I'm trying to invert and flip a two-dimensional array, but something goes wrong! Flipping works ok, but inverting is not.
Can't find a mistake right here:
public int[][] flipAndInvert(int[][] A) {
int row = -1;
int col = -1;
int[][] arr = A;
for (int i = 0; i < arr.length; i++) {
row++;
col = -1;
for (int j = arr[i].length - 1; j >= 0; j--) {
col++;
arr[row][col] = A[i][j];
}
}
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j] == 1) {
arr[i][j] = 0;
} else {
arr[i][j] = 1;
}
}
}
return arr;
}
int[][] A = { { 0, 1, 1 },{ 0, 0, 1 },{ 0, 0, 0 } };
After proceeding the output should be:
After inverting:
{1,1,0},{1,0,0},{0,0,0}
After flipping:
{0,0,1,},{0,1,1},{1,1,1}
Thanks to all a lot, the problem was here:
int[][] arr = A;
The reference of the array is being passed to arr.
What I think is that since you are using this line:
int[][] arr = A;
The reference of the array is being passed to arr, and hence the line:
arr[row][col] = A[i][j];
is equivalent to:
A[row][col] = A[i][j];
as arr has an reference to A and they both now refer to the same memory location (or they are both different names to a single variable)
You can fix this by either using the new keyword with arr and then initializing it:
int[][] arr = new int[someRows][someCols];
//use for loop to assign the value to each element of arr
Or you can run the for loop till arr[i].length/2 - 1:
for (int i = 0; i < arr.length; i++) {
row++;
col = -1;
for (int j = arr[i].length / 2 - 1; j >= 0; j--) { //here changed arr[i].length to arr[i].length / 2
col++;
arr[row][col] = A[i][j]; //for this you do not need arr and you can directly work on A and return it
}
}
The problem of your code should be this line:
int[][] arr = A;
You are assigning the reference of array A to arr and from then when you modify one you modify both of the arrays or better they are modified together because they refer to the same address.
Using apache commons lang:
int[][] matrix = new int[3][3];
for (int i = 0; i < matrix.length; i++) {
matrix[i][0] = 3 * i;
matrix[i][1] = 3 * i + 1;
matrix[i][2] = 3 * i + 2;
}
System.out.println(Arrays.toString(matrix[0]));
System.out.println(Arrays.toString(matrix[1]));
System.out.println(Arrays.toString(matrix[2]));
ArrayUtils.reverse(matrix);
System.out.println(Arrays.toString(matrix[0]));
System.out.println(Arrays.toString(matrix[1]));
System.out.println(Arrays.toString(matrix[2]));
Hope this helps.
Related
I am stuck with an exercise which tells me to create a reversed array from the given one.
After some thinking I made such code:
public int[] reverse3(int[] nums) {
int[] nums2 = new int[3];
for (int i = nums.length - 1; i >= 0; i--) {
for (int j = 0; j < nums.length; j++) {
nums2[j] = nums[i];
}
}
return nums2;
}
But it is throwing out three exact same numbers.
You don't need a nested for loop - just iterate over the source array and fill the result array in the opposite order:
public int[] reverse(int[] nums) {
int len = nums.length;
int[] result = new int[len];
for (int i = 0; i < len; ++i) {
result[len - i - 1] = nums[i];
}
}
From a first look, your code should be more like this:
public int[] reverse3(int[] nums)
{
// initialize a second array with the same length
int[] nums2 = new int[nums.length];
// initialize the nums2 index
int index = 0;
// you only need one loop for this (since we'll be incrementing the index of nums2)
for (int i = nums.length - 1; i >= 0; i--) {
nums2[index] = nums[i];
index++;
}
return nums2;
}
Swap the symetric values in the array like this :
public static void reverse(int[] nums) {
for (int i = 0; i < nums.length / 2; i++) {
int temp = nums[i];
nums[i] = nums[nums.length - 1 - i];
nums[nums.length - 1 - i] = temp;
}
To reverse an array you only have to swap the elements until the midpoint:
public int[] reverse(int[] nums) {
int numsLength = nums.length;
for (int i = 0; i < numsLength / 2; i++) {
int temp = nums[i];
nums[i] = nums[numsLength - i - 1];
nums[numsLength - i - 1] = temp;
}
return nums;
}
This way is much more optimized.
