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How to print first five elements of an matrix in Java

My goal is to implement the following method in parallel:
public static double[][] parallelAddMatrix(double[][] a, double[][] b), then test my program on randomly generated two lists of size 2000 x 2000. Finally I have to output the first 5 elements of matrix a and matrix b, and also the first five elements of the result matrix, which is what I'm having trouble with.
This is the part of my code where I create the first and second matrix.
public static void main(String[] args) {
int var1, var2;
final int matrices = 2000;
// creates first matrix
double[][] matrixA = new double[matrices][matrices];
for(var1 = 0; var1 < matrixA.length; var1++)
for (var2 = 0; var2 < matrixA[var1].length; var2++)
matrixA[var1][var2] = 1;
// creates second matrix
double[][] matrixB = new double[matrices][matrices];
for (var1 = 0; var1 < matrixB.length; var1++)
for (var2 = 0; var2 < matrixB[var1].length; var2++)
matrixB[var1][var2] = 1;
And then later created a function to create the result matrix...
public static double[][] parallelAddMatrix( double [][] a, double[][] b) {
//creates output matrix
double[][] resultMatrix = new double[a.length][a[0].length];
RecursiveAction task = new multiProcess(a, b, resultMatrix);
ForkJoinPool joinPool = new ForkJoinPool();
joinPool.invoke(task);
return resultMatrix;
}
How can I print out the first five elements for each of the three matrices?
I've tried stuff for the first and second matrix such as initializing var3, then under the "matrixA(orB)[var1][var2] = 1;", I put
for (var3 = 0; var3 < 5; var3++) {
System.out.println(var3);
}
and also tried
for (var3 = 0; var3 < 5; var3++) {
System.out.print(matrixA[var1][var2] + "");
}
System.out.println();
Please help on this, and please tell where it would be placed for each one (I might have trouble with brackets).

You'll need a nested for loop to iterate through the matrix, and a counter to see how many entries you've printed. Let's start with the easiest part: iterating over the matrix. I'll assume that the matrix is simply called matrix.
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
System.out.println(matrix[i][j]);
}
}
You probably already figured that out. Now we need a counter to count how many times we've printed out an entry from the matrix.
int num_printed = 0;
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
System.out.println(matrix[i][j]);
num_printed ++;
}
}
Ok. So now we need to stop once we've reached the end. We can't just use one break statement, because, we have two for loops.
int num_printed = 0;
for (int i = 0; i < matrix.length; i++) { // iterate over the rows
for (int j = 0; j < matrix[i].length; j++) { // iterate over the columns
if (num_printed == 5) { // if we've already printed five items, stop
break;
} else { // otherwise, print the next item
System.out.println(matrix[i][j]);
num_printed ++; // increment the counter
}
}
if (num_printed == 5) { // so that we don't go to the next row
break;
}
}
It's worth noting that you could create your own separate method, and only use a return statement:
public void print_five_elements() {
int num_printed = 0;
for (int i = 0; i < matrix.length; i++) { // iterate over the rows
for (int j = 0; j < matrix[i].length; j++) { // iterate over the columns
if (num_printed == 5) { // if we've already printed five items, stop
return;
} else { // otherwise, print the next item
System.out.println(matrix[i][j]);
num_printed ++; // increment the counter
}
}
}
}
More Specialized Approach
This approach allows you to use matrices that have less than five columns. However, since your matrix is 2000x2000, you could go for a much simpler approach. Use zero as the first index, and then just iterate up to five. Just keep in mind that this won't work if you have less than five columns:
public void print_five_elements_for_wide_matrix() {
for (int i = 0; i < 5; i++) {
System.out.println(matrix[0][i]);
}
}

Since the matrices are of size 2000 x 2000, you do not need nested loops to display first 5 elements from each of them.
int i;
//Display first 5 elements of matrixA
for(i=0; i<5; i++) {
System.out.print(matrixA[0][i] + " ");
}
System.out.println();
//Display first 5 elements of matrixB
for(i=0; i<5; i++) {
System.out.print(matrixB[0][i] + " ");
}
System.out.println();
double[][] result = parallelAddMatrix(matrixA, matrixB);
//Display first 5 elements of result
for(i=0; i<5; i++) {
System.out.print(result[0][i] + " ");
}
System.out.println();
Note that the above loops print the first 5 elements of the first row (i.e. row at index, 0) of each matrix. However, if you want to print the first element of the first 5 rows, just swap the indices e.g.
System.out.println(matrixA[i][0] + " ");

