Program on pointers- explanation needed

thelink123 · 07-18-2009, 09:01 AM

Hi

Code:

   main ()
   {
      int a[] = {10,60,30,40,50};
      char *p;
      p = (char *)a;
      printf("%d",*((int *)(p+4)));
   }

output is 60

Code:

   main ()
   {
      int a[] = {10,60,30,40,50};
      char *p;
      p = (char *)a;
      printf("%d",*((int *)p+4));
   }

output is 50

Please explain how both outputs are obtained.

Regards
thelink

SciYro · 07-18-2009, 09:48 AM

This smells like homework, so Ill wont say exactly what the awnser is.

Ill assume you can figure out where the intresting part of the program is, so Ill just say that the key is in the order of operations and the effects they have on the datatype the compiler sees (importent because diffrent types have diffrent sizes).

Sergei Steshenko · 07-18-2009, 11:47 AM

Quote:

Originally Posted by thelink123

Hi

Code:

   main ()
   {
      int a[] = {10,60,30,40,50};
      char *p;
      p = (char *)a;
      printf("%d",*((int *)(p+4)));
   }

output is 60

Code:

   main ()
   {
      int a[] = {10,60,30,40,50};
      char *p;
      p = (char *)a;
      printf("%d",*((int *)p+4));
   }

output is 50

Please explain how both outputs are obtained.

Regards
thelink

You have to exactly understand how

Code:

pointer + offset

is calculated - it is not just an arithmetic sum of pointer, offset.

The answer is in any "C" textbook in the chapters on pointers.

rkski · 07-18-2009, 02:23 PM

Hi

Like SciYro said, it has to do with order of operations. Without giving an exact answer, you have to determine the data type of p in each case before the pointer addition takes place.
In one case, p is still char* and in other case it's int* and that will affect the pointer addition (ie how many bytes you are moving).

paulsm4 · 07-18-2009, 02:39 PM

the issue here isn't so much "pointers" as "precedence".

Here's another (completely different) example of (exactly the same) issue:

Quote:

int i = 3 + 5 * 2;
int j = (3 + 5) * 2;
printf ("i=%d, j=%d\n", i, j);

In other words,

Quote:

(int *)(p+4) != (int *)p+4

'Hope that helps .. PSM