100

main(){{}{}{}

   {;;;;;;}
 {          }
{            }
{            }
{            }
 {          }
   {;;;;;;}

   {;;;;;;}
 {          }
{            }
{            }
{            }
 {          }
   {;;;;;;}}

Cannot open that file

Cannot open that file.

int main()
{
  char filename[80];
  FILE *fp;
  printf("input file name: ");
  fgets(filename, 79, stdin);
  fp = fopen(filename, "r");
  assert(fp != NULL);
  fclose(fp);
}

C:\SVN\book\C>ls -l fgets.c
-rw-rw-rw-   1 user     group         225 Dec 29 17:23 fgets.c
C:\SVN\book\C>a.exe
input file name: fgets.c
Assertion failed: fp != NULL, file fgets.c, line 10

變數靈異事件

#include <stdio.h>
#include <string.h>
int main(void)
{
  char source[] = "This is a string.";
  int i = 5;
  char destination[4];
  strcpy(destination, source);
  printf("i is %d\n", i);
  printf("source is [%s]\n", source);
  printf("destination is [%s]\n", destination);
  return 0;
}

The output.

i is 544434464
source is [a string.]
destination is [This is a string.]

CSIE rules!

struct csie {
  char c;
  short s;
  int i;
  double e;
};  
struct ceis {
  char c;
  double e;
  int i;
  short s;
};  
int main(void)
{
  printf("csie = %d\n", sizeof(struct csie));
  printf("ceis = %d\n", sizeof(struct ceis));
  return 0;
}

The output.

csie = 16
ceis = 24

一個打十個

之前的恆等式可能不好了解. 換這個試看看.

#include <stdio.h>
int main()
{
  int a[10];
  if (a == &a)
    printf("yes\n");
  else
    printf("no\n");

  if (a + 10 == &a + 1)
    printf("yes\n");
  else
    printf("no\n");
}

輸出變這樣.

yes

yes

7 * 7 = 19

The output is 19, not 49.

#include <stdio.h>
#define square(x) (x * x)
int main()
{
  int i = 3;
  int j = 4;
  printf("%d\n", square(i + j));
}

為何不用 char c 寫檔

有朋友問到之前 255 and 256 的例子為何不用 char c 寫檔.
如果你的膽子和硬碟都很大, 請自行嘗試. 後果P老師概不負責.

#include <stdio.h>

int main(void)
{
  FILE *fp;
  char c;
  fp = fopen("file", "wb");
  for (c = 0; c < 256; c++)
    fputc(c, fp);
  fclose(fp);
  return 0;
}