While循环后C中的空白值（LinkedList）

Question

I am working on a problem, that has to do with the Barnes-Hut algorithm for solving the N-Body problem.

I believe this is a C issue. But to give you some context, this method is used to get the new X and Y positions for each body.

Here's the problem, it seems to be doing the calculations correctly, and when I print out the values of which I want to see from within the loop they are properly populated.

However, when I go to print the values of the node outside the loop, they are all zeros.

I need to C programming, by can someone explain to me why this is happening, or how to go about fixing this?

I have attached the code below, and below that, I have attached the print out I am seeing from the console. Any and all help is greatly appreciated!

void reposition (struct Node *node)
{
struct Node *referenceNode = node;
int numberOfBodies = getAmountOfBodies(node->body);
float xValues [numberOfBodies];
float yValues [numberOfBodies];
int index = 0;

while (node->body->next != NULL)
{
    struct Forces * forces = getForce(node->body, node);

    //NOT SURE IF DIVIDING BY BODY MASS, OR NODE MASS
    float xVelocity = node->body->pos_x + (kDeltaTime * forces->x)/node->body->mass;
    float yVelocity = node->body->pos_y + (kDeltaTime * forces->y)/node->body->mass;
    xValues[index] = node->body->pos_x + xVelocity * kDeltaTime;
    yValues[index] = node->body->pos_y + yVelocity * kDeltaTime;
    node->body->v_x = xVelocity;
    node->body->v_y = yVelocity;
    index++;

    printf("RED: %d | GREEN: %d | BLUE: %d | xVelocity: %f | yVelocity: %f \n", node->body->red, node->body->green, node->body->blue, xVelocity, yVelocity);
    node->body = node->body->next;
}
node = referenceNode;

printf("\n");
printf("RED: %d | GREEN: %d | BLUE: %d | xVelocity: %f | yVelocity: %f \n", node->body->red, node->body->green, node->body->blue, xValues[0], yValues[0]);
}

BEFORE LOOP

RED: 255 | GREEN: 255 | BLUE: 0 | xVelocity: 0.000000 | yVelocity: 0.000000 
 RED: 0 | GREEN: 255 | BLUE: 0 | xVelocity: 57899999232.000000 | yVelocity: 0.000000 
 RED: 255 | GREEN: 0 | BLUE: 255 | xVelocity: 108200001536.000000 | yVelocity: 0.000000 
 RED: 0 | GREEN: 50 | BLUE: 255 | xVelocity: 149599993856.000000 | yVelocity: 0.000000 
 RED: 255 | GREEN: 0 | BLUE: 0 | xVelocity: 227899998208.000000 | yVelocity: 0.000000 

AFTER LOOP

RED: 0 | GREEN: 0 | BLUE: 0 | xVelocity: 0.000000 | yVelocity: 0.000000 

Answer 1

循环发布后的输出似乎表明LinkedList的最后一个非空节点中的值（因为循环末尾的位置）为零，这就是为什么要获得该输出的原因。

Answer 2

The last statement in your loop permanently changes the node's body pointer. So, after the loop referenceNode has been trashed.

Here's your code with annotations to show the problem [please pardon the gratuitous style cleanup]:

void
reposition(struct Node *node)
{
    struct Node *referenceNode = node;
    int numberOfBodies = getAmountOfBodies(node->body);
    float xValues[numberOfBodies];
    float yValues[numberOfBodies];
    int index = 0;

    while (node->body->next != NULL) {
        struct Forces *forces = getForce(node->body, node);

        // NOT SURE IF DIVIDING BY BODY MASS, OR NODE MASS
        float xVelocity = node->body->pos_x + (kDeltaTime * forces->x) / node->body->mass;
        float yVelocity = node->body->pos_y + (kDeltaTime * forces->y) / node->body->mass;

        xValues[index] = node->body->pos_x + xVelocity * kDeltaTime;
        yValues[index] = node->body->pos_y + yVelocity * kDeltaTime;
        node->body->v_x = xVelocity;
        node->body->v_y = yVelocity;
        index++;

        printf("RED: %d | GREEN: %d | BLUE: %d | xVelocity: %f | yVelocity: %f \n",
            node->body->red, node->body->green, node->body->blue,
            xVelocity, yVelocity);

        // NOTE/BUG: on the first iteration this is trashing referenceNode and
        // each subsequent node on each subsequent iteration
        node->body = node->body->next;
    }

    node = referenceNode;

    printf("\n");
    printf("RED: %d | GREEN: %d | BLUE: %d | xVelocity: %f | yVelocity: %f \n",
        node->body->red, node->body->green, node->body->blue,
        xValues[0], yValues[0]);
}

Here's the corrected code. Note I had to guess at the struct definition for what body points to [I called it Body ], but otherwise it should be fine:

void
reposition(struct Node *node)
{
    struct Node *referenceNode = node;
    struct Body *body;
    int numberOfBodies = getAmountOfBodies(node->body);
    float xValues[numberOfBodies];
    float yValues[numberOfBodies];
    int index = 0;

    body = node->body;
    while (body->next != NULL) {
        struct Forces *forces = getForce(body, node);

        // NOT SURE IF DIVIDING BY BODY MASS, OR NODE MASS
        float xVelocity = body->pos_x + (kDeltaTime * forces->x) / body->mass;
        float yVelocity = body->pos_y + (kDeltaTime * forces->y) / body->mass;

        xValues[index] = body->pos_x + xVelocity * kDeltaTime;
        yValues[index] = body->pos_y + yVelocity * kDeltaTime;
        body->v_x = xVelocity;
        body->v_y = yVelocity;
        index++;

        printf("RED: %d | GREEN: %d | BLUE: %d | xVelocity: %f | yVelocity: %f \n",
            body->red, body->green, body->blue,
            xVelocity, yVelocity);

        // NOTE/BUGFIX: here we're only advancing the pointer but _not_
        // permanently changing the node's body pointer
        body = body->next;
    }

    node = referenceNode;
    body = node->body;

    printf("\n");
    printf("RED: %d | GREEN: %d | BLUE: %d | xVelocity: %f | yVelocity: %f \n",
        body->red, body->green, body->blue,
        xValues[0], yValues[0]);
}