I wonder what is the real use of passing a closure object to a method.
Lets say I have a closure :
def a = {
println it
}
(Consider it is doing some sort of operation rather than just printing)
Now I'm just passing this closure a to a method :
def testMethod(def input,Closure a){
a(input)
}
testMethod "MethodPointer", a //passing closure.
Now the question is why this level of indirection? Why can't testMethod directly process its input? Yes, here I'm making input to be processed in closure a, but why one should do so?. What is the real use of passing closure around?
Thanks in advance.
How would you write the collect method without Closures for parameters? It would be a lot more complex than it is currently. The same goes for Builders, inject, each, with and many more...
It allows you to define something generic, and then make it more specific at a later date. For example, the collect method could be described as "takes a collection or iterable, and for each element DO SOMETHING and add it to a newly created collection". Without Closures to specify this DO SOMETHING at a later date, the value of collect would be minimal.
Slowly, you come to a more functional way of thinking. Rather than writing a specific function to perform a specific task, could I write a function which takes more of a generalist approach applicable to multiple problems, and put the specifics for each individual case in a Closure?
Edit
As an example, consider this procedural method which returns a List of the numbers between min and max which are multiples of mult:
List<Integer> filter( int min, int max, int mult ) {
List<Integer> multiples = new ArrayList<Integer>() ;
for( int i = min ; i < max ; i++ ) {
if( i % mult == 0 ) {
multiples.add( i ) ;
}
}
multiples
}
println filter( 1, 200, 15 )
If we write this in Groovy using Closures (for the filtering), we get:
List<Integer> filter( int min, int max, int mult ) {
(min..max).findAll { it % mult == 0 }
}
println filter( 1, 200, 15 )
(I accept that this example is basically mirroring the functionality of findAll, so probably isn't a great example -- and is also somewhat contrived)
Now consider that we want to filter based on some other criteria (that the integer exists in a database or something)... We could first rewrite our filter method to:
List<Integer> filter( int min, int max, Closure<Boolean> filter ) {
(min..max).findAll filter
}
println filter( 1, 200 ) { it % 15 == 0 }
Then, as you can see, we can pass any closure to this method that returns true if we want to keep the element, or false if we want to drop it.
To do this in an imperative style, you're going to (probably) end up with multiple methods all doing very similar tasks...
The idea is to write modular and structured clear code.
I wrote a blog post about Closure Design Patterns. You can find some examples of patterns using closures.
Related
I stumbled upon this challenge on stackoverflow while learning about property based testing in scala using ScalaCheck.
Find the smallest positive integer that does not occur in a given sequence
I thought of trying to write a generator driven property based test for this problem to check the validity of my program but can't seem to be able to think of a how to write a relevant test case. I understand that I could write a table driven property based testing for this use case but that limit the number of properties I could test this algo with.
import scala.annotation.tailrec
object Solution extends App {
def solution(a: Array[Int]): Int = {
val posNums = a.toSet.filter(_ > 0)
#tailrec
def checkForSmallestNum(ls: Set[Int], nextMin: Int): Int = {
if (ls.contains(nextMin)) checkForSmallestNum(ls, nextMin + 1)
else nextMin
}
checkForSmallestNum(posNums, 1)
}
}
Using Scalatest's (since you did tag scalatest) Scalacheck integration and Scalatest matchers, something like
forAll(Gen.listOf(Gen.posNum[Int]) -> "ints") { ints =>
val asSet = ints.toSet
val smallestNI = Solution.solution(ints.toArray)
asSet shouldNot contain(smallestNI)
// verify that adding non-positive ints doesn't change the result
forAll(
Gen.frequency(
1 -> Gen.const(0),
10 -> Gen.negNum[Int]
) -> "nonPos"
) { nonPos =>
// Adding a non-positive integer to the input shouldn't affect the result
Solution.solution((nonPos :: ints).toArray) shouldBe smallestNI
}
// More of a property-based approach
if (smallestNI > 1) {
forAll(Gen.oneOf(1 until smallestNI) -> "x") { x =>
asSet should contain(x)
}
} else succeed // vacuous
// Alternatively, but perhaps in a less property-based way
(1 until smallestNI).foreach { x =>
asSet should contain(x)
}
}
Note that if scalatest is set to try forAlls 100 times, the nested property check will check values 10k times. Since smallestNI will nearly always be less than the number of trials (as listOf rarely generates long lists), the exhaustive check will in practice be faster than the nested property check.
The overall trick, is that if something is the least positive integer for which some predicate applies, that's the same as saying that for all positive integers less than that something the predicate does not apply.