Try this:
Think of the first set of brackets as the row and the second set as the column.
for (int row = 0; row < 5; row++) {
for (int col = 0; col < 5; col++) {
System.out.print(matrixA[row][col] + " ");
}
System.out.println();
}
Since "multi-dimensional" arrays are really arrays of arrays you can do it like this if you wanted to print out the whole matrix
for (double[] row : matrixA) {
System.out.println(Arrays.toString(row));
}
Because of this, each row can be a different length. So you may have to get the length to print them out like you first wanted to.
for (int row = 0; row < matrixA.length; row++) {
for (int col = 0; col < matrixA[row].length; col++) {
System.out.print(matrixA[row][col] + " " );
}
}
Rows of different length of a "2D" array are known as ragged-arrays.

Select element j in each column of an array in JAVA

I have the array
String[] test_=new String[] {"a b c d", "f g h i","j k l s gf"};
Now i want to create another array that has the elements
{"b d", "g i","k s"}
how can I do this?
I've managed to separate the array into rows using
String split_test[] = null;
for (int j = 0 ; j <= 2 ; j++) {
split_test=test_[j].split("\\s+");
System.out.println(Arrays.toString(split_test));
}
But now I want to separate each of those rows, I tried the solution of
How to Fill a 2d array with a 1d array? Combined with something like this split_test=test_[j].split("\s+"), but I haven't been able to solve it.
Also If I do what they say I have to make the array split_test have a number of specific columns, but what I want is the size of the columns of split_test depend of the array test_. For example in case I want to have an array with the elements {"b d", "g i", "k s gf"}
String[][] split_test = new String[3][2];
for(int row = 0; row < split_test.length; row++) {
for(int col = 0; col < split_test[row].length; col++) {
split_test[row][col] = test_[row];/*I still don't understand how to use the split within the for*/
System.out.println(split_test[row][col]);
}
}
Is there a simpler and more efficient way of doing this?
Thanks

Here is another one.
You can use the substring method of String class.
Or use indexes of the array returned by the split method.
String output[] = new String[test_.length];
String split_test[] = null;
for (int j = 0; j < test_.length(); j++) {
split_test = test_[j].split("\\s+");
// use direct index
// output2[j] = split_test[1] + " " + split_test[3];
// or based on length
output[j] = split_test[1] + " " + split_test[split_test.length - 2];
}
System.out.println(Arrays.toString(output));
Output:
b d
g i
k s

I have used a different approach that works as well. I noticed you only take the uneven indexes, hence my modulo approach:
String[] array = new String[] {"a b c d", "f g h i","j k l s gf"};
String[] result = new String[array.length];
for(int i = 0; i < array.length; i++) {
String subresult = "";
String[] array2 = array[i].split(" ");
for(int j = 0; j < array2.length; j++) {
if(j % 2 == 1)
subresult += array2[j] +" ";
}
result[i] = subresult.trim();
}

You should use a 2 dimentional array, you can create one by doing:
String[][] input=new String[][] {{"a","b","c","d"}, {"f","g","h","i"},{"j","k","l","s"}};
You can then do something like this to retrieve {{"b","d"}, {"g","i"},{"k","s"}}:
String[][] output = new String[input.length][2];
for(int i = 0; i<input.length; i++)
{
output[i] = new String[]{input[i][1],input[i][3]};
}
System.out.println(Arrays.deepToString(output));