Source: How do I reverse an int array in Java?
I am new to Java development and sorry if this question will be too
silly, but it seems like I am stuck with the exercise which tells me
to create a reversed array from the given one.
After some thinking I made such code:
public int[] reverse3(int[] nums) {
int[] nums2 = new int[3];
for (int i = nums.length - 1; i >= 0; i--) {
for (int j = 0; j < nums.length; j++) {
nums2[j] = nums[i];
} } return nums2; }
But it is throwing out three exact same numbers. Could I please count
on some help? Thank you
Use one loop only. If you want to use 2 arrays(wich i do not see the point.) this will work:
int j = 0;
for(int i = nums.length -1; i >= 0; i--){
nums2[j] = nums[i];
j++;
}
But if you want to use only one array, you can do this:
for (int i = 0; i < nums.length/2; i++) {
int aux = nums[i];
nums[i] = nums[nums.length-i-1];
nums[nums.length-i-1] = aux;
}
There are so many efficient ways to do it but to make you understand i am gonna modify your own code
public int[] reverse3(int[] nums) {
int[] nums2 = new int[3];
for (int i = nums.length - 1; i >= 0; i--) {
for (int j = (nums.length-1) - i; j < nums.length; j++) {
nums2[j] = nums[i];
}
}
return nums2;
}
or let's do a a little bit modification rather than using nums.length() again and again we can put it inside a variable
public int[] reverse3(int[] nums) {
int[] nums2 = new int[3];
int length = nums.length;
for (int i = length - 1; i >= 0; i--) {
for (int j = (length-1) - i; j < length; j++) {
nums2[j] = nums[i];
}
}
return nums2;
}
Remember it is not an efficient way but to make you understand i just modify the logic. Using nested loops like that will decrease the performance so better avoid it and try to do it in much more optimized way..
I have an ArrayList and I want to create a method that will turn it into a 2d array, int[][].
This new 2d array will represent a matrix and it has to be square, so for example if I use [8, 2, 3, 0] the ressult will be {8,2}
{3,0}
public static int[][] convertIntegers(ArrayList<Integer> integers){
int m = (int) Math.sqrt(integers.size());
int[][] ret = new int[m][m];
int cont = 0;
for(int i=0; i<m+1 ; i++)
{
for(int j=0; j<m; j++)
{
cont = cont + 1;
ret[i][j] = integers.get(cont);
;
}
}
return ret;}
Your implementation is almost ok, except for some off-by-one errors:
You need to increment cont after the integers.get call, not before. If you increment before, then the first element of the list will be skipped. An easy way to fix that is to move the incrementing inside the inner loop, counting it together with j.
The outer loop should go until i < m instead of i < m + 1
With the errors fixed:
for (int i = 0, cont = 0; i < m; i++) {
for (int j = 0; j < m; j++, cont++) {
ret[i][j] = integers.get(cont);
}
}
Btw, another way is without using cont at all,
calculating the correct position using i, j and m:
for (int i = 0; i < m; i++) {
for (int j = 0; j < m; j++) {
ret[i][j] = integers.get(i * m + j);
}
}
I have a 2-dimensional array in java.
For example,
double count=0;
double[][] arr1 =new double[3][3];
for(int i=0;i<3;i++)
{
for(int j=0;j<3;j++)
{
arr1[i][j]=count;
count++;
}
}
Now, I want to remove all elements where i or j value was 1.
arr1[1][0], arr1[1][1], arr1[1][2], arr[0][1], arr[2][1] ...
How can I achieve this?
Copies into arr2.
