c - 程序比较两个整数数组 a 和 b 的元素，并将元素存储在数组 c 中，这些元素在 a 或 b 中，但不在 a 和 b 中

Question

我正在做的项目说要编写一个程序来比较两个整数数组 a 和 b 的元素，并将元素存储在数组 c 中，这些元素要么在 a 要么在 b 中，但不在 a 和 b 中。例如，数组 a 包含元素 {1, 2, 3}，数组 b 包含元素 {3, 2, 6, 7}。数组 c 应包含 {1, 6, 7}。

该函数应该使用指针算法——而不是下标——来访问数组元素。换句话说，消除循环索引变量和函数中所有 [] 运算符的使用。

现在我被困在用户输入上，但我也不知道如何做逻辑。

/* This is what I have so far I'm very stuck and would love guidance.
*/
#include <stdio.h>
#include <stdlib.h>

void find_elements(int *a, int n1, int *b, int n2, int *c, int*size);
int main()
{
    //User inputs length of arrays
    int n1 = 0;
    printf("Enter the length of the first array: ");
    scanf("%d", &n1);
    int *a;
    int *p;
    p = a;
    printf("Enter %d numbers: ", n1);
    for(p = a; p < a + n1; p++){
        scanf("%d", p);
    }
    printf("asdf");
    int n2 = 0;
    printf("Enter the length of the second array: ");
    scanf("%d", &n2);
    int *b;
    int *pb;
    pb = b;
    printf("Enter %d numbers: ", n2);
    for(pb = b; pb < b + n2; pb++){
        scanf("%d", pb);
    }
    printf("Output: \n");


}
void find_elements(int *a, int n1, int *b, int n2, int *c, int*size)
{


    return;
}

标签： c

Answer 1

正如在评论和其他答案中所说的那样，两组

int *a;
int *p;
p = a;

int *b;
int *pb;
pb = b;

是有问题的，因为您在需要分配内存时只是复制未初始化的值，而scanf写入未知地址

---

提案 ：

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

/* fill an array and return it or NULL on error, update sz if ok */
int * init(const char * nth, size_t * sz)
{
  printf("Enter the length of the %s array: ", nth);
  if ((scanf("%d", sz) != 1) || ((*sz) < 1)) {
    puts("invalid size");
    return NULL;
  }

  int * r = malloc((*sz) * sizeof(int));

  if (r == 0) {
    puts("not enough memory");
    return NULL;
  }

  printf("Enter %d numbers: ", *sz);

  int * p;

  for (p = r; p != r + (*sz); ++p) {
    if (scanf("%d", p) != 1) {
      puts("invalid value");
      free(r);
      return NULL;
    }
  }

  return r;
}

/* return 1 if v not in p, else 0 */
int absent(int v, int * p, size_t sz)
{
  int * sup = p + sz;

  while (p != sup)
    if (*p++ == v)
      return 0;

  return 1;
}

/* fill c and return number of values */
size_t find_elements(int *a, int sza, int *b, int szb, int * c)
{
  size_t szc = 0;
  int * pc = c;
  int * p, * sup;

  for (p = a, sup = a + sza; p != sup; ++p) {
    if (absent(*p, b, szb) && absent(*p, c, szc)) {
      *pc++ = *p;
      szc += 1;
    }
  }

  for (p = b, sup = b + szb; p != sup; ++p) {
    if (absent(*p, a, szb) && absent(*p, c, szc)) {
      *pc++ = *p;
      szc += 1;
    }
  }

  return szc;
}

int main()
{
  size_t sza;
  int * a = init("first", &sza);

  if (a == NULL)
    return -1;

  size_t szb;
  int * b = init("second", &szb);

  if (b == NULL)
    return -1;

  /* allocate with the worst case */
  int * c = malloc((sza + szb) * sizeof(int));

  if (c == 0) {
    puts("not enough memory");
    return -1;
  }

  size_t szc = find_elements(a, sza, b, szb, c);

  /* it is possible to use realloc to decrease the allocation size of c */

  for (int * p = c; p != c + szc; ++p)
    printf("%d ", *p);
  putchar('\n');

  free(a);
  free(b);
  free(c);
}

编译和执行

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra a.c
pi@raspberrypi:/tmp $ ./a.out
Enter the length of the first array: 3
Enter 3 numbers: 1 2 3
Enter the length of the second array: 4
Enter 4 numbers: 3 2 6 7
1 6 7 

在valgrind下：

pi@raspberrypi:/tmp $ valgrind ./a.out
==5346== Memcheck, a memory error detector
==5346== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==5346== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==5346== Command: ./a.out
==5346== 
Enter the length of the first array: 3
Enter 3 numbers: 1 2 3
Enter the length of the second array: 4
Enter 4 numbers: 3 2 6 7
==5346== Invalid read of size 4
==5346==    at 0x106C8: absent (in /tmp/a.out)
==5346==  Address 0x49d1894 is 0 bytes after a block of size 12 alloc'd
==5346==    at 0x4847568: malloc (vg_replace_malloc.c:299)
==5346==    by 0x105C3: init (in /tmp/a.out)
==5346== 
1 6 7 
==5346== 
==5346== HEAP SUMMARY:
==5346==     in use at exit: 0 bytes in 0 blocks
==5346==   total heap usage: 5 allocs, 5 frees, 2,104 bytes allocated
==5346== 
==5346== All heap blocks were freed -- no leaks are possible
==5346== 
==5346== For counts of detected and suppressed errors, rerun with: -v
==5346== ERROR SUMMARY: 2 errors from 1 contexts (suppressed: 6 from 3)