C puzzles

Post by jonsen » Thu, Nov 10, 2016, 4:35 PM UTC

mikeash wrote: post #17

ohyoutravel wrote: post #15

Earlier quoted context omitted.

This one, for example. It's just a typo, not really a "puzzle." (apparently I don't know how to write code on hn) #include int main() { int a=10; switch(a) { case '1': printf("ONE\n"); break; case '2': printf("TWO\n"); break; defa1ut: printf("NONE\n"); } return 0; } If you expect the output of the above program to be NONE, I would request you to check it out!!

Place two or more spaces at the beginning of a line to get code formatting. Edit: two, not four. Thanks, jonsen.

  Text after a blank line 
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Post by 0xmohit » Thu, Nov 10, 2016, 4:43 PM UTC

fisherjeff wrote: post #20

ohyoutravel wrote: post #15

Earlier quoted context omitted.

This one, for example. It's just a typo, not really a "puzzle." (apparently I don't know how to write code on hn) #include int main() { int a=10; switch(a) { case '1': printf("ONE\n"); break; case '2': printf("TWO\n"); break; defa1ut: printf("NONE\n"); } return 0; } If you expect the output of the above program to be NONE, I would request you to check it out!!

It is just a typo. What makes it "puzzling", though, is that it compiles .

  It is just a typo.

Post by swah » Thu, Nov 10, 2016, 5:01 PM UTC

0xmohit wrote: post #10

adamnemecek wrote: post #7

I found this presentation pretty useful http://www.slideshare.net/olvemaudal/deep-c

Alternate link: http://www.pvv.org/~oma/DeepC_slides_oct2011.pdf

Post by asamy45 » Thu, Nov 10, 2016, 5:14 PM UTC

mikeash wrote: post #14

hatsunearu wrote: post #13

That IA-64 one is really puzzling, anyone know what the heck is happening?

There's no #include first, so you get an implicit prototype for malloc. With an implicit prototype, the function is assumed to return int. The cast then converts the returned int to int*. This works on 32-bit where int is the size of a pointer, but on 64-bit with 32-bit ints, the top half of the pointer gets chopped off and you end up with a nonsense value. This is why it's considered bad form to cast the result of m…

Post by nickff » Thu, Nov 10, 2016, 5:21 PM UTC

CapacitorSet wrote: post #3

I really dislike his choice of coding style. Compare: int CountBits (unsigned int x ) { static unsigned int mask[] = { 0x55555555, 0x33333333, 0x0F0F0F0F, 0x00FF00FF, 0x0000FFFF } ; int i ; int shift ; /* Number of positions to shift to right*/ for ( i =0, shift =1; i > shift) & mask[i]); return x; } as opposed to: int countBits (unsigned int x) { static unsigned int mask[] = { 0x55555555, 0x33333333, 0x0F0F0F0F, 0x0…

  unsigned int countBits (unsigned int x)
  {
      int i;
      int shift; // number of positions to shift to the right
      static unsigned int mask[] = {
      0x55555555,
      0x33333333,
      0x0F0F0F0F,
      0x00FF00FF,
      0x0000FFFF};

      for (i = 0, shift = 1; i > shift) & mask[i]);

      return x;
  }