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};
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9 Answers
With Guava, you can use either
int[] all = Ints.concat(originalArray);
or
int[] all = Ints.concat(a, b, c);
3 Comments
Kevin Bourrillion
Downvoted why? Who wants to write all that code above when they need to do this?
gvlasov
Is there any method to do the same for an array of Objects?
Kevin Bourrillion
ObjectArrays.concatWith 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();
Comments
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);
3 Comments
phihag
The example arrays don't have the same length. a.length == 5, b.length == 4, c.length == 4.
Kevin K
Also might be worth mentioning that the two examples end up with different orderings for the final flattened array, if that matters. In the first example, the arrays are 'weaved', while in the second, they are placed 'end-to-end', if that makes sense
rsp
@Kevin, I think I did mention that:
different order, a, b, c can be of different lengthsThere 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;
}
Comments
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++;
}
Comments
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;
}
5 Comments
dclowd9901
Wow, I'm quite surprised recursion isn't necessary.
Jessy
is it possible not to use loop?
phihag
@Jessy Well, you could use recursion, but that would it make way messier and slower.
Jessy
I think, I should use arraycopy :-)
nicerobot
Note: though it wasn't specified as a requirement, with this technique, there's no way to return to the original arrays without retaining their lengths. The solution by @rsp is close a technique capable of reversing the process. Just pointing out that the requirements of the flattening will dictate the solution.
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]
Comments
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()]);
}
Comments
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.
answered Apr 2, 2010 at 21:29
Thorbjørn Ravn Andersen
75.4k35 gold badges203 silver badges359 bronze badges
Comments
