i want to write a function that checks for equality of lists in SML
for instance :
[1,2,3]=[1,2,3];
val it = true : bool
So instead of writing down the whole thing, i want to make a function that takes two predefined lists, and compare them, so that if list01 is [1,2,3] and list09 is [1,2,3]
then fun equal (list01, list09); will return -val it = true : bool;
You seem to be aware that = works on lists, so (as I already said in my comment) I don't see why you need to define an equal function.
That being said, you can just write:
fun equal (a, b) = (a = b);
Here is a not checked sample:
fun compare ([], []) = true # both empty
| compare (x::xs, y::ys) = (x = y) and compare(xs,ys)
| compare (_, _) = false # different lengths
Related
I am currently trying to make a comparable object and working on the compareTo() function, for which I wrote the following code
class InfoAcad(e: String, m: String, c: Int): Comparable<InfoAcad> {
override operator fun compareTo(other: InfoAcad): Int {
if (this.e < other.e) return -1
if (this.e > other.e) return 1
if (this.e == other.e && this.m < other.m) return -1
if (this.e == other.e && this.m > other.m) return 1
return 0
}
}
The idea is that e is an ID number inputted as a string, which always follows the format XX-XXX where every X character is an integer between 0 and 9, and m is a course code following the format LL-XXX where each L character is a capital letter between A and Z and the X characters are integers between 0 and 9 like in the ID numbers. The objects are first compared by their ID number, and if the ID numbers are equal they are then compared by the course code, if both values are the same then the objects are equal, the c parameter is not taken into account in the comparison. I found out yesterday that I could compare strings directly in Kotlin in < and > relations, so I decided to try using that to make the task of comparing the InfoAcad objects a bit easier on myself, however when I make a main function to test the comparisons, the equality always returns a false value independently of what is in the string values of the InfoAcad objects. Here's said main function:
fun main() {
var A = InfoAcad("18-10125", "CI-2526", 3)
var B = InfoAcad("18-10125", "CI-2526", 5)
println("A = B: " + (A == B).toString()) //true
println("A < B: " + (A < B).toString()) //false
ptintln("A > B: " + (A > B).toString()) //false
}
When I change characters in the ID and course code values the inequality relations work just as intended, so what could be causing the equality relation to always return false? I appreciate and thank any responses in advance.
Note: I have also tried giving A and B the same c value, the equality part still returned false.
Override the equals function as well, or use a data class.
compareTo is only used for the < and > operators. The == operator is implemented by the separate equals function.
You can find the available operators, and the functions you need to override for each, in the Operator overloading section of the Kotlin docs.
If you don't override the equals function, the default behaviour is for it to use object identity. That means that two different objects, even if they contain the same fields, will never be considered equal.
There is however a nice shortcut for what you want to do! Kotlin will automatically generate an equals function for you if you make your class a data class. It's a good fit for classes like yours, whose main purpose is to hold data.
Because == and != translates to a call to equals(other: Any?): Boolean method, from kotlinlang
Expression Translated to
a == b a?.equals(b) ?: (b === null)
a != b !(a?.equals(b) ?: (b === null))
These operators only work with the function equals(other: Any?): Boolean, which can be overridden to provide custom equality check
implementation. Any other function with the same name (like
equals(other: Foo)) will not be called.
So I have a custom programming language, and in it I am doing some math formalization/modeling. In this instance I am doing basically this (a pseudo-javascript representation):
isIntersection([1, 2, 3], [1, 2], [2, 3]) // => true
isIntersection([1, 2, 3], [1, 2, 3], [3, 4, 5]) // => false
function isIntersection(setTest, setA, setB) {
i = 0
while (i < setTest.length) {
let t = setTest[i]
if (includes(setA, t) || includes(setB, t)) {
i++
} else {
return false
}
}
return true
}
function includes(set, element) {
for (x in set) {
if (isEqual(element, x)) {
return true
}
}
return false
}
function isEqual(a, b) {
if (a is Set && b is Set) {
return isSetEqual(a, b)
} else if (a is X... && b is X...) {
return isX...Equal(a, b)
} ... {
...
