I am new to Scala and working on a project. I am writing a function that is suppose to return the smallest Int in an array. However when I run it I get a type error that it is returning unit instead of int.
Here is my code:
def minWhile(r: Array[Int]): Int = {
var pos = 1
var minInt = r(0)
while (pos < r.length) {
if (r(pos) < minInt)
minInt = r(pos)
pos += 1
}
minInt
}
Thank you very much!
Your code is compiled correctly, because it always returns correct type Int. But it may cause some runtime exceptions, if we passed empty array into your function: minWhile(Array()).
def minWhile(r: Array[Int]): Int =
{
var pos = 1
var minInt = r(0) /* potential runtime exception */
while( pos < r.length){
if(r(pos) < minInt)
minInt = r(pos)
pos+=1
}
minInt
}
You have to check arrays bounds working with it.
Or you can use a shorter way:
def minWhile(r: Array[Int]): Option[Int] = if (r.nonEmpty) Some(r.min) else None
Your code should be compiled correctly but it may rise an exception when passed empty container. You are using variables which is discouraged.
I would do it using some recursion calls or scala collections API like so:
array.reduceLeft(a: Somehitng, b: Something => Something)
Check thia link:
http://m.alvinalexander.com/scala/scala-use-reduceleft-get-max-min-from-collection
Consider this more functional style of conveying the semantics of minWhile, as follows,
def minWhile(r: Array[Int]): Int = {
(r zip r.drop(1)).takeWhile(t => t._1 > t._2).last._2
}
where we zip consecutive items and take those monotonically decreasing. The desired result is found in the second part of the last duple.
I'm trying to write a class where when you call a function defined in the class, it will store it in an array of functions instead of executing it right away, then user calls exec() to execute it:
class TestA(val a: Int, newAction: Option[ArrayBuffer[(Int) => Int]]) {
val action: ArrayBuffer[(Int) => Int] = if (newAction.isEmpty) ArrayBuffer.empty[(Int) => Int] else newAction.get
def add(b: Int): TestA = {action += (a => a + b); new TestA(a, Some(action))}
def exec(): Int = {
var result = 0
action.foreach(r => result += r.apply(a))
result
}
def this(a:Int) = this(a, None)
}
Then this is my test code:
"delayed action" should "delay action till ready" in {
val test = new TestA(3)
val result = test.add(5).add(5)
println(result.exec())
}
This gives me a result of 16 because 3 was passed in twice and got added twice. I guess the easy way for me to solve this problem is to not pass in value for the second round, like change val a: Int to val a: Option[Int]. It helps but it doesn't solve my real problem: letting the second function know the result of the first execution.
Does anyone have a better solution to this?? Or if this is a pattern, can anyone share a tutorial of it?
Just save the result of the action in the 'result' variable (instatiate it with 'a') and use the previous result as input for the current iteration
def exec(): Int = {
var result = a
action.foreach(r => result = r.apply(result))
result
}
or use the more functional oriented solution that does the same
def exec(): Int = {
action.foldLeft(a)((r, f) => f.apply(r))
}
I have this code UniqueKeyGenerator, there is a mutable variable number to be incrementally added, because I want to make the generated value to be predictable, I wonder how can I change it to be value instead of variable
object UniqueKeyGenerator {
var number = 0
def generate(suiteClass: Class[_]): String = {
number = number + 1
suiteClass.getCanonicalName + "." + this.number
}
}
Many thanks in advance
If you're just trying to express this with a val instead of a var, you could use an Iterator.
object UniqueKeyGenerator {
val numbers = Iterator.iterate(0)(_ + 1)
def generate(suiteClass: Class[_]): String =
s"${suiteClass.getCanonicalName}.${numbers.next()}"
}
Otherwise, I'm not sure what you're asking - maybe provide some more context?
If you have all of the inputs up front, then you can write something like this:
Seq(classOf[String], classOf[Int], classOf[String]).zipWithIndex
.map({ case (suiteClass, i) => s"${suiteClass.getCanonicalName}.$i" })
// res: List(java.lang.String.0, int.1, java.lang.String.2)
Aside from an Iterator you can also use a closure, for example if the id you want to generate is not just the next natural number, but a result of a more complex expression.
val nextId = {
var id = 0
() => {
id = id + 1 // change it to something more complex if you like
id
}
}
The "id" here is still a variable, but it is not accessible from the outside. The only way you can use it is by calling nextId:
def generate(suiteClass: Class[_]): String = {
suiteClass.getCanonicalName + "." + nextId()
}
I'm still learning Scala, but one thing I thought was interesting is that Scala blurs the line between methods and fields. For instance, I can build a class like this...
class MutableNumber(var value: Int)
The key here is that the var in the constructor-argument automatically allows me to use the 'value' field like a getter/setter in java.
