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Write a function called arraySum that returns the sum (as an int) of all the values in a two-dimensional array of int values (Java)

I am trying to sum all the int values of a 2D array. I named it array. my function is then called arraySum. If arraySum is null, I return 0. Otherwise, it goes through two for-loops, summing the values of these arrays.
int i = 0;
int j = 0;
static int sum = 0;
int[][] array = new int[i][j];
static int arraySum(int[][] array) {
if (array == null) { // test if the incoming param is null
return 0;
} else {
for (int i = 0; i < array.length; i++) { // length of the outer array
for (int j = 0; j < array[i].length; j++) { // length of the inner array
sum += array[i][j];
}
}
return sum; // moved out of the loop
}
}
my error message:
java.lang.AssertionError: Incorrect result: expected [-217085] but found [-308126]

Fixing the method signature is the first step.
Then you'll need to fix the null check.
Then your loops need to check the size of the inner and outer arrays.
Move the return statement.
Here's the fixed code:
static int arraySum(int[][] array) { // fix the signature
if (array == null) { // test if the incoming param is null
return 0;
} else {
int sum = 0; // you need this in the scope of the method - it will be returned at the end
for (int i = 0; i < array.length; i++) { // length of the outer array
for (int j = 0; j < array[i].length; j++) { // length of the inner array
sum += array[i][j];
}
}
return sum; // moved out of the loop
}
}
Edit: I've concentrated on just the method now - how you call it is up to you. Please note that the method will not affect any externally defined sum variable. It will return the sum and it's up to the caller to store that value somewhere.

Avoid using primitive for statement. See following:
static int arraySum(int[][] array) {
int sum = 0;
if (array == null) return 0;
for(int[] row: array){
for(int col : row) {
sum += col;
}
}
return sum;
}

Streams can do this too:
//Create 2D array
int[][] array = {{1,2,3},{4,5},{6}};
//Sum all elements of array
int sum = Stream.of(array).flatMapToInt(IntStream::of).sum();
System.out.println(sum);
Picking apart the summing code:
Stream.of(array)
Turns the 2D array of type int[][] into a Stream<int[]>. From here, we need to go one level deeper to process each child array in the 2D array as the stream only streams the first dimension.
So at this point we'll have a stream that contains:
an int[] array containing {1,2,3}
an int[] array containing {4,5}
an int[] array containing {6}
.flatMapToInt(IntStream::of)
So far we have a Stream of int[], still two-dimensional. Flat map flattens the structure into a single stream of ints.
flatMapToInt() expands each int[] element of the stream into one or more int values. IntStream.of() takes an array int[] and turns it into an int stream. Combining these two gives a single stream of int values.
After the flattening, we'll have an int stream of:
{1,2,3,4,5,6}
.sum()
Now we have an IntStream with all elements expanded out from the original 2D array, this simply gets the sum of all the values.
Now the disclaimer - if you are new to Java and learning about arrays, this might be a little too advanced. I'd recommend learning about nesting for-loops and iterating over arrays. But it's also useful to know when an API can do a lot of the work for you.

How to fill a 2D array with random non duplicate numbers in Java

I'm having a hard time figuring out how to fill my 2D array with random numbers without duplicates. I currently have it filed with random numbers within the correct range, but I just cant think of a solution to have non duplicates. How could i do this using very basic java methods? I have not yet learned anything such as arraylists, or anything like that, only the very basic methods.

Given a MxN integer array, you could fill the array with numbers from 1 to M*N using two for-loops, and then swap them using the Fisher-Yates algorithm.
EDIT:
I changed the algorithm so that it now does not create a new integer-array every time the algorithm is called. It uses one loop, and calculates m, n, i j from a random value and the iterating varaible l. Assuming the given array is not null, rectangular and at least 1x0 in size:
public static void fillRandomlyUniqe(int[][] a) {
/*
fill up the array with incrementing values
if the values should start at another value, change here
*/
int value = 1;
for (int i = 0; i < a.length; i++) {
for (int j = 0; j < a[i].length; j++)
a[i][j] = value++;
}
// swap them using Fisher-Yates algorithm
Random r = new Random();
int max = a.length * a[0].length;
for (int l = max - 1; l > 0; l--) {
//calculate a two dimensional index from random number
int index = r.nextInt(l + 1);
int m = index % a.length;
int n = index / a.length;
//calculate two dimensional index from the iterating value
int i = l % a.length;
int j = l / a.length;
int temp = a[i][j];
a[i][j] = a[m][n];
a[m][n] = temp;
}
}

