I am new to coding in scala and I am curious about something and it has been hard to find an answer for online. I have this array that takes multiple arguments of different types (:Any)
val arguments= Array("Monday",10,20,Array("test","test2"), if(4 == 4){ "true"})
I iterated and printed the content inside of it. Everything prints properly except the Array at index 3. I get the object memory address I believe, which is understandable-- same thing with Java would happen. But I am curious, how would you access it?
I tried saving the value of arguments(3) in an array (val arr:Array[String] = arguments(3)) but it didn't work because there is a type mismatch (any != Array[String])
Any tips? It might be a gap in my understanding of functional programming.
What you are iterating through is an Array[Any], so you are able to perform functions that are available to an Any type. You can access the items in your array using pattern matching, which uses the unapply methods under the hood to see if it can turn your Any into something more specific:
val arguments= Array("Monday",10,20,Array("test","test2"), if(4 == 4){ "true"})
arguments foreach { arg =>
arg match {
case a:Array[String] => println(s"This is the array: ${a.mkString(",")}, and I can do array functions ${a.contains("test")}")
case _ => println(s"Otherwise I have this: $arg")
}
}
// stdout:
// Otherwise I have this: Monday
// Otherwise I have this: 10
// Otherwise I have this: 20
// This is the array: test,test2, and I can do array functions true
// Otherwise I have this: true
I'm having some trouble around arrays in OCaml.
Namely, I want to just shift the elements to the right or left based on a value I pass.
Example: # let a = [|1;2;3;4;5|], # shift_array a 7;;- : int array array = [|4;5;1;2;3|]
I wrote the following code but I keep getting a syntax error where the second array is formed.
let shift_array arr x =
let size = Array.length arr in
let sec_arr = Array.make size 0 in
for i = 0 to size - 1 do
if i < x
then (sec_arr.(size - x + 1) <- arr.(i))
else (sec_arr.(i-x) <- arr.(i))
done;;
I'm just not 100% sure how to print out the new array.
EDIT: fixed it by adding in to the second and third line.
The problem now is that the function has type int array -> int -> unit and the one I'm trying to get is 'a array -> int -> 'a array. Is there some way to work around that ?
It should be let size = Array.length arr in, notice the in, which you're missing.
The let expression in OCaml has the form let <var> = <expr> in <body> and should not be (but commonly is) confused with the let definition that can occur only on the top-level (as an element of a module), which has form let <name> = <body>.
In your example, you have both, the top-level definition, let shift_array = <body> and two let expressions (though, you have used the wrong syntax for them.
EDIT:
Since OP edited the post, here is the corresponding edit.
You function doesn't return anything, it creates a new array, does the cycle, but doesn't return anything but the unit value (which is the value to which the for cycle evaluates). So you have to add one more line, that will contain the expression, to which the whole function will evaluate. Hint the sequencing operator ; is what you need, when you have expression x;y;z the computer evaluates x, then y, and finally z and the value of the whole expression of x;y;z is the value of z.
I am doing a project in scala and i am struggling with a certain thing. I am making a matrix DSL so i am overloading some operators like +,- or * so that i can do :
matrixMult = matrix1*matrix2
The thing is i made this class where the matrix was represented as a Array[Array[Double]] type but i would like to make it generic: Array[Array[T]]
The thing is i do not know how to handle this in the class methods since for operations like +,- and *. It should work for doubles or ints, but strings should throw an error. Here is my current code:
def +(other : Matrix[Double]): Matrix[Double] = {
var array = new Array[Array[Double]](rows)
for (i <- 0 to (rows - 1)) {
var arrayRow = new Array[Double](columns)
for (j <- 0 to (columns - 1)) {
arrayRow(j) = this.array(i)(j) + other.array(i)(j)
}
array(i) = arrayRow
}
return new Matrix(array)
}
I get an error on the arrayRow(j) =... line which is normal because it does not know what type the "this" object is.
What should i do to make this work? Like i would like this method only to be accessible to doubles (or ints) and not strings, if this method was to be invoked on a Matrix[String] object it should throw an error. I tried pattern matching with isInstanceOf() but that doesn't remove the error and i can't compile.
If kind of have the same issue with all of my methods in my class, so i'd like a generic answer if possible.
Any help is appreciated,
Thank you very much!
Not sure which version of Scala you are using, but if you're on 2.8, I found this thread on Scala-lang, and it looks like you may be able to use T:Numerics to limit it to Int, Long, Float, Double.
