问题
i have a code that create 2d array using user input and it work fine but now i have 2 questions
the first one: how to convert 2d array to 1d array?
second question: how to choose or trace the elements above the right diagonal in the 2d array?
anyone can help me to fix this code?
this my code
package question3;
import java.util.Arrays;
import java.util.Collection;
import java.util.Scanner;
public class Array2d {
public static void main(String[] args) {
int[][] matrix = new int[3][3];
int[] array = new int[matrix.length * matrix.length];
Scanner sc = new Scanner(System.in);
System.out.print("Please enter 9 integers separated by spaces:");
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix.length; j++) {
matrix[i][j] = sc.nextInt();
}
}
int idx = 0;
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix.length; column++) {
System.out.print(matrix[row][column] + " "); // Outputs the // array in a // 5x5 grid.
}
System.out.println();
}
for (int column = 0; column < matrix.length; column++) {
for (int row = column + 1; row < matrix.length+column ; row++){
// populate your array here
array[idx] = matrix[row][column];
// increment index
idx++;
System.out.println(matrix[row][column]);
}
}
}
}
output
Please enter 9 integers separated by spaces: 1 2 3 4 5 6 7 8 9
1 2 3
4 5 6
7 8 9
4 7 8
but what i expect 2 , 3 , 6
where the change that i need to make because i am stuck and i know that is in the third for loop
回答1:
Well if you run the following code,
public class Main {
public static void main(String[] args) {
int[][] matrix = new int[5][6];
System.out.println(matrix.length);
int[] matrix2 = matrix[4];
System.out.println(matrix2.length);
}
}
You will see that it prints out
5
6
So initially you have an array that has a length of 5, and there contains 5 int[] that have a length of ˛6 each.
Therefore it is stored in the pattern of
1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25
26 27 28 29 30
So what do you want to do in order to put these into an array? You need to go from topleft to right, and then down a row each time.
int newArray[] = new int[matrix.length*matrix[0].length];
for(int i = 0; i < matrix.length; i++) {
int[] row = matrix[i];
for(int j = 0; j < row.length; j++) {
int number = matrix[i][j];
newArray[i*row.length+j] = number;
}
}
And that should work.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Complete code to see for yourself:
public class Main {
public static void main(String[] args) {
int[][] matrix = new int[5][6];
int counter = 1;
for(int i = 0; i < 5; i++) {
for(int j = 0; j < 6; j++) {
matrix[i][j] = counter++;
}
}
int newArray[] = new int[matrix.length*matrix[0].length];
for(int i = 0; i < matrix.length; i++) {
int[] row = matrix[i];
for(int j = 0; j < row.length; j++) {
int number = matrix[i][j];
newArray[i*row.length+j] = number;
}
}
for(int i = 0; i < newArray.length; i++) {
System.out.print(newArray[i] + " ");
}
}
}
回答2:
matrix.length and array.length both will return 3 , So your 1D array will be of size 3 whereas you have total of 9 elements
So you cannot use this , Now if your 2D array is a square matrix then you have to create 1 DArray of size
int[] array = new int[matrix.length * matrix.length];
Then Simply traverse the 2 D Array and insert each element in 1 D Array as @Prashant suggested with a little bit of modification
int k = 0;
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
array[k++] = matrix[i][j];
}
}
回答3:
use loop and assign 1-d array as:
int[] array = new int[matrix.length * matrix[0].length];
int k = 0;
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix.length; j++) {
array[k++] = matrix[i][j];
}
}
回答4:
You have defined the array size equal to the number of rows of the matrix. It needs to have row * column elements. Populate the array as you traverse the matrix in the order you see fit. This is an example:
package question3;
import java.util.Arrays;
import java.util.Scanner;
public class Array2d {
public static void main(String[] args) {
int[][] matrix = new int[3][3];
// array must have row * col elements to hold the entire matrix
int[] array = new int[matrix.length * matrix[0].length];
Scanner sc = new Scanner(System.in);
System.out.println("Please enter 9 integers separated by spaces:");
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix.length; j++) {
matrix[i][j] = sc.nextInt();
}
}
// Index to step through array
int idx = 0;
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix.length; column++) {
System.out.print(matrix[row][column] + " ");
// populate your array here
array[idx] = matrix[row][column];
// increment index
idx++;
}
System.out.println();
}
System.out.println("the Matrix becomes " + Arrays.toString(array));
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length - row - 1; column++) {
// Work with matrix above right diagonal here, matrix[row][column];
System.out.println(matrix[row][column]);
}
}
}
}
The first index of a two-dimensional array is the row, the second is the column. You have them named incorrectly in the second for loop.
To access the everything above the diagonal that runs from bottom-left to top-right, excluding the diagonal, step through each row and then each column up to but not including row length - row index. Something like this:
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length - row - 1; column++) {
// Work with matrix above right diagonal here, matrix[row][column];
System.out.println(matrix[row][column]);
}
}
回答5:
With Kotlin, you could just do;
fun Array<IntArray>.toVector(): IntArray {
if (this.isEmpty()) return IntArray(0)
return this.reduce { firstArr, nextArr ->
firstArr.plus(nextArr)
}
}
This would return an int array comprising of all rows of the 2D array.
来源：https://stackoverflow.com/questions/28305359/how-to-convert-2d-array-into-1d