}
}
function isSetEqual(a, b) {
i = 0
while (i < a.length) {
let x = a[i]
let y = b[i]
if (!isEqual(x, y)) {
return false
}
i++
}
return true
}
The isIntersection is checking isEqual, and isEqual is configured to be able to handle all kinds of cases of equality check, from sets compared to sets, objects to objects, X's to X's, etc..
The question is, how can we make the isEqual somehow ignorant of the implementation details? Right now you have to have one big if/else/switch statement for every possible type of object. If we add a new type, we have to modify this gigantic isEqual method to add support for it. How can we avoid this, and just define them separately and cleanly?
I was thinking initially of making the objects be "instances of classes" so to speak, with class methods. But I like the purity of having everything just be functions and structs (objects without methods). Is there any way to implement this sort of thing without using classes with methods, instead keeping it just functions and objects?
If not, then how would you implement it with classes? Would it just be something like this?
class Set {
isEqual(set) {
i = 0
while (i < this.length) {
let x = this[i]
let y = set[i]
if (!x.isEqual(y)) {
return false
}
i++
}
return true
}
}
This would mean every object would have to have an isEqual defined on it. How does Haskell handle such a system? Basically looking for inspiration on how this can be most cleanly done. I want to ideally avoid having classes with methods.
Note: You can't just delegate to == native implementation (like assuming this is in JavaScript). We are using a custom programming language and are basically trying to define the meaning of == in the first place.
Another approach is to pass around an isEqual function along with everything somehow, though I don't really see how to do this and if it were possible it would be clunky. So not sure what the best approach is.
Haskell leverages its type and type-class system to deal with polymorphic equality.
The relevant code is
class Eq a where
(==) :: a -> a -> Bool
The English translation is: a type a implements the Eq class if, and only if, it defines a function (==) which takes two inputs of type a and outputs a Bool.
Generally, we declare certain "laws" that type-classes should abide by. For example, x == y should be identical to y == x in all cases, and x == x should never be False. There's no way for the compiler to check these laws, so one typically just writes them into the documentation.
Once we have defined the typeclass Eq in the above manner, we have access to the (==) function (which can be called using infix notation - ie, we can either write (==) x y or x == y). The type of this function is
(==) :: forall a . Eq a => a -> a -> Bool
In other words, for every a that implements the typeclass Eq, (==) is of type a -> a -> Bool.
Consider an example type
data Boring = Dull | Uninteresting
The type Boring has two proper values, Dull and Uninteresting. We can define the Eq implementation as follows:
instance Eq Boring where
Dull == Dull = True
Dull == Uninteresting = False
Uninteresting == Uninteresting = True
Uninteresting == Dull = False
Now, we will be able to evaluate whether two elements of type Boring are equal.
ghci> Dull == Dull
True
ghci> Dull == Uninteresting
False
Note that this is very different from Javascript's notion of equality. It's not possible to compare elements of different types using (==). For example,
ghci> Dull == 'w'
<interactive>:146:9: error:
* Couldn't match expected type `Boring' with actual type `Char'
* In the second argument of `(==)', namely 'w'
In the expression: Dull == 'w'
In an equation for `it': it = Dull == 'w'
When we try to compare Dull to the character 'w', we get a type error because Boring and Char are different types.
We can thus define
includes :: Eq a => [a] -> a -> Bool
includes [] _ = False
includes (x:xs) element = element == x || includes xs element
We read this definition as follows:
includes is a function that, for any type a which implements equality testing, takes a list of as and a single a and checks whether the element is in the list.
If the list is empty, then includes list element will evaluate to False.
If the list is not empty, we write the list as x : xs (a list with the first element as x and the remaining elements as xs). Then x:xs includes element iff either x equals element, or xs includes element.
We can also define
instance Eq a => Eq [a] where
[] == [] = True
[] == (_:_) = False
(_:_) == [] = False
(x:xs) == (y:ys) = x == y && xs == ys
The English translation of this code is:
Consider any type a such that a implements the Eq class (in other words, so that (==) is defined for type a). Then [a] also implements the Eq type class - that is, we can use (==) on two values of type [a].
