如何从2D java数组中获取列？

I know that 2d arrays are arrays of arrays. To get a row you can do:

我知道2d数组是数组的数组。要获得一行，你可以做：

rowArray = my2Darray[row]

Since each row can be a different size, I'm assuming it's not built in to get a column from a 2D array. It leads me to believe you'd have to do something like:

由于每行可以是不同的大小，我假设它不是为了从2D数组中获取列而内置的。这让我相信你必须做的事情如下：

for(int row = 0; row < numRows; row++)
{
    colArray[row] = m2Darray[row][columnOfInterest];
}

Is this correct? Is it the only way?

它是否正确？这是唯一的方法吗？

11 个解决方案

#1

If you are locked down to using a 2d array, then yes, this is it afaik. However, a suggestion that may help you (if possible):

如果你被锁定使用二维数组，那么是的，这就是它。但是，建议可以帮助您（如果可能）：

Wrap the array in a class that handles the column fetching.

将数组包装在处理列提取的类中。

Good luck.

祝你好运。

#2

Commons math has some tools you might want to check out:

Commons math有一些你可能想要查看的工具：

double[][] data = new double[10][10];
BigMatrix matrix = MatrixUtils.createBigMatrix(data);
matrix.getColumnAsDoubleArray(0);

Commons Math Library

#3

Your way is the way to go. However, if you have to do that many times, I may recommended storing it in columns. (or both ways)

你的方式是要走的路。但是，如果您必须多次这样做，我可能建议将其存储在列中。（或两种方式）

#4

int[][] array = new int[rows][coloumn];

for (int i = 0 ; i < array.length ; i++) {
    for (int j = 0 ; j < array[].length; j++) {
        int col = array[j][i]; 
    }
}

#5

Well actually I'd write this as a comment, but my reputation is still to low, so I have to answer:

那么实际上我会把它写成评论，但我的声誉仍然很低，所以我必须回答：

Guess you mean:

猜猜你的意思是：

for(int row = 0; row < numRows; row++)
{
    colArray[row] = m2Darray[row][columnOfInterest];
}

BTW: I suppose you are right. There is no easier way.

顺便说一句：我想你是对的。没有更简单的方法。

#6

Actually the newest version of Apache Commons (3.5) doesn't have BigMatrix class. Instead of this we can use RealMatrix

实际上最新版本的Apache Commons（3.5）没有BigMatrix类。而不是这个，我们可以使用RealMatrix

double[][] data = new double[10][10];
RealMatrix rm = new Array2DRowRealMatrix(data);
rm.getColumn(i);

#7

Another way is to store the rows as columns and vice versa. e.g. I needed to do exactly the same thing and I was originally planning to have an array with 10 rows and 2 cols. Because of this limitation, I just swapped my rows and columns and created an array with 10 columns and 2 rows. Then I can use array[0] to get the row from the new array (which would be a column of my original array). Of course you have this flexibility only if you are the creator of that array.

另一种方法是将行存储为列，反之亦然。例如我需要做同样的事情，我原本计划有一个包含10行和2列的数组。由于这个限制，我只是交换了我的行和列，并创建了一个包含10列和2行的数组。然后我可以使用array [0]从新数组中获取行（这将是我原始数组的一列）。当然，只有当您是该阵列的创建者时，才具有这种灵活性。

Hope that helps...

希望有帮助......

#8

    /**
     input data
    **/
    int[][] r = {
            {1, 2},
            {3, 4}
    };
    //the colum index
    int colIndex = 1;
    Integer[] col = Arrays.stream(r).stream().map(arr -> arr[colIndex]).toArray(size -> new Integer[size]);
    //col data
    for (Integer integer : col) {
        System.out.println(integer);
    }

Here it will print one whole column data in 2D matrix.

这里它将以2D矩阵打印一个完整的列数据。

#9

-1

You have to use StringBuilder class to append new character at the end of a string

您必须使用StringBuilder类在字符串的末尾追加新字符

    StringBuilder s=new StringBuilder();
    for(int i=0;i<n;i++)
        s.append(arr[i][column]);

#10

-2

Just came across this post by chance. Another way to perform operations such as array copying or manipulation on column arrays is to transpose your array/matrix.

刚碰到这篇文章的机会。在列阵列上执行阵列复制或操作等操作的另一种方法是转置阵列/矩阵。

Shortly speaking

简短地说

a. transpose 2Darray / matrix (i.e. 6x5 ==> 5x6 2Darray)
一个。转置2D阵列/矩阵（即6x5 ==> 5x6 2Darray）
Perform operations on column arrays
对列数组执行操作
transpose again ==> get back to your original 2Darray.
再次转置==>回到原来的2D阵列。

This approach have been used in seam carving - image cropping technique

这种方法已被用于接缝雕刻 - 图像裁剪技术

#11

-4

try this

尝试这个

int column = 3;
double result = array[][column];

Good Luck

祝你好运

#1