If your 2D array is NxM, and you want numbers from (say) 1 to NxM randomly placed in your 2D array, the simplest is to create an array/list with the numbers from 1 to NxM, shuffle it, then fill in your 2D array sequentially from the shuffled data. You are guaranteed to not have any duplicates because the original non-shuffled data is full of unique values.
List<Integer> data = IntStream.rangeClosed(1, M * N).boxed().collect(Collectors.toList());
Collections.shuffle(data);
Iterator<Integer> iter = data.iterator();
for (int i = 0; i < M; i++) {
for (int j = 0; j < N; j++) {
array[i][j] = iter.next();
}
}
There is probably a way to do the second half with the stream API too, but it escapes me at the moment.

Java two dimensional array notation and default access

From the following code:
/** Array for internal storage of elements.
#serial internal array storage.
*/
private double[][] A;
/** Row and column dimensions.
#serial row dimension.
#serial column dimension.
*/
private int m, n;
public Matrix times (Matrix B) {
if (B.m != n) {
throw new IllegalArgumentException("Matrix inner dimensions must agree.");
}
Matrix X = new Matrix(m,B.n);
double[][] C = X.getArray();
double[] Bcolj = new double[n];
for (int j = 0; j < B.n; j++) {
for (int k = 0; k < n; k++) {
Bcolj[k] = B.A[k][j];
}
for (int i = 0; i < m; i++) {
double[] Arowi = A[i];
double s = 0;
for (int k = 0; k < n; k++) {
s += Arowi[k]*Bcolj[k];
}
C[i][j] = s;
}
}
return X;
}
double[] Arowi = A[i];
What is this line doing?
A is a two dimensional Matrix, why is A[i] allowed? and what exactly is it doing?
Is that the same thing as A[i][0]?
This notation is confusing.
Also could someone translate that line to .NET, how would you do that with a double[,] Matrix?
I know there is a Matrix.GetLength() which gives you a specified length of a dimension.
But comparison to that line of Java code, how would that be translated?
EDIT
I believe I fixed it by just replacing that line and passing in my matrix to copy the first row into a new one dimensional matrix and returning it
public double[] ConvertFirstRowOfMultiArrayToOneArray(double[,] p)
{
double[] array = new double[p.Length];
for (int i = 0; i < p.GetLength(0); i++)
{
array[i] = p[0, i];
}
return array;
}

What is this line doing?
It's setting Arowi, that's declared as one dimension array, to the array A[i]
Suppose this case:
double[][] A = new double[2][2];
A[0][0] = 1.5;
A[0][1] = 2.5;
A[1][0] = 3.5;
A[1][0] = 4.5;
double[] B = A[0]; // Imagine A[0] as A[i]
/* then value of B is now A[0]
so B is now a unidimensional array containing value:
B[0] == 1.5 == A[0][0]
B[1] == 2.5 == A[0][1] */
A is a two dimensional Matrix, why is A[i] allowed?
Because as you can see, you are setting the unidimensional array value (B||Arowi) as the second dimension that's containing the A[i] array.
Imagine A[i] containing another array in itself (bidimensional). So we are saving this dimension into a new variable. In this case, it would be referencing the memory address, and not the value itself, if I'm not confused, and that if you edit the value in Arowi||B for example, it will change in the original array.
Example:
B[0] == 1.5 == TRUE
B[0] = 1.75
A[0][0] == 1.5 == FALSE
A[0][0] == 1.75 == TRUE
and what exactly is it doing?
I explain it in the example case.
The application is doing the following if I'm not confused:
It saves the result of adding (plus operation, s += Arowi[k]*Bcolj[k]) the values of multiplied A array (current array member/attribute in class (A) or maybe it's the Matrix class itself, I can't see all of the code) by the values of B columns (Matrix object taken by parameter). In each main loop it's going through a new column in B, storing all the values of rows in that column, and then looping the whole A array or class array, to store the results in C and then returning it as a Matrix object. I hope it is understandable and that I'm not confused.
Is that the same thing as A[i][0]?
No
A[i][0] sets the value. A[i] sets an unidimensional array
Sorry for my bad english, I hope you can understand and that the description and ideas are correct.
Edit
Look the example:
static void Main(string[] args)
{
double[][] A = new double[2][] { new double[2], new double[2] };
A[0][0] = 1.5;
A[0][1] = 2.5;
A[1][0] = 3.5;
A[1][1] = 4.5;
System.Console.WriteLine(A[0][0]);
// Prints 1,5
Console.Read();
double[] B = A[0];
System.Console.WriteLine(B[0]);
// Prints 1,5
System.Console.Read();
System.Console.Read();
}
So the line: double[] Arowi = A[i]; would translate to double[] Arowi = A[i];, just the same.
Only renember to create a jagged array double[][] A instead of array like double[,] A, because there seems to not be a way to reference a dimension with not jagged arrays (A[m,n]; being double[] Arowi = A[i]; not possible to be done like in Java). I can't tell why exactly right now. Just tried in my VS2013.
I recommend you to look at MSDN Documentation: http://msdn.microsoft.com/en-us/library/aa288453%28v=vs.71%29.aspx
I also recommend you this thread: Multidimensional Array [][] vs [,]