If you had
[1][2][3]
[4][5][6]
[7][8][9]
You'd get
[1][3]
[7][9]
If you wanted
[1][-][3]
[-][-][-]
[7][-][9]
See shijima's answer
double[][] arr2 = new double[arr1.length][arr1[0].length];
int ti = 0, tj = 0;
for(int i=0; i<arr1.length - 1; i++) {
if (i > 0)
ti = i+1;
else
ti = i;
for(int j=0; j<arr1[0].length - 1; j++) {
if (j > 0)
tj = j+1;
else
tj = j;
arr2[i][j] = arr1[ti][tj];
}
}
for(int i=0;i<3;i++)
{
for(int j=0;j<3;j++)
{
if(i==1 || j==1){
arr1[i][j]=0;
}
}
}
or with much better efficiency :
for(int i=0;i<3;i++){
arr1[i][1]=0;
}
for(int i=0;i<3;i++){
arr1[1][i]=0;
}
ArrayList would make your life a lot easier since you could just call remove(int), but if you insist on arrays you could do this.
double[] tmp = arr1[0];
arr1 = Arrays.copyOfRange(arr1, 1, arr1.length);
arr1[0] = tmp;
You could just avoid adding anything if i = 1, that way you don't need to worry about removing anything, since it will be empty later anyway...
double count = 0;
double[][] arr1 = new double[3][3];
outerLoop: for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (i == 1) {
continue outerLoop;
}
arr1[i][j] = count;
count++;
}
}
for (int x = 0; x < 3; x++) {
for (int y = 0; y < 3; y++) {
System.out.println("arr1[" + x + "][" + y + "]=" + arr1[x][y]);
}
}
I try to convert 1D to 2D array, but I keep getting java.lang.ArrayIndexOutOfBoundsException, and I have tried whatever I could find on the stackoverflow or internet, but I do not understand why I have this issue?
public class Arrayto2DArray {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
int[] a = {0,1, 6, 83, 4, 5, 12, 7};
int[][] b = new int[4][4];
for (int i = 0; i < b.length; i++) {
for (int j = 0; j < b[i].length; j++) {
b[i][j]=0;
System.out.print(b[i][j]);
}
System.out.println();
}
System.out.println("--------------------------");
for (int i = 0; i < 4; i++) {
for (int j = 0; j < b[i].length; j++) {
try{
b[i][j] = a[i+j*4];
}catch(Exception e){
e.printStackTrace();
System.out.println(e);
}
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
System.out.print(b[i][j]);
}
System.out.println();
}
}
}
I kind of know why I get this error and it because of this line
b[i][j] = a[i+j*4];
but I cannot come up any formula better than this.
Consider the second for-loop
Lets say when i = 3 and j= 3
a[i+j*4] evaluates to a[15] which is out of the array
When you declare you 2-d array, you specified int[][] b = new int[4][];, meaning that the first inner for loop for (int j = 0; j < b[i].length; j++) should result in a NullPointerException since b[i].length has no predefined length. Before intering the inner for loop, you should define the size of each b[i] like b[i] = new int[somenumber]
In regards to convert the 1d loop to a 2d, you need to define the rule around spliting it into the 2-d array. Then accordingly the second for loop need modification
EDIT:You modified your code to have an int[4][4] array, which means you have 16 placeholders. Your 1-d array contain only 8 placeholders. It depends on how you want to sort the array, like it can be
b 0 1 2 3
0 0 1 6 83
1 4 5 12 7
2 0 0 0 0
3 0 0 0 0
or any other pattern
Assuming the length of 1-d array is 8 and the total index of 2-d array is 16, the following is more of a general solution:
public static void main(String[] args) {
// TODO code application logic here
int[] a = { 0, 1, 6, 83, 4, 5, 12, 7 };
int[][] b = new int[4][4];
for (int i = 0; i < b.length; i++) {
for (int j = 0; j < b[i].length; j++) {
b[i][j] = 0;
System.out.print(b[i][j] + "\t");
}
System.out.println();
}
System.out.println("--------------------------");
for (int i = 0; i < 4; i++) {
for (int j = 0; j < b[i].length; j++) {
try {
if ((j + i * 4) < a.length)
b[i][j] = a[j + i * 4];
} catch (Exception e) {
e.printStackTrace();
System.out.println(e);
}
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
System.out.print(b[i][j] + "\t");
}
System.out.println();
}
}
What is wrong is your 2D array is a 4*4, so that means for each index, there will be 4 elements i.e. say values. So there would be a total of 16 values, so you would need a 1D array with 16 elements for the above code to work. Right now you have only 8 values in your 1D array.
