C puzzles详解【34-37题】

The following is a piece of C code, whose intention was to print a minus sign 20 times. But you can notice that, it doesn‘t work. 
  #include <stdio.h>
  int main()
  {
      int i;
      int n = 20;
      for( i = 0; i < n; i-- )
          printf("-");
      return 0;
  }
Well fixing the above code is straight-forward. To make the problem interesting, you have to fix the above code, by changing exactly one character. There are three known solutions. See if you can get all those three.

for( i = 0; i < n; i-- )

for( i = 0; i < n; n-- )

What‘s the mistake in the following code? 
  #include <stdio.h>
  int main()
  {
      int* ptr1,ptr2;
      ptr1 = malloc(sizeof(int));
      ptr2 = ptr1;
      *ptr2 = 10;
      return 0;
  }

int* ptr1,ptr2;

int *ptr1,*ptr2;

What is the output of the following program? 
  #include <stdio.h>
  int main()
  {
      int cnt = 5, a;

      do {
          a /= cnt;
      } while (cnt --);

      printf ("%d\n", a);
      return 0;
  }

What is the output of the following program? 
  #include <stdio.h>
  int main()
  {
      int i = 6;
      if( ((++i < 7) && ( i++/6)) || (++i <= 9))
          ;
      printf("%d\n",i);
      return 0;
  }