Как найти сумму четных чисел java

asked May 6, 2014 at 2:14

Since you have a 2D array, you’ll need to iterate through rows and columns:

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:

int sum = 0;

for (int[] innerArr : arr)
    for (int i : innerArr)
        if(i % 2 == 0)
            sum += i;

return sum;

answered May 6, 2014 at 2:20

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.
So basically a for loop inside a for loop. Likewise:

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. Enhanced for loops are great for printing output, but not for assigning/checking values — especially in multi dimensional arrays.

answered May 6, 2014 at 2:18

Using modulus and branches is expensive and I expect you will find a formula to be quite a bit faster. Try

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. This would effective only add odd numbers. 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.

~i flips all the bits of i and is the same as i ^ -1 or -i - 1 or more accurately -i == ~i + 1

answered May 6, 2014 at 2:19

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);
    }
}

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class task7 {
    public static void main(String[] args) {
        int[] a;
        Scanner z = new Scanner(System.in);
        System.out.println("Enter the number of elements in the array");
        int n = z.nextInt();
        a = new int[n];
        for (int i = 0; i <= (n - 1); i++) { //Принимаем все числа массива
            Scanner x = new Scanner(System.in);
            System.out.println("Enter" + " " + i + " " + "number of the array");
            int b = x.nextInt();
            a[i] = b;
        }



        List<Integer> even = new ArrayList<Integer>();
        List<Integer> odd = new ArrayList<Integer>();
        for(int i = 0; i< n; i++){
            if (a[i]%2==0) {
                even.add(i);
            }
            else {
                odd.add(i);
         }
        }
        int sumEven = 0;
        int sumOdd = 0;
        for (int i = 0; i < n; i++){
            sumEven +=odd.get(i);
        }
        for (int i = 0; i < n; i++){
          sumOdd+=even.get(i);
        }
        System.out.println("Array of even numbers is: " + even);
        System.out.println("The sum of even numbers is " + sumEven);
        System.out.println("Array of odd numbers is: " + odd);
        System.out.println("The sum of odd numbers is " + sumOdd);
    }

}

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.println("Enter the number of elements in the array");
        int command = s.nextInt();
        int numbers[] = new int[command];
        for (int i = 0; i < numbers.length; i++) { //Принимаем все числа массива
            System.out.println("Enter" + " " + i + " " + "number of the array");
            numbers[i] = s.nextInt();
        }

        List<Integer> even = new ArrayList<Integer>();
        List<Integer> odd = new ArrayList<Integer>();
        for (int i = 0; i < numbers.length; i++) {
            if (numbers[i] % 2 == 0) {
                even.add(numbers[i]);
            } else {
                odd.add(numbers[i]);
            }
        }

        int sumEven = 0;
        int sumOdd = 0;
        for (int i = 0; i < even.size(); i++) {
            sumEven += even.get(i);
        }
        for (int i = 0; i < odd.size(); i++) {
            sumOdd += odd.get(i);
        }

        System.out.println("Array of even numbers is: " + even);
        System.out.println("The sum of even numbers is " + sumEven);
        System.out.println("Array of odd numbers is: " + odd);
        System.out.println("The sum of odd numbers is " + sumOdd);
    }
}

Не надо обявлять 2 сканнера, один можно использовать везде в классе.
Выучи Forech loop, он поможет в итерации массивов (я сам не использовал чтобы тебе был понятен код).
И я бы посоветовал давать переменным логичные имена (а не «a, b, n, x и тд.), чтобы ты сам и другой человек не терялся в коде. Если будут вопросы по моему коду задавай, не стесняйся.

ответ дан 15 мая 2018 в 22:42