Question :
from the list of an integer get the square of odd number and half the even number and then return the list of value.
Ans : 1> i will write the logic and if else condition inside the map() method.
List<Integer> output = intArray.stream().map(x-> {
if(x%2 ==0){
x=x/2;
}else{
x= x*x;
}
}).collect(Collectors.toList());
Is their any better way to do this specially using Filter?
Try using map:
map(x -> x % 2 == 0? x / 2: x * x);
Let me know if this works for you.
You can learn more about map and filter here
As you are transforming data (performing a math operation), you cannot use Filter here. Filter is used to filter out elements in your stream. For example if you only want to preserve the even numbers, you could use a Filter
What you need to use is the Map, as you already did. Do note that a map should always return data. Your code is missing this return statement.
To make it more readable, you could split your mapping logic in a method. This makes your stream easy to read and easy to follow (when you give the method a good name ofcourse).
Code example
List<Integer> output = intArray.stream()
.map(test::divideOrPow)
.collect(Collectors.toList());
private int divideOrPow(intx) {
if (x % 2 == 0) {
return x / 2;
} else {
return x * x;
}
}
I have a list of A class objects
data class A{
val abc: Abc
val values: Int?
}
val list = List<A>
If I want to count how many objects I have in list I use:
val count= a.count()
or val count= a.count(it -> {})
How to append all values in the list of objects A avoiding for loop? Generaly Im looking for proper kotlin syntax with avoiding code below
if (a!= null) {
for (i in list) {
counter += i.values!!
}
}
Either use sumBy or sum in case you have a list of non-nullable numbers already available, i.e.:
val counter = list.sumBy { it.values ?: 0 }
// or
val counter = extractedNonNullValues.sum()
The latter only makes sense if you already mapped your A.values before to a list of non-nullable values, e.g. something like:
val extractedNonNullValues= list.mapNotNull { it.values } // set somewhere else before because you needed it...
If you do not need such an intermediate extractedNonNullValues-list then just go for the sumBy-variant.
I don't see you doing any appending to a list in the question. Based on your for loop I believe what you meant was "How do I sum properties of objects in my list". If that's the case you can use sumBy, the extension function on list that takes a labmda: ((T) -> Int) and returns an Int like so:
val sum = list.sumBy { a -> a.values ?: 0 }
Also, calling an Int property values is pretty confusing, I think it should be called value. The plural indicates a list...
On another note, there is a possible NPE in your original for loop. Avoid using !! on nullable values as, if the value is null, you will get an NPE. Instead, use null coalescing (aka elvis) operator to fall back to a default value ?: - this is perfectly acceptable in a sum function. If the iteration is not to do with summing, you may need to handle the null case differently.
I'm wanting to remove all of a possibly duplicated value in an array. At the moment I'm using the remove(x:T):Bool function in a while loop, but I'm wondering about the expression part.
I've started by using:
function removeAll(array:Array<String>, element:String):Void
while (array.remove(element)) {}
but I'm wondering if any of these lines would be more efficient:
while (array.remove(element)) continue;
while (array.remove(element)) true;
while (array.remove(element)) 0;
or if it makes any kind of difference.
I'm guessing that using continue is less efficient because it actually has to do something, true and 0 are slightly more efficient, but still do something, and {} would probably be most efficient.
Does anyone have any background information on this?
While other suggested filter, it will create a new instance of list/array which may cause your other code to lose reference.
If you loop array.remove, it is going to loop through all the elements in the front of the array every time, which is not so performant.
IMO a better approach is to use a reverse while loop:
var i = array.length;
while(--i >= 0)
if(array[i] == element) array.splice(i, 1);
It doesn't make any difference. In fact, there's not even any difference in the generated code for the {}, 0 and false cases: they all end up generating {}, at least on the JS target.
However, you could run into issues if you have a large array with many duplicates: in that case, remove() would be called many times, and it has to iterate over the array each time (until it finds a match, that is). In that case, it's probably more efficient to use filter():
function removeAll(array:Array<String>, element:String):Array<String>
return array.filter(function(e) return e != element);
Personally, I also find this to be a bit more elegant than your while-loop with an empty body. But again, it depends on the use case: this does create a new array, and thus causes an allocation. Usually, that's not worth worrying about, but if you for instance do it in the update loop of a game, you might want to avoid it.
In terms of the expression part of the while loop, it seems that it's just set to empty brases ({}) when compiled so it doesn't really matter what you do.