Creating a multi-dimensional String array from a method array parameter

I am attempting to solve a semi-difficult problem in which I am attempting to create an array and return a 3 dimensional array based on the parameter which happens to be a 2 dimensional int array. The array I'm attempting to return is a String array of 3 dimensions. So here is the code:
public class Displaydata {
static String[][][] makeArray(int[][] dimensions) {
String myArray[][][];
for (int i = 0; i < dimensions.length; i++) {
for (int j = 0; j < dimensions[i].length; j++) {
myArray[][][] = new String[i][j][]; //getting error here.
}
}
return myArray;
}
static void printArray(String[][][] a) {
for (int i = 0; i < a.length; i++) {
System.out.println("\nrow_" + i);
for (int j = 0; j < a[i].length; j++) {
System.out.print( "\t");
for (int k = 0; k < a[i][j].length; k++)
System.out.print(a[i][j][k] + " ");
System.out.println();
}
}
}
public static void main(String[] args) {
int [][] dim = new int[5][];
dim[0] = new int[2];
dim[1] = new int[4];
dim[2] = new int[1];
dim[3] = new int[7];
dim[4] = new int[13];
dim[0][0] = 4;
dim[0][1] = 8;
dim[1][0] = 5;
dim[1][1] = 6;
dim[1][2] = 2;
dim[1][3] = 7;
dim[2][0] = 11;
for (int i = 0; i < dim[3].length;i++)
dim[3][i] = 2*i+1;
for (int i = 0; i < dim[4].length;i++)
dim[4][i] = 26- 2*i;
String[][][] threeDee = makeArray(dim);
printArray(threeDee);
}
}
As you can see from the source code, I'm getting an error when I try to create an instance of my 3-dimensional array which I'm attempting to return. I'm supposed to create a three dimensional array with the number of top-level rows determined by the length of dimensions and, for each top-level row i, the number of second-level rows is determined by the length of dimensions[i]. The number of columns in second-level row j of top-level row i is determined by the value of dimensions[i][j]. The value of each array element is the concatenation of its top-level row index with its second-level row index with its column index, where indices are represented by letters : ‘A’ for 0, ‘B’ for 1 etc. (Of course, this will only be true if the indices don’t exceed 25.) I don't necessarily know where I'm going wrong. Thanks!

You should not be initializing the array on every iteration of the loop. Initialize it once outside the loop and then populate it inside the loop.
static String[][][] makeArray(int[][] dimensions) {
String[][][] myArray = new String[25][25][1];
for (int i = 0; i < dimensions.length; i++) {
for (int j = 0; j < dimensions[i].length; j++) {
myArray[i][j][0] = i + "," + j;
}
}
return myArray;
}
I just plugged in values for the size of the first two dimensions, you will need to calculate them based on what you put in there. The 'i' value will always be dimensions.length but the 'j' value will be the largest value returned from dimensions[0].length -> dimensions[n-1].length where 'n' is the number of elements in the second dimension.
Also you will need to set up a way to convert the numbers in 'i' and 'j' to letters, maybe use a Map.

I guess you should initialize the array as
myArray = new String[i][j][]; //getting error here.

I think
myArray[][][] = new String[i][j][]; //getting error here.
should be:
myArray[i][j] = new String[5]; // I have no idea how big you want to go.
And then you can fill in each element of you inner-most array like such:
myArray[i][j][0] = "first item";
myArray[i][j][1] = "second string";
...

I think you should just change that line to:
myArray = new String[i][j][]; //look ma! no compiler error
Also, you would need to initialize myArray to something sensible (perhaps null?)

Shuffle the order of a 2D array in java

I'm creating my own memory game. Everything is going well so far. Just to let you know I'm using processing for Java. I have created a 2 dim PImage array. This is the code for filling the 2D array:
int g = 0;
for(int i = 0; i < 4; i++) {
for (int j = 0; j < 6; j++) {
if (j % 2 == 0) {
kaart[i][j] = loadImage( g + ".jpg" );
kaart[i][j].resize(vlakGrootte - 1, vlakGrootte - 1);
g++;
} else if (j % 2 == 1) {
kaart[i][j] = kaart[i][j-1];
}
}
}
I want the items in this array to be shuffled. It seems like java collections does not support to shuffle a 2D PImage array? Please correct me if im wrong.
Thanks to you all for helping me out.

