Purpose of an 'Identity Function'? - prototypejs

I came across this subject when I was reading through PrototypeJS's docs: its Identity Function. I did some further searching&reading on it and I think I understand its mathematical basis (e.g. multiplication by 1 is an identity function (or did I misinterpret this?)), but not why you would write a JS(or PHP or C or whatever)-function that basically takes X as a parameter and then just does something like return X.
Is there a deeper insight connected to this? Why does Prototype supply this function? What can I use it for?
Thanks :)

Using the Identity function makes the library code slightly easier to read. Take the Enumerable#any method:
any: function(iterator, context) {
iterator = iterator || Prototype.K;
var result = false;
this.each(function(value, index) {
if (result = !!iterator.call(context, value, index))
throw $break;
});
return result;
},
It allows you to check if any of the elements of an array are true in a boolean context. Like so:
$A([true, false, true]).any() == true
but it also allows you to process each of the elements before checking for true:
$A([1,2,3,4]).any(function(e) { return e > 2; }) == true
Now without the identity function you would have to write two versions of any function, one if you pre process and one if you dont.
any_no_process: function(iterator, context) {
var result = false;
this.each(function(value, index) {
if (value)
throw $break;
});
return result;
},
any_process: function(iterator, context) {
return this.map(iterator).any();
},

I do not know about that library, but normally, you optimize formuals or code or whatever by factoring common part out, like if (add) (a + b) + x else a + b should be rewritten into a + b + (add ? x : 0). You are tempted to do the same with
if (!initialized) initialize(callback_with_very_long_name) else callback_with_very_long_name
Looks pretty similar. You can easily factor out a common factor or term but how do we factor out a function application? If you understand mathematiscs or Hascel, you should see that
a ? x + v : v
looks very much like
a ? f value : value
You add x in one case but not in the other. You apply function in one case but not in the other. You optimize the former into (a ? x : 0) + v because 0 is additive identity (it does not change anything when added to it) and v is a common factor here, which comes always, regardless of application of x. In case of function application (or not application), the callback is the common factor. We want to factor it out. What is the identity function that we should apply to it so that nothing changes? Identity function!
(a ? f : identity) value
is what we are looking for. Our original example looks like the following then
(initialized ? identity : initialize) (callback_with_very_long_name)
Please note that it fits into one row of page now.

Related

Property based test with generators for algorithm: "Find the smallest positive integer that does not occur in a given sequence"

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.

how to get the if else condition using filter operation of java stream?

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;
}
}

Using the Haxe While Loop to Remove All of a Value from an Array

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?

Concise notation for assigning `unique_ptr`?

I have a pointer to a parent class and I want to assign a new child object to that pointer conditionally. Right now, the syntax I have is rather lengthly:
std::unique_ptr<ParentClass> parentPtr;
if (...) {
parentPtr = std::unique_ptr<ParentClass>(new ChildClass1());
} else {
parentPtr = std::unique_ptr<ParentClass>(new ChildClass2());
}
Is there a good way of making this more readable / less lengthly?
Two possibilities would be:
std::unique_ptr<ParentClass> parentPtr(condition ?
(ParentClass*)new ChildClass1() :
(ParentClass*)new ChildClass2());
If condition is complicated, just assign a boolean to it and then write the construction. This solution only works for a binary condition though.
Another is to embrace C++14, and use
parentPtr = std::make_unique<ChildClass>();
First off, the "obvious" solution C ? new X : new Y does not work, since even if X and Y have a common base class A, the types X * and Y * have no common type. This is actually not so surprising after all if you consider that a class can have many bases (direct or indirect) and a given type may appear as a base multiple times.
You could make the conditional operator work by inserting a cast:
A * = C ? static_cast<A *>(new X) : static_cast<A *>(new Y);
But this would quickly get long and tedious to read when you try to apply this to your real situation.
However, as for std::unique_ptr, it offers the reset function which can be used to good effect here:
std::unique_ptr<A> p;
if (C)
{
p.reset(new X);
}
else
{
p.reset(new Y);
}
Now even if the actual new expressions are long, this is still nicely readable.

Early return statements and cyclomatic complexity

I prefer this writing style with early returns:
public static Type classify(int a, int b, int c) {
if (!isTriangle(a, b, c)) {
return Type.INVALID;
}
if (a == b && b == c) {
return Type.EQUILATERAL;
}
if (b == c || a == b || c == a) {
return Type.ISOSCELES;
}
return Type.SCALENE;
}
Unfortunately, every return statement increases the cyclomatic complexity metric calculated by Sonar. Consider this alternative:
public static Type classify(int a, int b, int c) {
final Type result;
if (!isTriangle(a, b, c)) {
result = Type.INVALID;
} else if (a == b && b == c) {
result = Type.EQUILATERAL;
} else if (b == c || a == b || c == a) {
result = Type.ISOSCELES;
} else {
result = Type.SCALENE;
}
return result;
}
The cyclomatic complexity of this latter approach reported by Sonar is lower than the first, by 3. I have been told that this might be the result of a wrong implementation of the CC metrics. Or is Sonar correct, and this is really better? These related questions seem to disagree with that:
https://softwareengineering.stackexchange.com/questions/118703/where-did-the-notion-of-one-return-only-come-from
https://softwareengineering.stackexchange.com/questions/18454/should-i-return-from-a-function-early-or-use-an-if-statement
If I add support for a few more triangle types, the return statements will add up to make a significant difference in the metric and cause a Sonar violation. I don't want to stick a // NOSONAR on the method, as that might mask other problems by new features/bugs added to the method in the future. So I use the second version, even though I don't really like it. Is there a better way to handle the situation?
Your question relates to https://jira.codehaus.org/browse/SONAR-4857. For the time being all SonarQube analysers are mixing the cyclomatic complexity and essential complexity. From a theoretical point of view return statement should not increment the cc and this change is going to happen in the SQ ecosystem.
Not really an answer, but way too long for a comment.
This SONAR rule seems to be thoroughly broken. You could rewrite
b == c || a == b || c == a
as
b == c | a == b | c == a
and gain two points in this strange game (and maybe even some speed as branching is expensive; but this is on the discretion of the JITc, anyway).
The old rule claims, that the cyclomatic complexity is related to the number of tests. The new one doesn't, and that's a good thing as obviously the number of meaningfull tests for your both snippets is exactly the same.
Is there a better way to handle the situation?
Actually, I do have an answer: For each early return use | instead of || once. :D
Now seriously: There is a bug requesting annotations allowing to disable a single rule, which is marked as fixed. I din't look any further.
Since the question is also about early return statements as a coding style, it would be helpful to consider the effect of size on the return style. If the method or function is small, less than say 30 lines, early returns are no problem, because anyone reading the code can see the whole method at a glance including all of the returns. In larger methods or functions, an early return can be a trap unintentionally set for the reader. If the early return occurs above the code the reader is looking at, and the reader doesn't know the return is above or forgets that it is above, the reader will misunderstand the code. Production code can be too big to fit on one screen.
So whoever is managing a code base for complexity should be allowing for method size in cases where the complexity appears to be problem. If the code takes more than one screen, a more pedantic return style may be justified. If the method or function is small, don't worry about it.
(I use Sonar and have experienced this same issue.)

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