Im trying to get multiple types of return (Int and Boolean) but i can only seem to get one at a time:
fun isValidPositions(p1: Int, p2: Int, pairs: List<Char>):Boolean{
if(p1 !in 0..9 || p2 !in 0..9 ){
println("Posições inválidas")
}else if (p1 == p2){
println("Posições inválidas")
}else if(pairs.get(p1) != '_' || pairs.get(p2) != '_' )
println("Posições inválidas")
return true
}
then in my funcion main i have this:
val first = readPosition("primeira")
val second = readPosition("segunda")
if ( isValidPositions(first, second, places) ) {
places = places.play(first, second, pairs)
and it gives me the error: Type mismatch: inferred type is unit but int is expected
i cant seem to understand how i can get 2 types of return when i need the true and an int
im really new to coding since i just got into uni
Your problem is most likely that readPosition doesn't return anything (in Kotlin, that means returning Unit). This means that you're assigning Unit to your first and second variables instead of the Int value you're expecting.
Then when you reach isValidPositions, it wants an Int but you're passing Unit.
You should post your readPosition function to get more help on this.
Related
I'm trying my hands on Kotlin. Being from a Python background is really giving me a tough time to get the knack of the Kotlin syntax. I'm trying to do a simple dictionary (Mutable Map) operation. However, its giving me exceptions.
This is what I tried. Kotlin compiler
Adding the code snippet for reference.
fun main() {
val openActivityMap = mutableMapOf<String, MutableMap<String, Any>>()
val packageName = "amazon"
val currentTime = 23454321234
if(openActivityMap.containsKey(packageName)){
if(openActivityMap[packageName]?.get("isAlreadyApplied")){
if((openActivityMap[packageName]?.get("lastAppliedAt") - currentTime) > 3600){
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
else{
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
}
I'm a bit late to the party, but I'd like to point out another solution here.
As I commented on the OP, heterogeneous maps with fixed string keys like this are usually better expressed with classes in Kotlin. For instance, in your case, the class for your main map's values could be the following:
data class PackageInfo(
var isAlreadyApplied: Boolean,
var lastAppliedAt: Long,
)
(you could obviously add more properties if need be)
This would save you all the casts on the final values.
Another point I'd like to make is that if you access the value for a key anyway, you don't need to check up front the existence of the key with containsKey. Maps return null for keys that are not associated with any value (this is why you need to check for null after getting the value).
The compiler cannot see the correlation between containsKey and the subsequent get or [] access. However, it's smart enough to understand a null check if you simply get the value first and then check for null.
This always applies unless you want to tell the difference between keys that aren't in the map and keys that are in the map but associated null values (which is quite rare).
All in all, I would write something like that:
fun main() {
val openActivityMap = mutableMapOf<String, PackageInfo>()
val packageName = "amazon"
val currentTime = 23454321234
val packageInfo = openActivityMap[packageName]
if (packageInfo != null) { // the key was found and the value is smart cast to non-null in the next block
if (packageInfo.isAlreadyApplied) {
if ((packageInfo.lastAppliedAt - currentTime) > 3600) {
packageInfo.isAlreadyApplied = false
}
} else {
packageInfo.isAlreadyApplied = false
}
}
}
data class PackageInfo(
var isAlreadyApplied: Boolean,
var lastAppliedAt: Long,
)
I would recommend writing tests first and working in small increments, but this should fix your compilation issues:
fun main() {
val openActivityMap = mutableMapOf<String, MutableMap<String, Any>>()
val packageName = "amazon"
val currentTime = 23454321234
if(openActivityMap.containsKey(packageName)){
if(openActivityMap[packageName]?.get("isAlreadyApplied") as Boolean){
if((openActivityMap[packageName]?.get("lastAppliedAt") as Long - currentTime) > 3600){
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
else {
openActivityMap[packageName]?.put("isAlreadyApplied", false)
}
}
}
EDIT: Also I prefer to avoid nullable variables and mutable objects in general, but I suppose there's an exception to every rule.