Not sure how it is in C# (.NET), but java has nothing like two (multi) dimensional arrays. It only has array of arrays (of arrays of arrays ...). So it is possible to have non-rectangular array of arrays.
double[] Arowi = A[i]; means declare new variable named Arowi of type array of doubles and set it to A[i] which is i-th (zero-based) member of A.
It is not the same as A[i][0], because this is 0-th member of i-th member of A.
EDIT: I haven't done any code review.

To understand what
double[] Arowi = A[i];
means, it is easiest to perceive A as an array of array of doubles instead of a matrix. Therefore, if you index A, it will return an array of doubles. Furthermore, if you index A at some index, it will return a double.
Hope this helps!

Transferring the contents of a one-dimensional array to a two-dimensional array

I'm trying to make an encryption program where the user enters a message and then converts the "letters into numbers".
For example the user enters a ABCD as his message. The converted number would be 1 2 3 4 and the numbers are stored into a one dimensional integer array. What I want to do is be able to put it into a 2x2 matrix with the use of two dimensional arrays.
Here's a snippet of my code:
int data[] = new int[] {10,20,30,40};
*for(i=0;i<2;i++)
{
for(j=0;j<2;j++)
{
for (int ctr=0; ictr<data.length(); ictr++){
a[i][j] = data[ctr];}
}
}
I know there's something wrong with the code but I am really lost.
How do I output it as the following?
10 20
30 40
(instead of just 10,20,30,40)

Here's one way of doing it. It's not the only way. Basically, for each cell in the output, you calculate the corresponding index of the initial array, then do the assignment.
int data[] = new int[] {10, 20, 30, 40, 50, 60};
int width = 3;
int height = 2;
int[][] result = new int[height][width];
for(int i = 0; i < height; i++) {
for(int j = 0; j < width; j++) {
result[i][j] = data[i * width + j];
}
}

Seems like you want to output a 2xn matrix while still having the values stored in a one-dimensional array. If that's the case then you can to this:
Assume the cardinality m of your set of values is known. Then, since you want it to be 2 rows, you calculate n=ceil(m/2), which will be the column count for your 2xn matrix. Note that if m is odd then you will only have n-1 values in your second row.
Then, for your array data (one-dimension array) which stores the values, just do
for(i=0;i<2;i++) // For each row
{
for(j=0;j<n;j++) // For each column,
// where index is baseline+j in the original one-dim array
{
System.out.print(data[i*n+j]);
}
}
But make sure you check the very last value for an odd cardinality set. Also you may want to do Integer.toString() to print the values.