1).Shuffle per-outter index :
YourType [][] kaart = new YourType [..][..];
List <YourType[]> list = (List<YourType[]> ) Arrays.asList(kaart);
Collections.shuffle(list);
kaart = (YourType[][]) list.toArray(new YourType[0][]);//convert back to a array
// just for checking
for(YourType[] k:kaart ){System.out.println(Arrays.toString(k));}
Replace YourType with the type of kaart.
2). Shuffle per-Outter+Inner index :
YourType[][] kaart = new YourType[..][..];
List<YourType[]> temp = new ArrayList<>();
for(YourType[] k:kaart ){
List <YourType> list = (List<YourType> ) Arrays.asList(k);
Collections.shuffle(list);//shuffle
YourType[] tempArray = (YourType[]) list.toArray();
temp.add(tempArray);
}
Collections.shuffle(temp);
kaart= (YourType[][]) temp.toArray(new YourType[0][]);//convert back to a array
// just for checking
for(YourType[] k:kaart ){System.out.println(Arrays.toString(k)); }
Replace YourType with the type of kaart.
3). Shuffle in The easiest way:
Just put all elements into a single List then call Collections.shuffle()

I would do this the same way you would deal those cards in real world. First you shuffle the deck:
ArrayList<Integer> pieces = new ArrayList<Integer>();
for (int i = 0; i < 4 * 6 / 2; i++) {
for (int j = 0; j < 2; j++) {
pieces.add(i);
}
}
Collections.shuffle(pieces);
Then you deal cards out of shuffled deck:
for(int i = 0; i < 4; i++) {
for (int j = 0; j < 6; j++) {
int g = pieces.remove(pieces.size()-1);
kaart[i][j] = loadImage( g + ".jpg" );
kaart[i][j].resize(vlakGrootte - 1, vlakGrootte - 1);
}
}

compare two array of string and store the result in another array

I want to compare two arrays and store the difference in another array
For example the two arrays might be
String[] a1 = { "cat" , "dog" };
String[] a2 = { "cat" , "rabbit" };
The resultant array would be like this
{ "rabbit" }
I use this code, but it does not work
int n = 0;
for (int k = 0; k <= temp.length; k++)
{
for (int u = 0; u <= origenal.length; u++)
{
if (temp[k] != origenal[u] && origenal[u] != temp[k])
{
temp2[n] = temp[k];
System.out.println(temp[u]);
n++;
}
}
}

This should do the trick.
String[] result = new String[100];
Int k = 0;
Boolean test = true;
for(i=0; i < a1.length; i++){
for(j=0; j < a2.length; j++){
if(a2[i].equals(a1[i])) continue;
test = false
}
if(test == false) result[k++] = a1[i];
}

I think that this may be what you are looking for. Note that it will only add to the third 'array' if the value exist in second array but not in first. In your example only rabbit will be stored, not dog (even though dog does not exist in both). This example could possibly be shortened but I wanted to keep it like this so it is easier to see what is going on.
First import:
import java.util.ArrayList;
import java.util.List;
Then do the following to populate and analyze the arrays
String a1[] = new String[]{"cat" , "dog"}; // Initialize array1
String a2[] = new String[]{"cat" , "rabbit"}; // Initialize array2
List<String> tempList = new ArrayList<String>();
for(int i = 0; i < a2.length; i++)
{
boolean foundString = false; // To be able to track if the string was found in both arrays
for(int j = 0; j < a1.length; j++)
{
if(a1[j].equals(a2[i]))
{
foundString = true;
break; // If it exist in both arrays there is no need to look further
}
}
if(!foundString) // If the same is not found in both..
tempList.add(a2[i]); // .. add to temporary list
}
tempList will now contain 'rabbit' as according to the specification. If you necessary need it to be a third array you can convert it to that quite simply by doing the following:
String a3[] = tempList.toArray(new String[0]); // a3 will now contain rabbit
To print the content of either the List or Array do:
// Print the content of List tempList
for(int i = 0; i < tempList.size(); i++)
{
System.out.println(tempList.get(i));
}
// Print the content of Array a3
for(int i = 0; i < a3.length; i++)
{
System.out.println(a3[i]);
}