How exactly does pointer subtraction work in the case of an integer array?
#include<stdio.h>
int main()
{
int arr[] = {10, 20, 30, 40, 50, 60};
int *ptr1 = arr;
int *ptr2 = arr + 5;
printf("Number of elements between two pointer are: %d.",
(ptr2 - ptr1));
printf("Number of bytes between two pointers are: %d",
(char*)ptr2 - (char*) ptr1);
return 0;
}
For the first statement, the printf()
output will be 5
according to the pointer subtraction confusion
What about the second operator printf()
, what's the output?
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To quote the C11
chapter §6.5.6, Additive Operators
When two pointers are subtracted, both must point to elements of the same array object, or one after the last element of the array object; the result is the difference between the indices of the two array elements.
So when you do
printf("Number of elements between two pointer are: %d.",
(ptr2 - ptr1));
two ptr1
and ptr2
are pointers to int
, so they give the difference in the index 5. In other words, the difference between the calculated address relative to sizeof(<type>)
.
OTOH,
printf("Number of bytes between two pointers are: %d",
(char*)ptr2 - (char*) ptr1);
Both ptr1
and ptr2
written to the pointer to char
the size of which is 1 byte. The corresponding calculation is performed. Result: 20
.
FWIW , note that subtracting two pointers produces the result as a type ptrdiff_t
, and you must use a format specifier %td
to print the result.
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If you have two type pointers T
that point to elements of the same array, then the difference in pointers gives the number of type elements T
between those pointers
So the first statement of the output
printf("Number of elements between two pointer are: %d.",
(ptr2 - ptr1));
outputs 5 - number of type elements int
between pointers ptr1
and ptr2
.
This is the so-called pointer arithmetic.
Pointers (char*)ptr1
and (char*)ptr2
have the same meanings as the original pointers ptr1
and ptr2
, but they treat (reinterpret) the amount of memory as an array of type char
, each element of which has a size equal to sizeof( char )
. In C, sizeof( char )
it is always 1. Thus, the difference ( char * )ptr2 - ( char * ) ptr1
gives the number of elements of the type char
that can correspond to the memory size. Obviously
sizeof( char )
no more sizeof( int )
. Thus, there can be more elements of a type in the same memory char
than of a type int
. If, for example, sizeof( int )
is equal 4
, then the amount of memory can contain 5 * sizeof( int )
elements of the type char
that 20
.
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