I think your code should change like this
public class Arrayto2DArray {
public static void main(String[] args) {
// TODO code application logic here
int[] a = {0,1, 6, 83, 4, 5, 12, 7};
int[][] b = new int[4][2];
for (int i = 0; i < b.length; i++) {
for (int j = 0; j < b[i].length; j++) {
b[i][j]=0;
System.out.print(b[i][j]);
}
System.out.println();
}
System.out.println("--------------------------");
for (int i = 0; i < 4; i++) {
for (int j = 0; j < b[i].length; j++) {
try{
b[i][j] = a[i+j*4];
}catch(Exception e){
e.printStackTrace();
System.out.println(e);
}
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 2; j++) {
System.out.print(b[i][j]+" ");
}
System.out.println();
}
}
}
Your correct formula should be like.
int[] a = {0,1, 6, 83, 4, 5, 12, 7};
int[][] b = new int[4][2];
int k=0;
for (int i = 0; i < b.length; i++) { // Column of 2D array
for (int j = 0; j < b[0].length; j++) { // Row of 2D array
b[i][j]= a[k++];
System.out.println(b[i][j]);
}
}
Edit:
For general case, If your row is fixed at 4 but column is not fixed than your 2D conversion formula should be like below.
int col = a.length / 4;
int remainder = a.length % 4;
if (remainder > 0) {
col = col + 1; // Get the correct column size.
}
int[][] b = new int[4][col];
int k = 0;
for (int i = 0; i < 4; i++) {
for (int j = 0; j < col; j++) {
if (k < a.length) {
b[i][j] = a[k];
System.out.println(b[i][j]);
}
k++;
}
}
This is the assignment: Write a method that sorts the elements of a matrix with 2 dimensions. For example
sort({{1,4}{2,3}})
would return a matrix
{{1,2}{3,4}}.
I dont know what im doing wrong in my code cause the output i get is 3.0, 3.0, 4.0, 4.0.
This is what i have so far any help would be appreciated.
public static void main(String[] args) {
double[][] array = { {1, 4}, {2, 3} };
double[][] new_array = sort(array);
for(int i = 0; i < array.length; i++) {
for(int j = 0; j < array.length; j++) {
System.out.print(new_array[i][j] + " ");
}
}
}
public static double[][] sort(double[][] array) {
double[] storage = new double[array.length];
for(int i = 0; i < array.length; i++) {
for(int j = 0; j < array.length; j++) {
storage[i] = array[i][j];
}
}
storage = bubSort(storage);
for(int i = 0; i < array.length; i++) {
for(int j = 0; j < array.length; j++) {
array[i][j] = storage[i];
}
}
return array;
}
public static double[] bubSort(double[] list) {
boolean changed = true;
double temp;
do {
changed = false;
for (int j = 0; j < list.length -1; j++)
if (list[j] > list[j + 1]) {
temp = list[j];
list[j] = list[j + 1];
list[j + 1] = temp;
changed = true;
}
} while (changed);
return list;
}
}
The main problem that you are experiencing is how you are copying the values from the 2d array into the 1d array. You are actually only copy two values into an array of length 2. The length of a 2d array is not the full m x n length.
I will give a small hint how you can go about copying from the 2d array into the 1d array, but it is up to you to figure out how to copy back from the 1d array into the 2d array. Also, how would you go about finding the full length of the array?
double[] storage = new double[4];//You should calculate this value
int k = 0;
for (int i = 0; i < array.length; i++) {
for (int j = 0; j < array.length; j++) {
storage[k++] = array[i][j];
}
}
Your bubble sort works fine, but then you are copying the values back wrong. Try printing the array storage after the sort and you will see that it is now correct.
You are overwriting your storage array you have it set to array[i]. Because it is in the for loop you are setting storage[0] = array[0][0] then setting storage[0] = array[0][1]. This causes you to only pick up the last number in that dimension of the array. Likewise, when you are reading them back out you are inserting the same number twice. Since 4 and 3 are the last two numbers in their respective dimensions this shows that you are sorting the array. You need a for loop for storage that is set < array.length and store your values inside that.