[英]calculate and return the sum of all the even numbers in the 2d-array in java
This Matrix class takes a int[][]
as a parameter and saves it in the instance variable.这个 Matrix 类将
int[][]
作为参数并将其保存在实例变量中。
I need to complete the sumOfEvenNumbers
method to calculate and return the sum of all the even numbers in the array.我需要完成
sumOfEvenNumbers
方法来计算并返回数组中所有偶数的总和。
public class Matrix
{
private int[][] matrix;
/**
* Gets the sum of all the even numbers in the matrix
* @return the sum of all the even numbers
*/
public int sumOfEvenNumbers()
{
int sum = 0;
for (int[] i: array)
if (i%2 == 0){
sum += i;
}
return sum;
// TODO: Return the sum of all the numbers which are even
}
}
Since you have a 2D array, you'll need to iterate through rows and columns:由于您有一个二维数组,您需要遍历行和列:
int sum = 0;
for (int i=0;i<arr.length;i++)
for (int j=0;j<arr[i].length;j++)
if(arr[i][j] % 2 == 0)
sum += arr[i][j];
return sum;
Alternatively, use a for-each to make this even simpler:或者,使用 for-each 使这更简单:
int sum = 0;
for (int[] innerArr : arr)
for (int i : innerArr)
if(i % 2 == 0)
sum += i;
return sum;
If you are looking to apply this to a two d array, try using a normal for loop which has a column counter and a row counter, then adding up the sums.如果您希望将其应用于两个 d 数组,请尝试使用具有列计数器和行计数器的普通 for 循环,然后将总和相加。 So basically a for loop inside a for loop.
所以基本上是一个 for 循环内的 for 循环。 Likewise:
同样地:
for(int col=0; col < matrix.length; col++)
{
for(int row=0; row < matrix.length; row++)
{
//your code to access display[row][col]
}
}
Your current code only supports 1 Dimensional arrays with an enhanced for loop.您当前的代码仅支持具有增强 for 循环的 1 维数组。 Enhanced for loops are great for printing output, but not for assigning/checking values - especially in multi dimensional arrays.
增强的 for 循环非常适合打印输出,但不适用于分配/检查值 - 特别是在多维数组中。
Using modulus and branches is expensive and I expect you will find a formula to be quite a bit faster.使用模数和分支是昂贵的,我希望你会发现一个公式要快得多。 Try
尝试
public long sumOfEvenNumbers() {
long sum =0;
for (int[] arr: array)
for(int i : arr)
sum += (~i & 1) * i;
return sum;
}
If you have (i & 1) * i
this will be 0
for even and i
for odd.如果你有
(i & 1) * i
这将是0
表示偶数, i
表示奇数。 This would effective only add odd numbers.这只会有效地添加奇数。 To flip this for even numbers, we can flip the bits of
i
and use (~i & 1) * i
which is 0
for odd and i
for even.要翻转偶数,我们可以翻转
i
的位并使用(~i & 1) * i
,其中0
表示奇数, i
表示偶数。
~i
flips all the bits of i
and is the same as i ^ -1
or -i - 1
or more accurately -i == ~i + 1
~i
翻转的所有位i
和是相同的i ^ -1
或-i - 1
或更准确地-i == ~i + 1
package Homeworks;
public class HW89SumEvenIndexEvenRow {
public static void main(String[] args) {
int[][] a = {
{-5,-2,-3,7},
{1,-5,-2,2},
{1,-2,3,-4}
};
int sum=0;
for (int i=0;i<a.length;i+=1){
for (int j=0;j<a[i].length;j++)
if(i%2==0 ||j%2==0) {
sum=sum+a[i][j];
}
}
System.out.println(sum);
}
}
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