Couldn't you just declare your Map<String, Any> to return a Boolean instead of Any? So,
val openActivityMap = mutableMapOf<String, MutableMap<String, Boolean>>()
It looks like you're trying to use your second Map to store both Booleans and Ints, which is complicating the logic. You'll need to typecast if you decide to approach it without Typing.
There's a problem with the 2 statement below
if(openActivityMap[packageName]?.get("isAlreadyApplied"))
if((openActivityMap[packageName]?.get("lastAppliedAt") - currentTime) > 3600)
As we all know, an IF statement requires a boolean value for it's param. The types of both statement are unknown at compilation time as they are of a Generic type, Any. As such,
openActivityMap[packageName]?.get("isAlreadyApplied") could be a null or of type Any (Not Boolean).
openActivityMap[packageName]?.get("lastAppliedAt") could be a null or of type Any (an Int was expected here for computation).
This would throw compilation errors as the compiler does not know the types to go with. What could be done is to cast to it's proper types.
Solution
openActivityMap[packageName]?.get("isAlreadyApplied") as Boolean ?: false
((openActivityMap[packageName]?.get("lastAppliedAt") as Int ?: 0) - currentTime)
Giving a default value if it's null.
maybe you can try something like this
if (openActivityMap.containsKey(packageName)) {
val packageMap = openActivityMap[packageName]!!
val applyRequired = (packageMap["lastAppliedAt"] as Long - currentTime) > 3600
packageMap["isAlreadyApplied"] = packageMap.containsKey("isAlreadyApplied") && !applyRequired
}
btw. do you really want to have lastAppliedAt to be in te future? otherewise it will never be > 3600
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.
I want to compare fields of a JSONObject.
Here is the code I am trying:
x = JSONObject()
x.put("num",1)
if(x["num"] as Int < 2){
print("Good work!")
}
Error shown by IDE:
Type expected
Expecting a '>
You need to put x["num"] as Int into parenthesis, otherwise the compiler will think that you are trying to do Int < 2 which doesn't really make any sense.
val x = JSONObject()
x.put("num", 1)
if ((x["num"] as Int) < 2) {
print("Good work!")
}
But btw, this will throw a ClassCastException if the value is not actually an Int. It is in your minimal case, but you don't always actually know. In that scenario I would do something like this:
val x = JSONObject()
x.put("num", "b")
val num = x["num"]
if (num is Int && num < 2) {
print("Good work!")
}
The current code doesn't work as-is because the compiler sees Int<42 and thinks that you're trying to use a type parameter for the Int type (like Int<T>) which obviously doesn't work for 2 reasons:
Type expected because 42 is not a type
Expecting a '>' because well, there is no >
As #Alex.T said, you can disambiguate this by putting (..) around the cast expression: (x["num"] as Int) < 2.
However, if you're OK with your expression failing at runtime like this, you should instead use a typed getter like getInt():
val x = JSONObject()
x.put("num", 1)
if (x.getInt("num") < 2) {
print("Good work!")
}
If the value is not an integer, this will fail at runtime with org.json.JSONException (as mentioned in the Javadoc linked above). You can catch this exception if you want to handle the error gracefully.
Alternatively you can use a safer option with a default value, like optInt(), which will default to the given value if the key is not present in the JSON OR if the value is not a number.
I am trying to find the find the result of num1 raised to the power num2:
This is my code ->
fun power(num1 : Int, num2: Int): Int {
var result = 1
while (num2 != 0) {
return result *= num1
num2--
}
}
But the above code is producing the following error -->
Calculator.kt:30:16: error: assignments are not expressions, and only expressions are allowed in this context
return result *= num1
^
Calculator.kt:33:5: error: a 'return' expression required in a function with a block body ('{...}')
}
^
I have read a number of articles but not able to understand. Any help will be highly appreciated.
Thank you
An expression is something that evaluates to a value. An assignment is something that assigns a value to a variable or property.
x *= y is an assignment that is shorthand for x = x * y.