The way that [a] implements the typeclass is as follows:
The empty list equals itself.
An empty list does not equal a non-empty list.
To decide whether two non-empty lists (x:xs) and (y:ys) are equal, check whether their first elements are equal (aka whether x == y). If the first elements are equal, check whether the remaining elements are equal (whether xs == ys) recursively. If both of these are true, the two lists are equal. Otherwise, they're not equal.
Notice that we're actually using two different ==s in the implementation of Eq [a]. The equality x == y is using the Eq a instance, while the equality xs == ys is recursively using the Eq [a] instance.
In practice, defining Eq instances is typically so simple that Haskell lets the compiler do the work. For example, if we had instead written
data Boring = Dull | Uninteresting deriving (Eq)
Haskell would have automatically generated the Eq Boring instance for us. Haskell also lets us derive other type classes like Ord (where the functions (<) and (>) are defined), show (which allows us to turn our data into Strings), and read (which allows us to turn Strings back into our data type).
Keep in mind that this approach relies heavily on static types and type-checking. Haskell makes sure that we only ever use the (==) function when comparing elements of the same type. The compiler also always knows at compile type which definition of (==) to use in any given situation because it knows the types of the values being compared, so there is no need to do any sort of dynamic dispatch (although there are situations where the compiler will choose to do dynamic dispatch).
If your language uses dynamic typing, this method will not work and you'll be forced to use dynamic dispatch of some variety if you want to be able to define new types. If you use static typing, you should definitely look into Haskell's type class system.
I'm studying kotlin, but I'm very disappointed, I can not compare two Strings.
What is the right way to compare.
btn_login.setOnClickListener {
val login = input_email.text.trim()
val pass = input_password.text.trim()
if( login.equals( pass ) ){
startActivity<MainActivity>()
}
if (login?.equals(other = pass)){
startActivity<MainActivity>()
}
if (login == pass){
startActivity<MainActivity>()
}
}
According to documentation for structual equality use ==. It is translated to a?.equals(b) ?: (b === null).
In you case convert login and pass from SpannableStringBuilder to String.
val login = input_email.text.trim().toString()
Here is the example for matching the two strings using kotlin.
If you are using == (double equals) for matching the string then it's compare the address & return maximum time wrong result as per java documentation so use equals for the same
If you want to use equal ignore case then pass the true in the equals method of String
if (s1.equals(s2,true))
other wise you can just use this without boolean like
if (s1.equals(s2,false)) or if (s1.equals(s2))
compleate code is below
fun main(args: Array<String>) {
val s1 = "abc"
val s2 = "Abc"
if (s1.equals(s2,true))
{
println("Equal")
}
else
{
println("Not Equal")
}
}
Covert both the SpannableStringBuilder to string with toString, this should work.
val login = input_email.text.trim().toString()
val pass = input_password.text.trim().toString()
if (login == pass){
startActivity<MainActivity>()
}
1. == :
if ( string1 == string2 ){...}
2. equals :
Indicates whether some other object is "equal to" this one.
Implementations must fulfil the following requirements:
Reflexive: for any non-null reference value x, x.equals(x) should
return true.
Symmetric: for any non-null reference values x and y, x.equals(y)
should return true if and only if y.equals(x) returns true.
Transitive: for any non-null reference values x, y, and z, if
x.equals(y) returns true and y.equals(z) returns true, then
x.equals(z) should return true
Consistent: for any non-null reference values x and y, multiple
invocations of x.equals(y) consistently return true or consistently
return false, provided no information used in equals comparisons on
the objects is modified.
/** * Returns `true` if this string is equal to [other], optionally ignoring character case. * * #param ignoreCase `true` to ignore character case when comparing strings. By default `false`. */
public fun String?.equals(other: String?, ignoreCase: Boolean = false): Boolean
3. compareTo :
public override fun compareTo(other: String): Int
Compares this object with the specified object for order. Returns zero
if this object is equal to the specified other object, a negative
number if it's less than other, or a positive number if it's greater
than other.
public fun String.compareTo(other: String, ignoreCase: Boolean = false): Int
Compares two strings lexicographically, optionally ignoring case
differences
i know this is way too late, but as a newbie learning Kotlin, i had the same doubts.
then i came across this wonderful article that articulates the various string comparison types in Kotlin and the differences between them all.
in short both == and .equals() can be used to compare the value of 2 strings in kotlin.
hopefully that helps
With case checking
String a=.....
