Java 中使用 2D ArrayLists 的矩阵乘法 [英] Matrix multiplication in Java using 2D ArrayLists
本文介绍了Java 中使用 2D ArrayLists 的矩阵乘法的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我正在尝试用 Java 实现一个简单的矩阵乘法,我将矩阵存储在二维 ArrayList 中.似乎错误是由嵌套 for 循环内的矩阵 Result
的设置引起的,但我不明白为什么.
I am trying to implement a simple matrix multiplication in Java, where I am storing the matrices in two-dimensional ArrayLists. It would seem the error is caused by the setting of matrix Result
, inside the nested for loop, but I do not understand why.
import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;
public class Test {
public static void main(String args[]) {
int n = 2;
ArrayList<ArrayList<Integer>> Result =
new ArrayList<ArrayList<Integer>>(
Collections.nCopies(n, new ArrayList<Integer>(
Collections.nCopies(n, 0))));
ArrayList<ArrayList<Integer>> A = new ArrayList<ArrayList<Integer>>();
ArrayList<ArrayList<Integer>> B = new ArrayList<ArrayList<Integer>>();
A.add(new ArrayList<Integer>(Arrays.asList(1, 6)));
A.add(new ArrayList<Integer>(Arrays.asList(2, 2)));
B.add(new ArrayList<Integer>(Arrays.asList(0, 9)));
B.add(new ArrayList<Integer>(Arrays.asList(5, 6)));
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
for (int k = 0; k < n; k++) {
int val = A.get(i).get(k) * B.get(k).get(j);
Result.get(i).set(j, Result.get(i).get(j) + val);
}
}
}
}
}
代码产生的结果:
A * B = [[40, 75], [40, 75]]
实际上应该是:
A * B = [[30, 45], [10, 30]]
推荐答案
Result
未正确初始化.
ArrayList<ArrayList<Integer>> Result = new ArrayList<ArrayList<Integer>>(
Collections.nCopies(n,
new ArrayList<Integer>(Collections.nCopies(n, 0))));
这将创建一个矩阵,其中两行(或列,取决于您如何看待)实际上是同一行(列),被引用两次.
This creates a matrix where two rows (or columns, depending how you look at that) are in fact same row (column), referenced twice.
你可以像这样初始化Result
:
List<List<Integer>> result = new ArrayList<>();
for (int i = 0; i < n; i++) {
result.add(new ArrayList<>(Collections.nCopies(n, 0)));
}
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