for example
val coordinates: Array<IntArray> = [[1, 2], [1, 3], [1, 4]]
coordinates
.map {it[0]}
.all {it == x[0]}
x means the map result of coordinates. I want use it in the 'all' step, how to do that?
You could just write this
coordinates
.map {it[0]}
.all {it == coordinates[0][0]}
Or if you don't want to directly refer to coordinates again, maybe insert a let like
coordinates
.map { it[0] }
.let { x -> x.all { it == x[0] } }
Although for your specific use case I probably would just do
coordinates
.all { it[0] == coordinates[0][0] }
If I understand correctly, you can just declare a val called x:
val x = coordinates.map { it[0] }
val result = x.all { it == x[0] }
Or if you want to do it in one expression, you can use the scope function run or let:
val result = coordinates.map { it[0] }.run {
all { it == this[0] }
}
or:
val result = coordinates.map { it[0] }.let { x ->
x.all { it == x[0] }
}
Though, if you just want to check if the whole list has exactly one unique value, I think this is more readable:
val result = coordinates.map { it[0] }.distinct().size == 1
The above doesn't short-circuit like all. A shortcircuiting version would need Sequence:
val result = coordinates
.asSequence()
.map { it[0] }
.distinct().take(2).count() == 1
(Kotlin newbie here) I have a text file with rows that look like these:
1-1-1
1-1-2
1-1-3
2-1-1
2-1-2
etc.
I have to transform these data to a map where the key is the first 2 elements and the value is a list of the third elements that that match the key. For example, the above records will transform into this JSON:
1-1: [1, 2, 3]
2-1: [1, 2]
etc.
I'm unable to increment the list. Here's a simplified version, I get stuck on the "else":
fun main () {
val l1 = mutableListOf("1-1-1", "1-1-2", "1-1-3", "2-1-1", "2-1-2")
val m = mutableMapOf<String, List<Int>>()
for (e in l1) {
val c = e.split("-")
val key = "${c[0]}-${c[1]}"
if (m[key] == null) m[key] = listOf(c[2].toInt())
else println("How do I append to the list?")
}
println(m)
}
Output:
{1-1=[1], 2-1=[1]}
But I want:
{1-1=[1, 2, 3], 2-1=[1, 2]}
Thank you (comments about idiomatic form are welcome!)
If we continue to follow your strategy, what you need is for the value type to be a MutableList. Then you can add to the existing MutableList when there's already an existing list for that key:
fun main() {
val l1 = mutableListOf("1-1-1", "1-1-2", "1-1-3", "2-1-1", "2-1-2")
val m = mutableMapOf<String, MutableList<Int>>()
for (e in l1) {
val c = e.split("-")
val key = "${c[0]}-${c[1]}"
if (m[key] == null) m[key] = mutableListOf(c[2].toInt())
else m[key]!!.add(c[2].toInt())
}
println(m)
}
This can be more natural using getOrPut(). It returns the existing MutableList or creates one and puts it in the map if it's missing. Then we don't have to deal with if/else, and can simply add the new item to the list.
fun main() {
val l1 = mutableListOf("1-1-1", "1-1-2", "1-1-3", "2-1-1", "2-1-2")
val m = mutableMapOf<String, MutableList<Int>>()
for (e in l1) {
val c = e.split("-")
val key = "${c[0]}-${c[1]}"
m.getOrPut(key, ::mutableListOf).add(c[2].toInt())
}
println(m)
}
But we can use the map and groupBy functions to create it more simply:
val m = l1.map { it.split("-") }
.groupBy(
{ "${it[0]}-${it[1]}" }, // keys
{ it[2].toInt() } // values
)
You can achieve your desired output with a single call to groupBy of the Kotlin standard library.
val input = listOf("1-1-1", "1-1-2", "1-1-3", "2-1-1", "2-1-2")
val result = input.groupBy(
{ it.substringBeforeLast("-") }, // extract key from item
{ it.substringAfterLast("-").toInt() } // extract value from item
)
The first lambda function extracts the key to group by of every list item. The second lambda function provides the value to use for each list item.
You can also do it by first mapping your values to Pairs and then group them as follows:
fun main(args: Array<String>) {
val input = listOf("1-1-1", "1-1-2", "1-1-3", "2-1-1", "2-1-2")
val result = input.map {
val values = it.split("-")
"${values[0]}-${values[1]}" to values[2]
}.groupBy ({ it.first }) { it.second }
println(result)
}
I have the example to show what I mean:
fun makeRange(i: Int) = Pair(i - 1, i + 1)
val listOfData = listOf(1, 2, 3, 4, 5, 6)
val pairs = listOfData
.map { makeRange(it) }
val leftRange = pairs.map { it.first }
val rightRange = pairs.map { it.second }
I have some list and function which returns a tuple. But the result I need is touple of two lists. I need something like that:
// can I get something like that ?
val (leftRange, rightRange) = listOfData.map { makeRange(it) } ...
