shifting a sequence of numbers in C?

Question

I have a question about some trying to wrap around a sequence of numbers that I'm trying to shift in the C programming language. The first value that is found in the sequence of numbers I calculate via a loop gets thrown out in the end. Here is what the code looks like right now:

numbers[d] = numbers[x];
for (d = y-1; d >=0; d --){
 numbers[d] = numbers[(d -  1) % y];
 printf(" numbers[d] = %d \n", numbers[d]);
}

Here are the numbers[x] I calculated from my previous loop:

1, 17, 3, 15, 14, 6, 12, 8, 10

Here is what the numbers[d] currently looks like:

17, 3, 15, 14, 6, 12, 8, 10, 10

...and here is what it should look like:

17, 3, 15, 14, 6, 12, 8, 10, 1

It seems like it doesn't wrap the 1 around to the end. Is there a conditional that I am missing in my loop? Thanks!

Answer 1

Code

#include <stdio.h>

static const int debug = 0;

static void dump_array(const char *tag, int n, const int array[n])
{
    printf("%s (%d)", tag, n);
    for (int i = 0; i < n; i++)
        printf("%3d", array[i]);
    putchar('\n');
}

static void rot1u(int n, int numbers[n])
{
    int v = numbers[n-1];
    for (int d = n - 1; d >= 0; d--)
    {
        numbers[d] = numbers[(n + d - 1) % n];
        if (debug)
            printf(" numbers[%d] = %d\n", d, numbers[d]);
    }
    numbers[0] = v;

    dump_array("Up After: ", n, numbers);
}

static void rot1d(int n, int numbers[n])
{
    int v = numbers[0];
    for (int d = 0; d < n; d++)
    {
        numbers[d] = numbers[(d + 1) % n];
        if (debug)
            printf(" numbers[%d] = %d\n", d, numbers[d]);
    }
    numbers[n-1] = v;

    dump_array("Dn After: ", n, numbers);
}

int main(void)
{
    int numbers[] = { 1, 17, 3, 15, 14, 6, 12, 8, 10 };
    enum { N_NUMBERS = sizeof(numbers) / sizeof(numbers[0]) };
    dump_array("-- Before:", N_NUMBERS, numbers);

    rot1u(N_NUMBERS, numbers);
    rot1d(N_NUMBERS, numbers);
    rot1d(N_NUMBERS, numbers);
    rot1d(N_NUMBERS, numbers);
    rot1u(N_NUMBERS, numbers);
    rot1u(N_NUMBERS, numbers);

    return 0;
}

Example output

-- Before: (9)  1 17  3 15 14  6 12  8 10
Up After:  (9) 10  1 17  3 15 14  6 12  8
Dn After:  (9)  1 17  3 15 14  6 12  8 10
Dn After:  (9) 17  3 15 14  6 12  8 10  1
Dn After:  (9)  3 15 14  6 12  8 10  1 17
Up After:  (9) 17  3 15 14  6 12  8 10  1
Up After:  (9)  1 17  3 15 14  6 12  8 10

Answer 2

Let's analyze your for loop, minus the printf statement.

for (d = y-1; d >=0; d --){
 numbers[d] = numbers[(d -  1) % y];
}

Before you start the loop, you have the following values in numbers .

1, 17, 3, 15, 14, 6, 12, 8, 10

The value of y is 9 .

In the first iteration of the loop, d = 8 . (d-1)%y = 7 . You replace the value of number[8] by number[7] . The array becomes:

1, 17, 3, 15, 14, 6, 12, 8, 8

In the next iteration of the loop, d = 7 . (d-1)%y = 6 . You replace the value of number[7] by number[6] . The array becomes:

1, 17, 3, 15, 14, 6, 12, 12, 8

When you reach the iteration where d=1 , (d-1)%y = 0 . You replace the value of number[1] by number[0] . The array becomes:

1, 1, 17, 3, 15, 14, 6, 12, 8

In the next iteration, d=0 , (d-1)%y = -1 . The statement

 numbers[d] = numbers[(d -  1) % y];

is equivalent to

 numbers[0] = numbers[-1];

This certainly leads to undefined behavior but it doesn't explain the other numbers in your output. Maybe the output that you posted corresponds to a different block of code.

I think the answer by @JonathanLeffler gives a solution to your algorithmic problem. I won't repeat that here.

Answer 3

您需要保存在循环之前应该旋转到另一侧的元素，然后才将其放入适当的位置（将它们放入适当的位置）。

Answer 4

#include <stdio.h>
#include <time.h>
#define METHOD 2
#define MAX_SKIERS 20
int starting_lineup[MAX_SKIERS+1];
int main(void)
{
int i, num_skiers = 20;
srand(time(NULL));
int pos1, pos2, temp;
for (i = 0; i <= num_skiers; i++)
starting_lineup[i] = i;
for (i = 0; i < num_skiers*2; i++) {
// Generate two random positions
pos1 = rand() % num_skiers + 1;
pos2 = rand() % num_skiers + 1;
// Swap the skiers at the two positions
temp = starting_lineup[pos1];
starting_lineup[pos1] = starting_lineup[pos2];
starting_lineup[pos2] = temp;
}
printf("The starting lineup (first to last):\n");
for (i = 1; i <= num_skiers; i++)
printf("%s%d", (i == 1 ? "" : ", "), starting_lineup[i]);
putchar('\n');
return 0;
}