In terms of performance, a much better solution is the Method 2 from the following:
class Test
{
static function main()
{
var thing:Array<String> = new Array<String>();
for (index in 0...1000)
{
thing.push("0");
thing.push("1");
}
var copy1 = thing.copy();
var copy2 = thing.copy();
trace("epoch");
while (copy1.remove("0")) {}
trace("check");
// Method 2.
copy2 = [
for (item in Lambda.filter(copy2, function(v)
{return v != "0";}))
item
];
trace("check");
}
}
which can be seen [here](https://try.haxe.org/#D0468"Try Haxe example."). For 200,000 one-character elements in an Array<String>, Method 2 takes 0.017s while Method 1 takes 44.544s.
For large arrays it will be faster to use a temporary array and then assign that back after populating ( method3 in try )?
OR
If you don't want to use a temp you can assign back and splice ( method4 in try )?
https://try.haxe.org/#5f80c
Both are more verbose codewise as I setup vars, but on mac seems faster at runtime, summary of my method3 approach:
while( i < l ) { if( ( s = copy[ i++ ] ) != '0' ) arr[ j++ ] = s;
copy = arr;
am I missing something obvious against these approaches?
I mean this:
bool passed = true;
for(int i = 0; i < collection.Length; i++)
{
if(!PassesTest(collection[i]))
{
passed = false;
break;
}
}
if(passed){/*passed code*/}
requires extra variable, extra test
for(int i = 0; i < collection.Length; i++)
{
if(!PassesTest(collection[i]))
{
return;
}
}
{/*passed code*/}
neat, but requires this to be it's own function, if this it's self is inside a loop or something, not the most performant way of doing things. also, writing a whole extra function is a pain
if(passed){/*passed code*/}
for(int i = 0; i < collection.Length; i++)
{
if(!PassesTest(collection[i]))
{
goto failed;
}
}
{/*passed code*/}
failed: {}
great, but you have to screw around with label names and ugly label syntax
for(int i = 0; ; i++)
{
if(!(i < collection.Length))
{
{/*passed code*/}
break;
}
if(!PassesTest(collection[i]))
{
break;
}
}
probably the nicest, but still a bit manual, kinda wasting the functionality of the for loop construct, for instance, you can't do this with a foreach
what is the nicest way to handle this problem?
it seems to me something like this would be nice:
foreach(...)
{
...
}
finally{...} // only executed if loop ends conventionally (without break)
am I missing something? because this is a very common problem for me, and I don't really like any of the solutions I've come up with.
I use c++ and C#, so solutions in either would be great.
but would also be interested in solutions in other languages. (though a design principle that avoids this in any language would be ideal)
If your language doesn't have this feature, write a function "forall," which takes two arguments: a list and a boolean-valued function which is to be true for all elements of the list. Then you only have to write it once, and it matters very little how idiomatic it is.
The "forall" function looks exactly like your second code sample, except that now "collection" and "passesTest" are the arguments to that function.
Calling forall looks roughly like:
if (forall(myList,isGood)) {
which is readable.
As an added bonus, you could implement "exists" by calling "forall" on the negated boolean function, and negating its answer. That is, "exists x P(x)" is implemented as "not forall x not P(x)".
You can use Linq in .NET.
Here's an example in C#:
if(collection.All(item => PassesTest(item)))
{
// do my code
}
C++ and STL
if (std::all_of(collection.begin(), collection.end(), PassesTest))
{
/* passed code */
}
Ruby:
if collection.all? {|n| n.passes_test?}
# do something
end
Clojure:
(if (every? passes-test? collection)
; do something
)
Groovy:
if (!collection.find { !PassesTest(it) }) {
// do something
}
Scala:
def passesTest(i: Int) = i < 5 // example, would likely be idiomatically in-line
var seq = List(1,2,3,4);
seq.forall(passesTest) // => True
Most, if not all of the answers presented here are saying: higher-order constructs -- such as "passing functions" -- are really, really nice. If you are designing a language, don't forget about the last 60+ years of programming languages/designs.
Python (since v2.5):
if all(PassesTest(c) for c in collection):
do something
Notes:
we can iterate directly on collection, no need for an index and lookup
all() and any() builtin functions were added in Python 2.5
the argument to any(), i.e. PassesTest(c) for c in collection, is a generator expression. You could also view it as a list comprehension by adding brackets [(PassesTest(c) for c in collection]
... for c in collection causes iteration through a collection (or sequence/tuple/list). For a dict, use dict.keys(). For other datatypes, use the correct iterator method.