Is there a way to do it?
If you really want to destructure it like this, I would also split up your makeRange-function, e.g.:
fun makeLeftRange(i: Int) = i - 1
fun makeRightRange(i: Int) = i + 1
fun makeRange(i: Int) = makeLeftRange(i) to makeRightRange(i) // if you still need it...
Then you can destructure as follows:
val (leftRange, rightRange) = listOfData.map(::makeLeftRange) to listOfData.map(::makeRightRange)
Or if it is really just such an easy function, why not just use the following instead:
val (leftRange, rightRange) = listOfData.map(Int::dec) to listOfData.map(Int::inc)
// or
val (leftRange, rightRange) = listOfData.map { it - 1 } to listOfData.map { it + 1 }
If you want to keep your makeRange as is and want to do it that way, it will get a bit uglier, e.g.:
val (leftRange, rightRange) = listOfData.map(::makeRange).let {
listOfPairs -> listOfPairs.map { it.first } to listOfPairs.map { it.second }
}
Basically reusing what you've shown in an additional let-statement.
Seems like kotlin unzip function is just what you're looking for.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/unzip.html
In your example the usage would look something like
val (leftRange, rightRange) = pairs.unzip()
What will be the syntax of creating a 3D matrix in Kotlin. It's Java equivalent is as follows:
public static final int[][][] data = {{{0,0},{0}},{{0,1},{0}},{{1,0},{0}},{{1,1},{1}}};
Thanks
Edit:
Also how can I print the Kotlin code using the simple println?
When working with arrays in most languages I find it nice to create a helper class, rather than working directly with an int[][][] type. This way you can ensure certain invariants hold (such as all rows having the same length), and ensure better data locality. It can also let you efficiently implement certain operations such as slicing, sub-matrices, transpose etc.
My usual set of classes would look something like this for 3D. (though I'd probably template on the stored type, rather than hard code it for Int)
Its pretty incomplete, but the main at the end shows how many of the functions work.
But to show how you can create a 3D array from values you can do
val V = /* .. as in mEQ5aNLrK3lqs3kfSa5HbvsTWe0nIu's answer */
val M = Matrix3D(NX,NY,NZ).transform( { v, ix, iy, iz -> V[ix][iy][iz] } )
Further examples are
fun main(args: Array<String>) {
// Create an empty matrix
val v = Matrix3D(4,4,2);
// We can access elements via [a,b,c] or [a][b][c]
v[0,1,1] = 7;
print(v)
println("v[0,1,1]=" + v[0,1,1])
println("v[0][1][1]=" + v[0][1][1])
println("-----")
// Make the matrix a little more interesting
v.transform({ w,ix,iy,iz -> ix+iy+iz})
print(v)
println("-----")
// Transform just the slice with ix=2
// Slices are fast, as they copy no elements.
// but if you change them you change the original
v[2].transform({w,iy,iz -> w+3})
print(v)
// If you dont want to change the original you can always
// create an independent copy
print(v[2].bake().transform({w,iy,iz -> w-3}))
println("-----")
// W is the slice of v with ix=0
// Can easily extend the slicing options to allow slicing along
// any axis - I'd like to add v[_,1,_] to mean the slice with iy=1
// but I've not got to that yet.
val W = v[0]
print("W=\n")
print(v[0])
print("W^T=\n")
// Fast transpose, no elements are copied.