// use number...
val num = new MutableNumber(5)
num.value = 6
println(num.value)
If I want to add constraints, I can do so by switching to using methods in place of the instance-fields:
// require all mutable numbers to be >= 0
class MutableNumber(private var _value: Int) {
require(_value >= 0)
def value: Int = _value
def value_=(other: Int) {
require(other >=0)
_value = other
}
}
The client side code doesn't break since the API doesn't change:
// use number...
val num = new MutableNumber(5)
num.value = 6
println(num.value)
My hang-up is with the named-parameter feature that was added to Scala-2.8. If I use named-parameters, my API does change and it does break the api.
val num = new MutableNumber(value=5) // old API
val num = new MutableNumber(_value=5) // new API
num.value = 6
println(num.value)
Is there any elegant solution to this? How should I design my MutableNumber class so that I can add constraints later on without breaking the API?
Thanks!
You can use the same trick that case classes do: use a companion object.
object Example {
class MutableNumber private (private var _value: Int) {
require (_value >= 0)
def value: Int = _value
def value_=(i: Int) { require (i>=0); _value = i }
override def toString = "mutable " + _value
}
object MutableNumber {
def apply(value: Int = 0) = new MutableNumber(value)
}
}
And here it is working (and demonstrating that, as constructed, you must use the object for creations, since the constructor is marked private):
scala> new Example.MutableNumber(5)
<console>:10: error: constructor MutableNumber cannot be accessed in object $iw
new Example.MutableNumber(5)
^
scala> Example.MutableNumber(value = 2)
res0: Example.MutableNumber = mutable 2
scala> Example.MutableNumber()
res1: Example.MutableNumber = mutable 0
Thanks for the answer! As an aside, I think the Scala-guys might be aware that there's an issue:
What's New in Scala 2.8: Named and Default Parameters
...
Until now, the names of arguments were a somewhat arbitrary choice for library developers, and weren't considered an important part of the API. This has suddenly changed, so that a method call to mkString(sep = " ") will fail to compile if the argument sep were renamed to separator in a later version.
Scala 2.9 implements a neat solution to this problem, but while we're waiting for that, be cautious about referring to arguments by name if their names may change in the future.
http://www.artima.com/scalazine/articles/named_and_default_parameters_in_scala.html
class MutableNumber {
private var _value = 0 //needs to be initialized
def value: Int = _value
def value_=(other: Int) {
require(other >=0) //this requirement was two times there
_value = other
}
}
you can modify all members of any class within curly braces
val n = new MutableNumber{value = 17}
Is there any reason for Scala not support the ++ operator to increment primitive types by default?
For example, you can not write:
var i=0
i++
Thanks
My guess is this was omitted because it would only work for mutable variables, and it would not make sense for immutable values. Perhaps it was decided that the ++ operator doesn't scream assignment, so including it may lead to mistakes with regard to whether or not you are mutating the variable.
I feel that something like this is safe to do (on one line):
i++
but this would be a bad practice (in any language):
var x = i++
You don't want to mix assignment statements and side effects/mutation.
I like Craig's answer, but I think the point has to be more strongly made.
There are no "primitives" -- if Int can do it, then so can a user-made Complex (for example).
Basic usage of ++ would be like this:
var x = 1 // or Complex(1, 0)
x++
How do you implement ++ in class Complex? Assuming that, like Int, the object is immutable, then the ++ method needs to return a new object, but that new object has to be assigned.
It would require a new language feature. For instance, let's say we create an assign keyword. The type signature would need to be changed as well, to indicate that ++ is not returning a Complex, but assigning it to whatever field is holding the present object. In Scala spirit of not intruding in the programmers namespace, let's say we do that by prefixing the type with #.
Then it could be like this:
case class Complex(real: Double = 0, imaginary: Double = 0) {
def ++: #Complex = {
assign copy(real = real + 1)
// instead of return copy(real = real + 1)
}
The next problem is that postfix operators suck with Scala rules. For instance:
def inc(x: Int) = {
x++
x
}
Because of Scala rules, that is the same thing as:
def inc(x: Int) = { x ++ x }
Which wasn't the intent. Now, Scala privileges a flowing style: obj method param method param method param .... That mixes well C++/Java traditional syntax of object method parameter with functional programming concept of pipelining an input through multiple functions to get the end result. This style has been recently called "fluent interfaces" as well.
The problem is that, by privileging that style, it cripples postfix operators (and prefix ones, but Scala barely has them anyway). So, in the end, Scala would have to make big changes, and it would be able to measure up to the elegance of C/Java's increment and decrement operators anyway -- unless it really departed from the kind of thing it does support.
In Scala, ++ is a valid method, and no method implies assignment. Only = can do that.
A longer answer is that languages like C++ and Java treat ++ specially, and Scala treats = specially, and in an inconsistent way.
