Not getting all results shifting my array - c

Question

I'm trying to make a program that shifts all the elements of an array to the right by one and move the last element into the first position. My problem is when I run my code it's giving me the number 5 twice. Can someone help me, maybe my logic or my for loop is not right?
#include <stdio.h>
int main ()
{
int array[6];
int x;
int temp;
printf("Enter six numbers.\n\n");
for (x = 0; x < 6; x++) {
printf("Enter a number : ", x + 1);
scanf("%d", &array[x]);
temp = array[x - 1];
}
for (x = 6 - 1; x > 0; x--) {
array[x] = array[x - 1];
}
array[0] = temp;
for (x = 0; x < 6; x++) {
printf("%d\n", array[x]);
}
return 0;
}

Answer 1

You could make a for loop like
for(i=0; i<SIZE; ++i)
{
scanf("%d", &arr[(i+1)%SIZE]);
}
The (i+1)%SIZE would evaluate to i+1 if i+1 is less than SIZE. Otherwise, it would wrap around.
Or you can
int t=arr[SIZE-1];
for(i=SIZE-1; i>0; --i)
{
arr[i]=arr[i-1];
}
arr[0]=t;
save the last element into a temporary variable, shift other elements towards right and finally assign the first element the value in the temporary variable.
As Gourav pointed out, in the first iteration of your first for loop, arr[x-1] would become arr[-1] as x is 0. You are trying to access memory which is not part of that array. This invokes undefined behavior.

Answer 2

I will try to explain it with the very easiest algorithm and yes easy means not efficient as from performance perspective.
For example, let's say you have an array of six elements:
1 2 3 4 5 6
From the question, all I understood is that you want to reverse this array to have a final array to be:
6 5 4 3 2 1
A naive way of doing is to store the first element in a temporary variable, assign the second element to the first element and after that assign the temporary variable to the second element and repeat this until all elements are swapped as shown below:
temp = arr[0]
arr[0] = arr[1]
arr[1] = temp
You will need two loops to do this, one decrementing and one incrementing
1 2 3 4 5 6 i=0; j=5
2 1 3 4 5 6 i=1; j=5
2 3 1 4 5 6 i=2; j=5
2 3 4 1 5 6 i=3; j=5
2 3 4 5 1 6 i=4; j=5
2 3 4 5 6 1 i=5; j=5
Next, you have to execute the above loop with one less iteration:
2 3 4 5 6 1 i=0; j=4
3 2 4 5 6 1 i=1; j=4
3 4 2 5 6 1 i=2; j=4
3 4 5 2 6 1 i=3; j=4
3 4 5 6 2 1 i=4; j=4
So, the loops would be:
for(i=5; i>0; i--)
{
for(j=0; j<i; j++)
{
temp = arr[j];
arr[j] = arr[j+1];
arr[j+1] = temp;
}
}

Answer 3

You can make things a bit more generic by adding a direction to your shift routine and putting the shift code in a function that takes the array, the number of members and the direction to shift as arguments. Then its just a matter of iterating in the proper direction and shifting the elements in the correct direction and putting the final value in the proper place. For example, you could write a simple function as follows:
/** shift array index in in circular manner by 1 in the
* 0 - left or 1 - right direction.
*/
void arrayshift (int *a, size_t nmemb, int dir)
{
if (dir) { /* shift to RIGHT */
int tmp = a[nmemb - 1];
for (size_t i = nmemb - 1; i > 0; i--)
a[i] = a[i - 1];
a[0] = tmp;
}
else { /* shift to LEFT */
int tmp = a[0];
for (size_t i = 1; i < nmemb; i++)
a[i - 1] = a[i];
a[nmemb - 1] = tmp;
}
}
A simple test routine could be:
#include <stdio.h>
enum { LEFT, RIGHT };
void arrayshift (int *a, size_t nmemb, int dir);
int main (int argc, char **argv) {
int a[] = { 1, 2, 3, 4, 5, 6 },
dir = argc > 1 ? LEFT : RIGHT;
size_t n = sizeof a / sizeof *a;
printf ("original:");
for (size_t i = 0; i < n; i++)
printf (" %d", a[i]);
putchar ('\n');
arrayshift (a, n, dir);
printf ("shifted :");
for (size_t i = 0; i < n; i++)
printf (" %d", a[i]);
putchar ('\n');
return 0;
}
/** shift array index in in circular manner by 1 in the
* 0 - left or 1 - right direction.
*/
void arrayshift (int *a, size_t nmemb, int dir)
{
if (dir) { /* shift to RIGHT */
int tmp = a[nmemb - 1];
for (size_t i = nmemb - 1; i > 0; i--)
a[i] = a[i - 1];
a[0] = tmp;
}
else { /* shift to LEFT */
int tmp = a[0];
for (size_t i = 1; i < nmemb; i++)
a[i - 1] = a[i];
a[nmemb - 1] = tmp;
}
}
Example Use/Output
$ ./bin/array_cir_shift_by_1
original: 1 2 3 4 5 6
shifted : 6 1 2 3 4 5
$ ./bin/array_cir_shift_by_1 0
original: 1 2 3 4 5 6
shifted : 2 3 4 5 6 1
You can also pass the number of element to shift the array by and use the modulo operator to help with the indexing. (that is left for another day)