val WT=v[0].transpose()
print(WT)
// Changing the transpose slice writes back into the original
WT[1,1]=5
print(V)
}
fun print(M:Matrix3D) {
for(iz in 0..(M.nz-1)) {
for(iy in 0..(M.ny-1)) {
for(ix in 0..(M.nx-1)){
print("%d ".format(M[ix,iy,iz]))
}
print("\n")
}
print("\n")
}
}
fun print(M:Matrix2D) {
for(iy in 0..(M.ny-1)) {
for(ix in 0..(M.nx-1)){
print("%d ".format(M[ix,iy]))
}
print("\n")
}
}
The library code looks like this:
class Matrix1D(
val v:Array<Int>,
val nx:Int,
val offset:Int,
val xstride:Int) {
// TODO: Check that the nx,offset,strides etc are valid
constructor(nx:Int) : this(Array(nx,{i->0}), nx, 0, 1) {
}
fun offsetof(ix:Int):Int {
return offset + ix*xstride
}
operator fun get(ix:Int): Int {
return v[offsetof(ix)]
}
operator fun set(ix:Int, v:Int) {
this.v[offsetof(ix)] = v
}
fun reverse() : Matrix1D {
return Matrix1D(v, nx, offsetof(nx-1), -xstride)
}
fun submatrix(startx:Int, newNX:Int) : Matrix1D {
return Matrix1D(v,newNX,offsetof(startx), xstride)
}
fun transform(body: (Int, Int) -> Int ) {
for(ix in 0..(nx-1)){
this[ix] = body(this[ix], ix)
}
}
fun bake() : Matrix1D {
val rv = Matrix1D(nx);
for(ix in 0..(nx-1)) {
rv[ix] = this[ix]
}
return rv
}
}
class Matrix2D(
val v:Array<Int>,
val nx:Int, val ny:Int,
val offset:Int,
val xstride:Int, val ystride:Int) {
// TODO: Check that the nx,ny,offset,strides etc are valid
constructor(nx:Int, ny:Int) : this(Array(nx*ny,{i->0}), nx, ny, 0, 1, nx ) {
}
fun offsetof(ix:Int,iy:Int): Int {
return offset + ix*xstride + iy*ystride
}
operator fun get(ix:Int,iy:Int): Int {
return v[offsetof(ix,iy)]
}
operator fun set(ix:Int,iy:Int,v:Int) {
this.v[offsetof(ix,iy)] = v
}
operator fun get(ix:Int): Matrix1D {
return Matrix1D(v, ny, offsetof(ix,0), ystride)
}
fun transpose(): Matrix2D {
return Matrix2D(v,ny,nx,offset,ystride,xstride)
}
fun submatrix(startx:Int, starty:Int, newNX:Int, newNY:Int) : Matrix2D {
return Matrix2D(v,newNX,newNY,offsetof(startx,starty), xstride, ystride)
}
fun transform(body: (Int, Int, Int) -> Int ) {
for(iy in 0..(ny-1)) {
for(ix in 0..(nx-1)){
this[ix,iy] = body(this[ix,iy], ix,iy)
}
}
}
fun bake() : Matrix2D {
val rv = Matrix2D(nx,ny);
for(ix in 0..(nx-1)) {
for(iy in 0..(ny-1)) {
rv[ix,iy] = this[ix,iy]
}
}
return rv
}
}
class Matrix3D(
val v:Array<Int>,
val nx:Int, val ny:Int, val nz:Int,
val offset:Int,
val xstride:Int, val ystride:Int, val zstride:Int) {
// TODO: Check that the nx,ny,nz,offset,strides etc are valid
constructor(nx:Int, ny:Int, nz:Int) : this(Array(nx*ny*nz,{i->0}), nx, ny, nz, 0, 1, nx, nx*ny ) {
}
operator fun get(ix:Int,iy:Int,iz:Int): Int {
return v[offset + ix*xstride + iy*ystride + iz*zstride]
}
operator fun set(ix:Int,iy:Int,iz:Int, v:Int) {
this.v[offset + ix*xstride + iy*ystride + iz*zstride] = v
}
operator fun get(ix:Int): Matrix2D {
return Matrix2D(v, ny, nz, offset + ix*xstride, ystride, zstride )
}
fun transform(body: (Int, Int, Int, Int) -> Int ) {
for(iz in 0..(nz-1)) {
for(iy in 0..(ny-1)) {
for(ix in 0..(nx-1)){
this[ix,iy,iz] = body(this[ix,iy,iz], ix,iy,iz)
}
}
}
}
fun bake() : Matrix3D {
val rv = Matrix3D(nx,ny,nz);
for(ix in 0..(nx-1)) {
for(iy in 0..(ny-1)) {
for(iz in 0..(nz-1)){
rv[ix,iy,iz] = this[ix,iy,iz]
}
}
}
return rv
}
}
Kotlin currently does not support array literals.
You can use a combination of arrayOf() and intArrayOf():
val data = arrayOf(
arrayOf(intArrayOf(0, 0), intArrayOf(0)),
arrayOf(intArrayOf(0, 1), intArrayOf(0)),
arrayOf(intArrayOf(1, 0), intArrayOf(0)),
arrayOf(intArrayOf(1, 1), intArrayOf(1))
)
You can cut down a little bit on the verbosity using import aliasing if needed:
import kotlin.arrayOf as arr
import kotlin.intArrayOf as iarr
val data = arr(
arr(iarr(0, 0), iarr(0)),
arr(iarr(0, 1), iarr(0)),
arr(iarr(1, 0), iarr(0)),
arr(iarr(1, 1), iarr(1))
)
Also note that you can auto-convert Java code to Kotlin
in IntelliJ IDEA: copy Java code into a Kotlin file, a confirmation prompt will open.
online: using http://try.kotlinlang.org.
Using Multik
Multik Multidimensional array library for Kotlin.
Syntax for creating 3D array
mk.d3array(2, 2, 3) { it * it }
//output
/*[[[0, 1, 4],
[9, 16, 25]],
[[1, 0, 0],
[1, 1, 1]]]
*/
Note:Multik supports up to 4 dimensions
The Multik project’s GitHub repository
For more info check Jetbrain blog post