You cannot return an assignment, because it does not evaluate to a value. An assignment contains an expression on the right side of the equals sign, but as a whole does not represent an expression.
There are some other syntax problems you have. You can't modify a function paramter's value (num2-- isn't allowed).
The logic also doesn't make sense. return returns an expression immediately. To fix your code, you need to create a local variable from num2, and move the return statement to the end.
fun power(num1 : Int, num2: Int): Int {
var result = 1
var count = num2
while (count != 0) {
result *= num1
count--
}
return result
}
FYI, there's a function called repeat that is simpler than using a while loop with a counter. It runs the code inside the brackets by the number of times you give.
fun power(num1 : Int, num2: Int): Int {
var result = 1
repeat(num2) {
result *= num1
}
return result
}
You function contains multiple errors, I suggest you to study Kotlin, here a reference. Kotlin website has some more material.
Back to your problem, I have modified your function:
fun power(num1 : Int, num2: Int): Int {
var result = 1
var num2_local = num2
while (num2_local != 0) {
result *= num1
num2_local--
}
return result
}
Problems with your version:
you return from the function immediately
basic types args passed to kotlin functions are passed by const copy, this means that you cannot modify them (e.g num2--)
If you keep a local modifiable reference (var keyword) withing your function, then you achieve your goal
Please help me understand this piece of code in the kotlin docs:-
val a: Int = 10000
print(a === a) // Prints 'true'
val boxedA: Int? = a
val anotherBoxedA: Int? = a
print(boxedA === anotherBoxedA) // !!!Prints 'false'!!!
Now, I understand that first int a = 10000 then in the next line it is comparing it using ===.
Now the question is why when it assigned boxedA=a, it checked if it is null using int?. Can it just be written like this:-
val boxedA: Int=a
Please if I'm understanding it the wrong way, someone guide to check the right place or explain it a bit for me.
First, the Int will be mapped to java int/Integer depending on its context. If Int is a generic argument, then its mapped type is Integer. Otherwise, it is a primitive type int. for example:
val a:Int = 1
//final int a = 1; //mapped to java code
val numbers:List<Int> = asList(1,2,3)
//final List<Integer> numbers = asList(1, 2, 3); //mapped to java code
Secondly, boxing an Int to an Int? is same behavior as java boxing an int to an Integer, for example:
val a:Int = 1 // int a = 1;
val b:Int? = a; // Integer b = a;
Why the boxed Integers are not the same?
This is because Integer only caching values in the range [-128, 127]. the the variable a above is out of the cache range, it will create a new Integer instance for each boxing rather than using a cached value. for example:
// v--- 1 is in cache range
val ranged: Int = 1
val boxedRanged1: Int? = ranged
val boxedRanged2: Int? = ranged
println(boxedRanged1 === boxedRanged2) //true
// v--- 128 is out of cache range
val excluded: Int = 128
val boxedExcluded1: Int? = excluded
val boxedExcluded2: Int? = excluded
println(boxedExcluded1 === boxedExcluded2) //false
when it assigned boxedA = a, it checked if it is null using int?
I have no idea what you mean by this. Making a variable have the type Int? makes it a variable that can either store an Int or null. There is no checking happening at this assignment. If you have a non-null value to assign to the variable, just make it non-nullable, without the ? in the type:
val copyOfA: Int = a
You can even omit the type, and get Int inferred:
val copyOfA = a
As for the comparisons:
== is used for comparing by value in Kotlin (this is the equivalent of using equals in Java), === is used for comparing references (this is == in Java).
When you create boxedA and anotherBoxedA, you create two Integer instances under the hood (because nullable variables can't be represented by primitives). These will be equal when compared with == (they have the same value), but not when compared with === (they are different instances).
You can see the relevant part of the offical docs here.
it checked if it is null using int?
That is not what it means.
Kotlin's null safety feature does not allow a variable to be set as null by default.
Check here.
val anotherBoxedA: Int? = a
This means that anotherBoxedA can be assigned null or is nullable.
val anotherBoxedA: Int? = null
This will be allowed.