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javaarrays
Jessy
Jessy

Reputation: 15661

How to flatten 2D array to 1D array?

How can I flatten the 2 dimensions array int originalArray[][] to 1 dimension array?

    int a [] = {1,2,6,7,2};
    int b [] = {2,44,55,2};
    int c [] = {2,44,511,33};

    int originalArray [][] = new int[][]{a,b,c};

Upvotes: 29

Views: 58095

Answers (9)

Kevin Bourrillion
Kevin Bourrillion

Reputation: 40851

With Guava, you can use either

int[] all = Ints.concat(originalArray);

or

int[] all = Ints.concat(a, b, c);

Upvotes: 46

Dean Jain
Dean Jain

Reputation: 2198

below code can merge varied 2D arrays (diff sizes of internal array) into a one dimensional array:

 public static Integer[] merge2DArrays(int[][] twoDArray){
        ArrayList<Integer> list = new ArrayList<>();
        for (int i = 0; i < twoDArray.length; i++) {
            for (int j = 0; j < twoDArray[i].length; j++) {
                list.add(twoDArray[i][j]);
            }
        }
    return list.toArray(new Integer[list.size()]);
    }

Upvotes: 0

rsp
rsp

Reputation: 23373

A simple for loop will do, it is not difficult, but will depend on the order in which you want to copy the values. For instance (based on the fact that in your example the arrays all have the same length):

int[] newArray = new int[3 * a.length];
int index = 0;
for (int n = 0; n < a.length; n++) {
    newArray[index++] = a[n];
    newArray[index++] = b[n];
    newArray[index++] = c[n];
}

or (different order, a, b, c can be of different lengths):

int[] newArray = new int[a.length + b.length + c.length];
System.arraycopy(a, 0, newArray, 0, a.length);
System.arraycopy(b, 0, newArray, a.length, b.length);
System.arraycopy(c, 0, newArray, a.length + b.length, c.length);

Upvotes: 9

Kaplan
Kaplan

Reputation: 3728

one-liner with IntStream

IntStream.concat(
    IntStream.concat( IntStream.of(originalArray[0]), IntStream.of(originalArray[1]) ),
        IntStream.of(originalArray[2]) ).toArray();

gets: [1, 2, 6, 7, 2, 2, 44, 55, 2, 2, 44, 511, 33]

Upvotes: 0

assylias
assylias

Reputation: 328598

With Java 8 you can "flatMap" the inner arrays:

int[] flatArray = Arrays.stream(originalArray)
        .flatMapToInt(Arrays::stream)
        .toArray();

or:

int[] flatArray = Stream.of(a, b, c)
        .flatMapToInt(Arrays::stream)
        .toArray();

Upvotes: 43

Nolesh
Nolesh

Reputation: 7018

int[] oneDArray = new int[arr.length*arr.length];
    //Flatten 2D array to 1D array...
    int s = 0;
    for(int i = 0; i < arr.length; i ++) 
          for(int j = 0; j < arr.length; j ++){                           
              oneDArray[s] = arr[i][j];
              s++;
          }

Upvotes: 4

phihag
phihag

Reputation: 287825

Since arrays can't be extended (i.e. you have to declare the size of an error upon initialization), you have to traverse the arrays twice:

int size = 0;
for (int[] ar : originalArray) size += ar.length;
int[] result = new int[size];
int pos = 0;
for (int[] ar : originalArray) {
    System.arraycopy(ar, 0, result, pos, ar.length);
    pos += ar.length;
}

Upvotes: 2

Roman
Roman

Reputation: 66156

There will be 2 steps:

1) find out total number of elements to create a new vector (1d array)

2) iterate through your 2d array in predefined order and copy its elements to the created vector

int elementsNumber = 0;

for (int i = 0; i < originalArray.length; i++) {
   elementsNumber += originalArray[i].length;
}

int[] newArray = new int[elementsNumber];
int j = 0;
for (int i = 0; i < originalArray.length; i++) {
   System.arrayCopy (originalArray[i], 0, newArray, j, originalArray[i].length);
   j += originalArray[i].length;
}

Upvotes: 4

Thorbj&#248;rn Ravn Andersen
Thorbj&#248;rn Ravn Andersen

Reputation: 75376

Count the total number of elements in originalArray. Create new array of that length. Copy elements one by one into the new array.

I am unfamiliar with any library function to do so.

Upvotes: -1

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