C: reverse array in subrange - c

Question

I am trying to implement left array rotation using the method described here: http://www.cs.bell-labs.com/cm/cs/pearls/s02b.pdf (under section Reversal algorithm)
I am having trouble reversing the array when the start index isn't 0.
Here is what I have so far:
void reverse_arr(int *a, int start, int end)
{
int i;
int len = end - start;
//printf("Len pre loop: %d\n", len);
int swap;
for(i = start; i < --len; i++)
{
//printf("start: %d\tlen: %d\n", start, len);
swap = a[i];
a[i] = a[len];
a[len] = swap;
}
}
This works great when the start index is 0, but when it is anything else it never reverses all of the elements.
ex:
int test[] = {1,2,3,4,5,6};
reverse_arr(test, 0, 2); //reverse the first 2 elements of the array
results in: {2,1,3,4,5,6} which is expected
int test[] = {1,2,3,4,5,6};
reverse_arr(test, 2, 6); //reverse the last 4 elements of the array
results in: {2,1,4,3,5,6} which is not expected, only the 3 and 4 were reversed.
Any help would be appreciated.

Answer 1

You need to adjust the other subscript in the swap:
void reverse_arr(int *a, int start, int end)
{
int len = end - start;
for (int i = start; i < --len; i++)
{
int swap = a[i];
a[i] = a[i+len];
a[i+len] = swap;
}
}
You could probably also use:
void reverse_arr(int *a, int start, int end)
{
int len = end - start;
end--;
for (int i = start; i < --len; i++, end--)
{
int swap = a[i];
a[i] = a[end];
a[end] = swap;
}
}
Working test code
#if defined(VERSION1)
static void reverse_arr(int *a, int start, int end)
{
int i;
int len = end - start;
int swap;
for (i = start; i < --len; i++)
{
swap = a[i];
a[i] = a[i+len];
a[i+len] = swap;
}
}
#else
static void reverse_arr(int *a, int start, int end)
{
int i;
int len = end - start;
end--;
for (i = start; i < --len; i++, end--)
{
int swap = a[i];
a[i] = a[end];
a[end] = swap;
}
}
#endif
#define DIM(x) (sizeof(x)/sizeof(x[0]))
#include <stdio.h>
static void print_array(int *array, size_t size)
{
for (size_t i = 0; i < size; i++)
printf(" %d", array[i]);
putchar('\n');
}
static void tester(int lo, int hi)
{
int test[] = {1,2,3,4,5,6};
printf("Before: (%d, %d)\n", lo, hi);
print_array(test, DIM(test));
reverse_arr(test, lo, hi); //reverse the first 2 elements of the array
printf("After: (%d, %d)\n", lo, hi);
print_array(test, DIM(test));
putchar('\n');
}
int main(void)
{
tester(0, 2);
tester(2, 6);
return(0);
}
And the results from compiling with -DVERSION1 or not are the same:
Before: (0, 2)
1 2 3 4 5 6
After: (0, 2)
2 1 3 4 5 6
Before: (2, 6)
1 2 3 4 5 6
After: (2, 6)
1 2 6 4 5 3

Answer 2

You should drop len altogether, and use end instead. While you are at it, you can also drop i, and use start. Your code would become a lot more readable that way:
void reverse_arr(int *a, int start, int end)
{
int swap;
while(start < end)
{
swap = a[start];
a[start++] = a[--end];
a[end] = swap;
}
}

Answer 3

It's because you're comparing i against len which is fine if it starts at 0 but not if it starts at anything else (the base values are different).
For example if you want to rotate three characters starting at offset 12, nothing will happen since the for loop begins in a exit state.
A quick fix is to simply adjust a before beginning the loop so that it effectively thinks the (local value) a is the start of the array:
a += start;
for (i = start; i < --len; i++) // No change on this line.