当你吃一个苹果时我的 Snake 游戏崩溃了，苹果在你采摘一个新的 position (C++, SDL) 后会在你体内生成

Question

I thought I'd attempt to make Snake since it is a pretty easy game to make. I was having an issue where the apple would spawn inside the snakes body, and so I came up with a way to prevent that from happening:

void getRandomApplePos() {
    // variable that tells the while loop whether the moving of the apple was successful
    bool success;

    // variable that is set to false if the apple is inside the snakes body
    bool appleNotInside;

    // Tells the collision to stop testing for collision until the apple has successfully moved
    bool appleHasMoved;

    // sets the variables 
    success = false;
    appleNotInside = false;
    appleHasMoved = false;

    // while the apple spawns inside the snake, it keeps generating new positions 
    while (!success) {
        // random seed
        srand((unsigned int)time(NULL));

        // gets a random position
        int randomX = rand() % 769;
        int randomY = rand() % 673;

        // resets the two variables if this while loop as ran again
        apple.delta_pos_x = 0;
        apple.delta_pos_y = 0;
    
    // checks to see if the apple has spawned in the same exact position
        while (apple.delta_pos_x == 0 && apple.delta_pos_y == 0) {
            // gets the previous poition of the apple
            apple.prevPos_x = apple.x;
            apple.prevPos_y = apple.y;

            // picks a new apple position
            apple.x = round((randomX) / 32) * 32;
            apple.y = round((randomY) / 32) * 32;

            // gets the new apple position
            apple.currentPos_x = apple.x;
            apple.currentPos_y = apple.y;

            // sets the difference between the positions, if it's 0, then it has spawned in the same exact location
            apple.delta_pos_x = (float)(apple.currentPos_x - apple.prevPos_x);
            apple.delta_pos_y = (float)(apple.currentPos_y - apple.prevPos_y);
        }

        // checks to see if the snake length is only one, as to make the list not go out of index
        if (snake.bodyLength == 1) {
            // if the apple happens to spawn inside the snake with a length of 1, it will add false to the appleInSnake vector, else it adds true
            if (apple.x == snakeBody[0][0] && apple.y == snakeBody[0][1]) {
                appleNotInside = false;
                appleInSnake.push_back(appleNotInside);
            }
            else {
                appleNotInside = true;
                appleInSnake.push_back(appleNotInside);
            }
        }
        else {
            // if the apple happens to spawn inside the currently compared snakeBodyPosition, it will add false to the appleInSnake vector, else it adds true
            for (int i = 0; i < snakeBody.size(); i++) {
                if (apple.x == snakeBody[i][0] && apple.y == snakeBody[i][1]){
                    appleNotInside = false;
                    appleInSnake.push_back(appleNotInside);
                }
                else {
                    appleNotInside = true;
                    appleInSnake.push_back(appleNotInside);
                }
            }
        }

        // if false appears inside the appleInSnake vector at all, it sets success to false and goes through the loop again. Else it breaks out.
        if (std::find(appleInSnake.begin(), appleInSnake.end(), false) != appleInSnake.end()) {
            success = false;
        }
        else {
            success = true;
        }

        //clears appleInSnake so that it can take in a new comparision
        appleInSnake.clear();
    }

    // tells the collision to start back up again
    appleHasMoved = true;

}

So, whenever the apple does end up spawning inside of the snakes body, it crashes, just outright. I suspect some kind of infinite loop, but I can't put my finger on why this happens.

Answer 1

You are initializing your random number generator within your loop. Note that the RNG is deterministic . It means that you will end up drawing the same numbers all over again as in the previous loop.

Initialize the RNG once at the start of your program. This way, the numbers drawn may be expected to be different within every loop.

You might wonder, that the crude use of time() should prevent this. A typical implementation of time() will have the granularity of seconds. So you would only expect the return value to change once a second, hence, you get the same initialization over and over again in your loop.