Your code is close but not quite right. Specifically, your innermost loop (the one with ctr) doesn't accomplish much: it really just repeatedly sets the current a[i][j] to every value in the 1-D array, ultimately ending up with the last value in the array in every cell. Your main problem is confusion around how to work ctr into those loops.
There are two general approaches for what you are trying to do here. The general assumption I am making is that you want to pack an array of length L into an M x N 2-D array, where M x N = L exactly.
The first approach is to iterate through the 2D array, pulling the appropriate value from the 1-D array. For example (I'm using M and N for sizes below):
for (int i = 0, ctr = 0; i < M; ++ i) {
for (int j = 0; j < N; ++ j, ++ ctr) {
a[i][j] = data[ctr];
}
} // The final value of ctr would be L, since L = M * N.
Here, we use i and j as the 2-D indices, and start ctr at 0 and just increment it as we go to step through the 1-D array. This approach has another variation, which is to calculate the source index explicitly rather than using an increment, for example:
for (int i = 0; i < M; ++ i) {
for (int j = 0; j < N; ++ j) {
int ctr = i * N + j;
a[i][j] = data[ctr];
}
}
The second approach is to instead iterate through the 1-D array, and calculate the destination position in the 2-D array. Modulo and integer division can help with that:
for (int ctr = 0; ctr < L; ++ ctr) {
int i = ctr / N;
int j = ctr % N;
a[i][j] = data[ctr];
}
All of these approaches work. Some may be more convenient than others depending on your situation. Note that the two explicitly calculated approaches can be more convenient if you have to do other transformations at the same time, e.g. the last approach above would make it very easy to, say, flip your 2-D matrix horizontally.

check this solution, it works for any length of data
public class ArrayTest
{
public static void main(String[] args)
{
int data[] = new int[] {10,20,30,40,50};
int length,limit1,limit2;
length=data.length;
if(length%2==0)
{
limit1=data.length/2;
limit2=2;
}
else
{
limit1=data.length/2+1;
limit2=2;
}
int data2[][] = new int[limit1][limit2];
int ctr=0;
//stores data in 2d array
for(int i=0;i<limit1;i++)
{
for(int j=0;j<limit2;j++)
{
if(ctr<length)
{
data2[i][j] = data[ctr];
ctr++;
}
else
{
break;
}
}
}
ctr=0;
//prints data from 2d array
for(int i=0;i<limit1;i++)
{
for(int j=0;j<limit2;j++)
{
if(ctr<length)
{
System.out.println(data2[i][j]);
ctr++;
}
else
{
break;
}
}
}
}
}

RealMatrix multiply without reassign

in my Java source i must execute following lines very often:
vecX = EigenMat.multiply(vecX);
vecY = EigenMat.multiply(vecY);
EigenMat is a N x N Matrix with N~40
vecX/vecY is a N x 1 vector (intern a RealMatrix to)
I used the "Sampler" from VisualFM to find some hotspots in my code and
org.apache.commons.math3.linear.Array2DRowRealMatrix.<init>()
org.apache.commons.math3.linear.Array2DRowRealMatrix.multiply()
have a very high runtime.
I'm not a java professional but i think every multiplication a new vector is created. Can i reassign the old one?
Maybe i should switch to JBLAS to speed it up?
Matyro
Edit:
Single core only

i think every multiplication a new vector is created
Yes, it is. Source code of multiply():
public Array2DRowRealMatrix multiply(final Array2DRowRealMatrix m) {
// Safety check.
MatrixUtils.checkMultiplicationCompatible(this, m);
final int nRows = this.getRowDimension();
final int nCols = m.getColumnDimension();
final int nSum = this.getColumnDimension();
final double[][] outData = new double[nRows][nCols];
// Will hold a column of "m".
final double[] mCol = new double[nSum];
final double[][] mData = m.data;
// Multiply.
for (int col = 0; col < nCols; col++) {
// Copy all elements of column "col" of "m" so that
// will be in contiguous memory.
for (int mRow = 0; mRow < nSum; mRow++) {
mCol[mRow] = mData[mRow][col];
}
for (int row = 0; row < nRows; row++) {
final double[] dataRow = data[row];
double sum = 0;
for (int i = 0; i < nSum; i++) {
sum += dataRow[i] * mCol[i];
}
outData[row][col] = sum;
}
}
return new Array2DRowRealMatrix(outData, false);
}
Input vector m is copied, as stated in a comment Copy all elements of column "col" of "m" so that will be in contiguous memory.
Can i reassign the old one?
Yes, you can perform the multiplication by yourself, writing two loops. Use getData() to get a reference to the underlying double[][] data.