In Scala when you write i += 1 the compiler first looks for a method called += on the Int. It's not there so next it does it's magic on = and tries to compile the line as if it read i = i + 1. If you write i++ then Scala will call the method ++ on i and assign the result to... nothing. Because only = means assignment. You could write i ++= 1 but that kind of defeats the purpose.
The fact that Scala supports method names like += is already controversial and some people think it's operator overloading. They could have added special behavior for ++ but then it would no longer be a valid method name (like =) and it would be one more thing to remember.
I think the reasoning in part is that +=1 is only one more character, and ++ is used pretty heavily in the collections code for concatenation. So it keeps the code cleaner.
Also, Scala encourages immutable variables, and ++ is intrinsically a mutating operation. If you require +=, at least you can force all your mutations to go through a common assignment procedure (e.g. def a_=).
The primary reason is that there is not the need in Scala, as in C. In C you are constantly:
for(i = 0, i < 10; i++)
{
//Do stuff
}
C++ has added higher level methods for avoiding for explicit loops, but Scala has much gone further providing foreach, map, flatMap foldLeft etc. Even if you actually want to operate on a sequence of Integers rather than just cycling though a collection of non integer objects, you can use Scala range.
(1 to 5) map (_ * 3) //Vector(3, 6, 9, 12, 15)
(1 to 10 by 3) map (_ + 5)//Vector(6, 9, 12, 15)
Because the ++ operator is used by the collection library, I feel its better to avoid its use in non collection classes. I used to use ++ as a value returning method in my Util package package object as so:
implicit class RichInt2(n: Int)
{
def isOdd: Boolean = if (n % 2 == 1) true else false
def isEven: Boolean = if (n % 2 == 0) true else false
def ++ : Int = n + 1
def -- : Int = n - 1
}
But I removed it. Most of the times when I have used ++ or + 1 on an integer, I have later found a better way, which doesn't require it.
It is possible if you define you own class which can simulate the desired output however it may be a pain if you want to use normal "Int" methods as well since you would have to always use *()
import scala.language.postfixOps //otherwise it will throw warning when trying to do num++
/*
* my custom int class which can do ++ and --
*/
class int(value: Int) {
var mValue = value
//Post-increment
def ++(): int = {
val toReturn = new int(mValue)
mValue += 1
return toReturn
}
//Post-decrement
def --(): int = {
val toReturn = new int(mValue)
mValue -= 1
return toReturn
}
//a readable toString
override def toString(): String = {
return mValue.toString
}
}
//Pre-increment
def ++(n: int): int = {
n.mValue += 1
return n;
}
//Pre-decrement
def --(n: int): int = {
n.mValue -= 1
return n;
}
//Something to get normal Int
def *(n: int): Int = {
return n.mValue
}
Some possible test cases
scala>var num = new int(4)
num: int = 4
scala>num++
res0: int = 4
scala>num
res1: int = 5 // it works although scala always makes new resources
scala>++(num) //parentheses are required
res2: int = 6
scala>num
res3: int = 6
scala>++(num)++ //complex function
res4: int = 7
scala>num
res5: int = 8
scala>*(num) + *(num) //testing operator_*
res6: Int = 16
Of course you can have that in Scala, if you really want:
import scalaz._
import Scalaz._
case class IncLens[S,N](lens: Lens[S,N], num : Numeric[N]) {
def ++ = lens.mods(num.plus(_, num.one))
}
implicit def incLens[S,N:Numeric](lens: Lens[S,N]) =
IncLens[S,N](lens, implicitly[Numeric[N]])
val i = Lens[Int,Int](identity, (x, y) => y)
val imperativeProgram = for {
_ <- i := 0;
_ <- i++;
_ <- i++;
x <- i++
} yield x
def runProgram = imperativeProgram ! 0
And here you go:
scala> runProgram
runProgram: Int = 3
It isn't included because Scala developers thought it make the specification more complex while achieving only negligible benefits and because Scala doesn't have operators at all.
You could write your own one like this:
class PlusPlusInt(i: Int){
def ++ = i+1
}
implicit def int2PlusPlusInt(i: Int) = new PlusPlusInt(i)
val a = 5++
// a is 6
But I'm sure you will get into some trouble with precedence not working as you expect. Additionally if i++ would be added, people would ask for ++i too, which doesn't really fit into Scala's syntax.
Lets define a var:
var i = 0
++i is already short enough:
{i+=1;i}
Now i++ can look like this:
i(i+=1)
To use above syntax, define somewhere inside a package object, and then import:
class IntPostOp(val i: Int) { def apply(op: Unit) = { op; i } }
implicit def int2IntPostOp(i: Int): IntPostOp = new IntPostOp(i)
Operators chaining is also possible:
i(i+=1)(i%=array.size)(i&=3)
The above example is similar to this Java (C++?) code:
i=(i=i++ %array.length)&3;
The style could depend, of course.