String b=.....
if(a==b){
}
IgnoreCase
if(a.equals(b,false))
KOTLIN:
if (editText1.text.toString() == editText2.text.toString() ) {
println("Should work now! The same value")
}
Try the following solution, see if it helps:
val passStr: String = textView.text.toString()
if( loginStr.compareTo(passStr, false) ){
startActivity<MainActivity>()
}
Try this surely will work.
val style = buildString { karthik}
val style2 = buildString { karthik }
var result = style.equals(style2)
if(result){//Do something}
Scala's List classes have indexWhere methods, which return a single index for a List element which matches the supplied predicate (or -1 if none exists).
I recently found myself wanting to gather all indices in a List which matched a given predicate, and found myself writing an expression like:
list.zipWithIndex.filter({case (elem, _) => p(elem)}).map({case (_, index) => index})
where p here is some predicate function for selecting matching elements. This seems a bit of an unwieldy expression for such a simple requirement (but I may be missing a trick or two).
I was half expecting to find an indicesWhere function on List which would allow me to write instead:
list.indicesWhere(p)
Should something like this be part of the Scala's List API, or is there a much simpler expression than what I've shown above for doing the same thing?
Well, here's a shorter expression that removes some of the syntactic noise you have in yours (modified to use Travis's suggestion):
list.zipWithIndex.collect { case (x, i) if p(x) => i }
Or alternatively:
for ((x,i) <- list.zipWithIndex if p(x)) yield i
But if you use this frequently, you should just add it as an implicit method:
class EnrichedWithIndicesWhere[T, CC[X] <: Seq[X]](xs: CC[T]) {
def indicesWhere(p: T => Boolean)(implicit bf: CanBuildFrom[CC[T], Int, CC[Int]]): CC[Int] = {
val b = bf()
for ((x, i) <- xs.zipWithIndex if p(x)) b += i
b.result
}
}
implicit def enrichWithIndicesWhere[T, CC[X] <: Seq[X]](xs: CC[T]) = new EnrichedWithIndicesWhere(xs)
val list = List(1, 2, 3, 4, 5)
def p(i: Int) = i % 2 == 1
list.indicesWhere(p) // List(0, 2, 4)
You could use unzip to replace the map:
list.zipWithIndex.filter({case (elem, _) => p(elem)}).unzip._2
For example, instead of
- op =;
val it = fn : ''a * ''a -> bool
I would rather have
- op =;
val it = fn : ''a -> ''a -> bool
for use in
val x = getX()
val l = getList()
val l' = if List.exists ((op =) x) l then l else x::l
Obviously I can do this on my own, for example,
val l' = if List.exists (fn y => x = y) l then l else x::l
but I want to make sure I'm not missing a more elegant way.
You could write a helper function that curries a function:
fun curry f x y = f (x, y)
Then you can do something like
val curried_equals = curry (op =)
val l' = if List.exists (curried_equals x) l then l else x::l
My knowledge of SML is scant, but I looked through the Ullman book and couldn't find an easy way to convert a function that accepts a tuple to a curried function. They have two different signatures and aren't directly compatible with one another.
I think you're going to have to roll your own.
Or switch to Haskell.
Edit: I've thought about it, and now know why one isn't the same as the other. In SML, nearly all of the functions you're used to actually accept only one parameter. It just so happens that most of the time you're actually passing it a tuple with more than one element. Still, a tuple is a single value and is treated as such by the function. You can't pass such function a partial tuple. It's either the whole tuple or nothing.
Any function that accepts more than one parameter is, by definition, curried. When you define a function that accepts multiple parameters (as opposed to a single tuple with multiple elements), you can partially apply it and use its return value as the argument to another function.