Question

比较数组元素的上一个和下一个

2019-03-16

212

javascript

这似乎非常简单，但我却一无所知。

我有几个 for 循环，我想检查 i 和 j 值 -1 和 +1 ，但显然由于我正在检查的数组没有 arr[0-1] 元素，因此它会返回错误。我该如何修复它？

var islandPerimeter = function(grid) {
let result = 0;

for(var i = 0; i < grid.length; i++) {
  for(var j = 0; j < grid[i].length; j++) {
        if(grid[i][j] === 1) {
            if(grid[i-1][j] !== 1) { //left
                result += 1;
            }
            if(grid[i+1][j] !== 1) { //right
                result += 1;
            }
            if(grid[i][j+1] !== 1) { //bottom
                result += 1;
            }
            if(grid[i][j-1] !== 1) { //top
                result += 1;
            }
        }
      }  
   }
   return result;
};

因此，在我有 //left 注释的地方，我收到错误：

Uncaught TypeError: Cannot read property '1' of undefined

我怎样才能最好地防止此问题？谢谢。

这是问题描述和 输入 以及预期的 输出 ：

You are given a map in form of a two-dimensional integer grid where 1 represents land and 0 represents water.

Grid cells are connected horizontally/vertically (not diagonally). The grid is completely surrounded by water, and there is exactly one island (i.e., one or more connected land cells).

The island doesn't have "lakes" (water inside that isn't connected to the water around the island). One cell is a square with side length 1. The grid is rectangular, width and height don't exceed 100. Determine the perimeter of the island.

Input:
[[0,1,0,0],
 [1,1,1,0],
 [0,1,0,0],
 [1,1,0,0]]

Output: 16

Answer 1

您可以使用以下行作为条件

!grid[i-1] || grid[i-1][j] !== 1

这将检查 grid[i-1] 是否存在，它将跳转到 if 语句。如果存在，它将以正常方式检查其他条件。

for(var i = 0; i < grid.length; i++) {
  for(var j = 0; j < grid[i].length; j++) {
        if(grid[i][j] === 1) {
            if(!grid[i-1] || grid[i-1][j] !== 1) { //left
                result += 1;
            }
            if(grid[i+1][j] !== 1) { //right
                result += 1;
            }
            if(grid[i][j+1] !== 1) { //bottom
                result += 1;
            }
            if(grid[i][j-1] !== 1) { //top
                result += 1;
            }
        }
      }  
   }
   return result;
}

Answer 2

你可以尝试这样做

var islandPerimeter = function(grid) {
let result = 0;

for(var i = 0; i < grid.length; i++) {
  for(var j = 0; j < grid[i].length; j++) {
    if(grid[i][j] === 1) {
    if (i > 0) {    
    if(grid[i-1][j] !== 1) { //left
            result += 1;
        }
     }
        if(grid[i+1][j] !== 1) { //right
            result += 1;
        }
        if(grid[i][j+1] !== 1) { //bottom
            result += 1;
        }
        if(grid[i][j-1] !== 1) { //top
            result += 1;
        }
    }
  }  
 }
  return result;
 };

Answer 3

最初的问题是，在第一次尝试访问 grid[i-1] 时。当 i 等于 0 时， grid[-1] 不存在。考虑到这一点，一些基本验证将确保只检查定义的单元格。

然后，您只需使用括号表示法访问属性，并使用所需的值。

var islandPerimeter = function(grid) {
  let result = 0;

  for(var i = 0; i < grid.length; i++) {
    for(var j = 0; j < grid[i].length; j++) {
      if(grid[i][j] === 1) {
        if(grid[i-1] && grid[i-1][j] !== 1) { //left
          result += 1;
        }

        if(grid[i+1] && grid[i+1][j] !== 1) { //right
          result += 1;
        }

        if(grid[i][j+1] && grid[i][j+1] !== 1) { //bottom
          result += 1;
        }

        if(grid[i][j-1] && grid[i][j-1] !== 1) { //top
          result += 1;
        }
      }
    }
  }
  return result;
};

var grid = [
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
];

console.log(islandPerimeter(grid)); // returns 5 ¯\_(ツ)_/¯