Simple Pointers Problem.

[tushars]

int a[]={0,1,2,3,4};
int *p;
for(p=a+4;p>=a;p--)
{
  printf("%d",a[p-a]);
  puts("");
}

by address airthematic...
let base address is 100 then a+4=108... because int usses 2 bytes....(100+4*2=108)...
so a[p-a] should be a[8]...
and should be error...
but output is...
4
3
2
1
0


why so please someone explain.....

[laserlight]

by address airthematic...
let base address is 100 then a+4=108... because int usses 2 bytes....(100+4*2=108)...
so a[p-a] should be a[8]...

You are being inconsistent

Consider the first iteration. We know that p = a + 4. Hence, by substitution, a[p-a] would be a[a+4-a], which is a[4].

Now, if we consider your argument with addresses: the value of p is 108, and the value of a is 100. p-a = 8. But then we have to divide by 2 (i.e., by sizeof(int)), thus 8/2 = 4. So, you remembered to multiply the constant by sizeof(int) when adding it to the pointer, but forgot to divide by sizeof(int) when subtracting pointers to get an index offset.

[tushars]

thanks for your help.... but i really was not knowing that we have to divide it by 2.... anyways thank you....do you know code for 0/1 KNAPSACK PROBLEM... BY DYNAMIC PROGRAMMINg....????

[tushars]

I Am Having A Subject Of Computer Graphics In this Sem.... In This Subject Practicals Are Done In 'C'..... But I dont Know Nothing about functions of <graphics.h>.... so please can you suggest some good BEGINNERS book for learning graphics programming using 'C'..... Thanks A LoT.....