javascript - JavaScript中二维数组的“无法设置未定义的属性'4'”

Question

我在下面的 JavaScript 代码中发现了“无法设置未定义的属性 '4'”的错误。

    const kLatticeSize = 10;
    const kLatticeHalfSize = 5;
        //A vector notation designating the 4 neighborhoods of the cell on which we put the grains.
    const kDx = [1,0,-1,0];
    const kDy = [0,1,0,-1];

    const LCoord = () => {
        return {x:-1,y:-1}
    }

    const MoveStandard_1Step = (n,ih) => {
        let z_lat =  new Array(kLatticeSize).fill().map(i => new Array(kLatticeSize).fill(ih));
            // models the standard lattice Z^2
        let v_sites = new Array(kLatticeSize).fill().map(i => new Array(kLatticeSize).fill(false));
            // vertices of Z^2 which were visited during the process//which means the neighborhood cells onto which the toppled grains fall.
        v_sites[kLatticeHalfSize][kLatticeHalfSize] = true;

        let to_be_moved = new Array(kLatticeSize).fill().map(i => new Array(kLatticeSize).fill(false));
            // vertices of Z^2 which are already in the walking stack//which means the cell from which a toppling occurs.
        let odometer = new Array(kLatticeSize).fill().map(i => new Array(kLatticeSize).fill(0));
            // total number of topplings of a given vertex of Z^2


        let top = -1;
        let walking = new Array(kLatticeSize*kLatticeSize).fill().map(i => LCoord());
            //"top" is the number of sites which need to topple in the present avalanche.
            //"top = 3" means "3 sites to topple are remaining"
            //When we find a site to topple, we increment "top",
            //and then stack the site into "walking[top]"
            //So,"top" can take  0,1,2,...,(kLatticeSize * kLatticeSize - 1).

        let x = 0, y = 0, lx = 0, ly = 0;
            //(x,y): a site to topple
            //(lx,ly): a next site to topple

        let max_of_top = 0;//max of top in a simulation.
        let n_of_moves = 0;//the number of moves occured on this simulation.

        for(let i = n;i>=0;i--){
            //focusing on the origin
            if(++z_lat[kLatticeHalfSize][kLatticeHalfSize] >= 4){// one grain out of 'n' grains being put on the origin, and evaluating if the height of the origin is bigger or equal to 4(the upper limit value of height with which the cell stays stable.).
                walking[++top].x = kLatticeHalfSize;
                walking[top].y = kLatticeHalfSize;
            }
            console.log(`i:${i}\n`);
                //avalanche
            while(top >= 0){
                n_of_moves += 1;
                console.log(`top=${top}\n`);
                if(max_of_top < top){
                    max_of_top = top;
                }

                    // designate the site which topples.(origin)
                x = walking[top].x;
                y = walking[top].y;

                z_lat[x][y] = z_lat[x][y] - 4;
                if (z_lat[x][y] < 4){
                    top--;
                    to_be_moved[x][y] = false;
                }

                    //designate sites to topple
                    //designate sites onto which a grain falls.
                    //topple.
                for (let k = 0; k < 4; ++k){
                    //designating the neighborhood cells onto which the toppled grains fall.
                    // 'k' denotes a direction out of 4.(k = 0,1,2,3 : right,up,left,down) (refering to "kDx","kDy" on "abel" on "Abel.h")
                    lx = x + kDx[k];
                    ly = y + kDy[k];
                    console.log(`k=${k}:(lx,ly) = (${lx},${ly})`);
                    v_sites[lx][ly] = true;//The falling grain lands on the "true" cell which we designated right above.
                    z_lat[lx][ly]++;//The grain has been piled.

                    if (to_be_moved[lx][ly] == false && z_lat[lx][ly] >= 4){//fires when a toppling successively occurs.(when "avalanche" continues)
                            //(lx,ly) cell is going to be the next one to topple.
                            //Here in this iteration, the integer "top" can take 0,1,2,3 (other than 0)
                        walking[++top].x = lx;
                        walking[top].y = ly;
                        to_be_moved[lx][ly] = true;
                    }
                }

            }


        }

        return z_lat;
    }

    MoveStandard_1Step(30,2);

当我尝试

    MoveStandard_1Step(10,2);

它可以工作并且不会出现任何错误。但，

    MoveStandard_1Step(30,2);

它在下面得到一个错误： Uncaught TypeError: Cannot set property '4' of undefined at MoveStandard_1Step (:78:24) at :1:1 MoveStandard_1Step @ VM1627:78 (anonymous) @ VM1667:1

我猜当我将一个大数字作为函数 "MoveStandard_1Step" 的第一个参数时可能会发生错误。错误发生在 for 循环中的这一行中：

    v_sites[lx][ly] = true;//The falling grain lands on the "true" cell which we designated right above.

在 Chrome Developer 的工具上使用 console.log，我发现了发生错误的“for loop”迭代次数。只需将上面的代码放到Chrome开发者工具的控制台中就可以看到。（我不能在这里显示，因为控制台中的结果行太长，并且受到字符数的限制。）

我不明白出了什么问题。我不明白“未定义的属性'4'”是什么意思。我的猜测是“4”表示二维数组“v_sites”的索引（在函数“MoveStandard_1Step”的开头说明），但在我看来，“v_sites”的第 4 个元素是正确定义的。你能告诉我有什么问题吗？

标签： javascriptarraysgoogle-chrome-devtoolstypeerror