAX_SIZE) {  /* invalid input > MAX_SIZE */ 
      fprintf (stderr, "error: invalid input (n > %d).\n", MAX_SIZE); 
      goto tryagain; 
     } 

     break;  /* if we are here - we have a good value, break */ 

     tryagain:; /* label for goto to jump over break */ 

     empty_stdin(); /* empty characters that remain in input buffer */ 
    } 

比较使用fgets和sscanf读/解析month，day，year输入。你可以做简单的东西如：

for (i = 0; i < n;) { /* loop until all elements filled */ 

     char buf[MAX_DAY + 1] = "", ans[MAXC] = ""; 

     /* if fgets return is NULL, EOF encountered */ 
     if (fgets (buf, MAX_DAY + 1, stdin) == NULL) { 
      fprintf (stderr, "note: user canceled input, exiting.\n"); 
      return 0; 
     } 

     /* parse with sscanf, validate 3 conversion took place */ 
     if (sscanf (buf, "%11s %u %u", ar[i].month, &ar[i].day, &ar[i].year) != 3) 
     { 
      fprintf (stderr, "error: invalid input.\n"); 
      continue; 
     } 

     i++; /* only increment if valid sscanf conversion took place */ 
    } 

没有必要通过int i作为参数的功能output，刚刚宣布在本地，例如：

/* output n elements of array of struct date */ 
void output (date *ar, int n) 
{ 
    int i; 

    printf ("\nOutput sorted by year:\n\n"); 

    for (i = 0; i < n; i++) 
     printf (" %s %d %d\n", ar[i].month, ar[i].day, ar[i].year); 
} 

接下来，当你的sort功能可以正常工作，您可以凝聚的排序一年，同时避免可能的溢出，通过使用不等式来代替：

/* sort struct date on year */ 
int sort (const void *a, const void *b) 
{ 
    date *date1 = (date *) a; 
    date *date2 = (date *) b; 

    if (date2->year != date1->year) 
     return (date1->year > date2->year) - (date1->year < date2->year); 

    return 0; 
} 

最后，如果您分配内存，它你有责任保存一个指向块开始的指针，然后在不再需要内存的时候保存到free。虽然它将在exit上释放，但要养成跟踪和释放所有分配内存的习惯。当你从事更复杂的项目时，良好的习惯会很好地为你服务。

干脆把它和添加提示退出，如果用户只需点击输入，而不用输入日期，你可以这样做以下：

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

/* constants for max chars, max day, max year, max size */ 
enum { MAXC = 12, MAX_DAY = 31, MAX_YEAR = 2017, MAX_SIZE = 1000 }; 

typedef struct { 
    char month[MAXC]; /* either make static or allocate separately */ 
    unsigned day; 
    unsigned year; 
} date; 

/* empty character remaining in stdin */ 
void empty_stdin() 
{ 
    int c; 
    while ((c = getchar()) != '\n' && c != EOF) {} 
} 

/* sort struct date on year */ 
int sort (const void *a, const void *b) 
{ 
    date *date1 = (date *) a; 
    date *date2 = (date *) b; 

    if (date2->year != date1->year) 
     return (date1->year > date2->year) - (date1->year < date2->year); 

    return 0; 
} 

/* output n elements of array of struct date */ 
void output (date *ar, int n) 
{ 
    int i; 

    printf ("\nOutput sorted by year:\n\n"); 

    for (i = 0; i < n; i++) 
     printf (" %s %d %d\n", ar[i].month, ar[i].day, ar[i].year); 
} 

int main (void) { 

    int i, n; 
    date *ar = NULL; 

    while (1) {  /* obtain valid 'n', compare with using fgets below */ 

     int rtn; /* varaible to save return of scanf -- always validate */ 

     printf ("Enter number of dates to be entered (between 1 & 1000): "); 
     if ((rtn = scanf ("%d", &n)) != 1) { /* if conversion failed */ 
      if (rtn == EOF) { /* test for user cancelation of input */ 
       fprintf (stderr, "note: user canceled input, exiting.\n"); 
       return 0; 
      }     /* otherwise simply an invalid input */ 
      fprintf (stderr, "error: invalid input.\n"); 
      goto tryagain; 
     } 

     if (n < 0) {   /* invalid input < 0 */ 
      fprintf (stderr, "error: invalid input (n < 0).\n"); 
      goto tryagain; 
     } 

     if (n > MAX_SIZE) {  /* invalid input > MAX_SIZE */ 
      fprintf (stderr, "error: invalid input (n > %d).\n", MAX_SIZE); 
      goto tryagain; 
     } 

     break;  /* if we are here - we have a good value, break */ 

     tryagain:; /* label for goto to jump over break */ 

     empty_stdin(); /* empty characters that remain in input buffer */ 
    } 

    empty_stdin();  /* empty characters that remain in input buffer */ 

    /* allocate array of struct ar, n elements */ 
    if ((ar = malloc (sizeof *ar * n)) == NULL) { 
     fprintf (stderr, "error: virtual memory exhausted.\n"); 
     return 1; 
    } 

    /* provide format instructions */ 
    printf ("Enter the date (month day year)\n" 
      " format, e.g.: Jan 18 2017\n\n"); 

    for (i = 0; i < n;) { /* loop until all elements filled */ 

     char buf[MAX_DAY + 1] = "", ans[MAXC] = ""; 

     printf (" date[%2d] : ", i + 1); /* prompt for input */ 

     /* if fgets return is NULL, EOF encountered */ 
     if (fgets (buf, MAX_DAY + 1, stdin) == NULL) { 
      fprintf (stderr, "note: user canceled input, exiting.\n"); 
      return 0; 
     } 

     if (*buf == '\n') { /* if first char is '\n', user just hit enter */ 
      printf ("no input provided, quit (y/n)? "); 
      if (fgets (ans, MAXC, stdin) && (*ans == 'y' || *ans == 'Y')) 
       return 0; 
      else if (!*ans) { /* if ans NULL, EOF encountered */ 
       fprintf (stderr, "note: user canceled input, exiting.\n"); 
       return 0; 
      } 
     } 

     /* parse with sscanf, validate 3 conversion took place */ 
     if (sscanf (buf, "%11s %u %u", ar[i].month, &ar[i].day, &ar[i].year) != 3) 
     { 
      fprintf (stderr, "error: invalid input.\n"); 
      continue; 
     } 

     i++; /* only increment if valid sscanf conversion took place */ 
    } 

    qsort (ar, n, sizeof (date), sort);  /* sort by year */ 

    output (ar, n);  /* output results */ 

    free (ar);  /* free ar - you allocate it, you free it */ 

    return 0; 
} 

注：有很多，很多方面几乎接近代码的每个部分。如果您查看大部分行的位置，则使用它们在验证输入。这只是最低限度的验证。您在理想情况下比较值为每个day和year打击最大/最小值，并且你会比较每个month针对查找（或哈希）表来验证每一个月是有效的月份（你也可以使用日期/时间的功能，但被留下的另外一个问题）

示例使用/输出

$ ./bin/qsortstruct 
Enter number of dates to be entered (between 1 & 1000): 4 
Enter the date (month day year) 
    format, e.g.: Jan 18 2017 

date[ 1] : September 11 2001 
date[ 2] : April 22 2010 
date[ 3] : June 2 1968 
date[ 4] : February 13 1979 

Output sorted by year: 

    June 2 1968 
    February 13 1979 
    September 11 2001 
    April 22 2010 

看东西了，一定要了解什么正在发生各个部分，并询问你是否有进一步的问题。