A little farther down in the thread, to limit it to JUST a subset of those (like Int, Double), they say to define your own generic Trait.
https://www.scala-lang.org/old/node/4787
Answer can be found in comments:
Matrix addition has been asked, and answered, before. Even though the question posses a different matrix implementation, I believe the answer from the redoubtable Rex Kerr is still applicable. – jwvh
I'm looking to calculate the range of a list or array and was frustrated by not being able to simply take the difference of the max() and min() functions due to null safety feature of Kotlin.
I came up with this solution which is forcing me to deal with the case of an ifEmpty() even if I know that is not possible in with this immutable array. I wonder if there is something simpler without the extra characters.
val array=arrayOf(1,2,3,4,5)
// doesn't compile... val range = array.max()-array.min()
val range = (array.max() ?: 0) - (array.min() ?: 0)
This isn't much different, but since you know that it isn't empty, you can use !! (the Hold My Beer operator), to tell Kotlin to ignore the nullability of the types.
val range = if(array.isEmpty()) 0 else array.max()!! - array.min()!!
It might also be handy to define an extension if you anticipate doing this a lot:
fun Array<Int>.range(): Int =
if(isEmpty()) 0 else max()!! - min()!!
// Usage
val range = array.range()
Updated Question; Original below.
I am trying to create an array which represents a grid of cells, which have tuples containing the walls they are surrounded by.
I have come up with this:
rooms(Array) ->
Size = array:size(Array),
if
Size == ?HSIZE * ?VSIZE ->
Array;
true ->
HFactor = Size rem ?VSIZE,
VFactor = Size div ?HSIZE,
Room = {1+HFactor+11*VFactor,
7+HFactor+11*VFactor,
12+HFactor+11*VFactor,
6+HFactor+11*VFactor},
rooms(array:set(Size, Room, Array))
end.
When I run this with rooms(array:new()). I get the following array back:
{array,25,100,undefined,
{{{1,7,12,6},
{2,8,13,7},
{3,9,14,8},
{4,10,15,9},
{5,11,16,10},
{12,18,23,17},
{13,19,24,18},
{14,20,25,19},
{15,21,26,20},
{16,22,27,21}},
{{23,29,34,28},
{24,30,35,29},
{25,31,36,30},
{26,32,37,31},
{27,33,38,32},
{34,40,45,39},
{35,41,46,40},
{36,42,47,41},
{37,43,48,42},
{38,44,49,43}},
{{45,51,56,50},
{46,52,57,51},
{47,53,58,52},
{48,54,59,53},
{49,55,60,54},
undefined,undefined,undefined,undefined,undefined},
10,10,10,10,10,10,10,10}}
Which is quite close to the desired result, but there are two things I can't quite put my finger on (The numbers are correct). Why does it look like it is split up into multiple subarrays? What are those undefineds and 10's doing there? These are mostly due to my lack of erlang knowledge, because array:get produces the expected results, but I couldn't find anything which explains where they come from.
Original Question
rooms(Array) ->
Size = array:size(Array),
if
Size == 5 ->
Array;
Size rem 5 == 0 ->
rooms(array:set(Size, array:new(), Array));
true ->
In_Array = array:get(array:size(Array), Array),
In_Size = array:size(In_Array),
Room = {1+In_Size+11*In_Size,
7+In_Size+11*In_Size,
12+In_Size+11*In_Size,
6+In_Size+11*In_Size},
New_In = array:set(In_Size, Room, In_Array),
rooms(array:set(Size, New_In, Array))
end.
I call it with rooms(array:new()). but the result is
** exception error: bad argument
in function array:size/1 (array.erl, line 317)
in call from framework_kamer:rooms/1 (framework_kamer.erl, line 195)
Which makes sense because In_Array is not an array, but undefined. However, I can't figure out why.
Side question, is there an easier/cleaner/better way to do this?
You are (in the second call of the recursion) trying
In_Array = array:get(array:size(Array), Array),
As array is zero-indexed this will always fail, as the access will be always be off by one. Change this line to
In_Array = array:get(array:size(Array) - 1, Array),
and you are fine.
A few comments on your code:
Conventional variable naming in Erlang would be CamelCase without underscores (i.e. InArray)
Expressing a 2-dimensional array as nested arrays is almost never a good idea. Linearalise it by writing simple wrappers that recalculate a one-dimensional index from x and y as index = y * max_x + x.