I'm new to Scala. I'm trying to create a test case for a simple factorial function.
I couldn't assign the result value in the assert statement. I'm getting
Integer number is out of range even for type Long error in IntelliJ.
test("Factorial.factorial6") {
assert(Factorial.factorial(25) == 15511210043330985984000000L)
}
I also tried to assign the value to val, using the 'L' literal, again it shows the same
message.
val b: BigInt = 15511210043330985984000000L
I'm clearly missing some basic stuff about Scala, I would appreciate your help, to solve this
The value you are giving is indeed larger than can be held in a Long, and that is the maximum size for a literal value in Scala. However you can initialise a BigInt using a String containing the value:
val b = BigInt("15511210043330985984000000")
and therefore
assert(Factorial.factorial(25) == BigInt("15511210043330985984000000"))
Related
The following simple code converts an Integer value to a string value and logs it.
module Main where
import Effect (Effect)
import Effect.Console (log)
import Prelude ((<>), Unit, discard)
import Data.Int (toStringAs, radix)
type CustomerFeedback = {
customerServiceScore :: Int,
productQualityScore :: Int,
onTimeDeliveryScore :: Int
}
feedback :: CustomerFeedback
feedback = {
customerServiceScore : 4,
productQualityScore : 2,
onTimeDeliveryScore : 6
}
stringifyCustomerFeedback :: CustomerFeedback -> String
stringifyCustomerFeedback feedback = "Service: " <> toStringAs (radix 10) feedback.customerServiceScore
main ∷ Effect Unit
main = do
log (stringifyCustomerFeedback(feedback))
However, running this code produces the following error:
Could not match type
Maybe Radix
with type
Radix
while checking that type Maybe Radix
is at least as general as type Radix
while checking that expression radix 10
has type Radix
in value declaration stringifyCustomerFeedback
Questions would be as follows:
How do you change the code above so it outputs a string as expected and not an error?
What's the point of a Maybe Radix type if using it where you would use a Radix causes an error? How do you use a Maybe value?
The idea of the radix function is that you give it a number and it creates a Radix from it. But not every number constitutes a valid Radix. For example, if you give it -5, it shouldn't work. And neither should 0 or 1 for example. For some technical reasons, radices above 32 are also deemed invalid.
So that's why it returns Maybe: it would be Nothing in case the number you gave it wasn't a "valid" radix.
And the use case for that function is when you don't actually know the number ahead of time. Like if you get it from the user. Or from some sort of config file or whatnot. In that case, if you get a Nothing, you would interpret that as "invalid user input" or "corrupted config file" and report an error accordingly. And you won't even get as far as calling toStringAs. This is one of the big selling points of static types: applied properly, they can force you to write a correct, reliable program, without ignoring edge cases.
However, in case you already know that you're interested in decimal radix, just use decimal. It's a Maybe-free constant provided by the library, along with some other frequently used ones, such as binary and octal.
stringifyCustomerFeedback feedback = "Service: " <> toStringAs decimal feedback.customerServiceScore
object LPrimeFactor {
def main(arg:Array[String]):Unit = {
start(13195)
start(600851475143)
}
def start(until:Long){
var all_prime_fac:Array[Int] = Array()
var i = 2
(compile:compileIncremental) Compilation failed
integer number too large
Even though I changed the arg type to Long, it's still not fixed.
Pass the argument as a Long (notice the L at the end of the number):
start(600851475143L)
// ^
To create a Long literal you must add L to the end of it.
start(600851475143L)
Please remember literals values, if you has not any type direct suffix, the compiler try to get your numeric type values, such as 600851475143 as type Int, which is 32-bit length, two complement representation
MIN_VALUE = -2147483648(- 2 ^ 31)
MAX_VALUE = 2147483647(2 ^ 31 - 1)
So please add right suffix on the literal value, as 600851475143L
I am coming from Java, and trying to understand the following Scala syntax
null.asInstanceOf[Double]
Why is this not a NullPointerException?
I was trying to do something like:
var d : Double = 0
if(d == null)
// do something
However, I got following error message:
comparing values of types Double and Null using `==' will always yield false
This got fixed when I changed null to null.asInstanceOf[Double] as per this answer, but this is a weird statement for me, how on earth this is working?
Scala's scala.Double does not correspond to Java's java.lang.Double. Scala's double inherits from AnyVal, the parent of all value types. It most closely corresponds to the primitive type double in Java, which can't be null. When you do null.asInstanceOf[Double], you're actually getting the double value 0.0, not a null double.
From the Scala language specification section 6.3
[the null value] implements methods in scala.AnyRef as follows
...
asInstanceOf[T] returns the default value of type T
And the default value of Double is zero.
So, in short, your value can't possibly be null because it's like a Java primitive. So you don't need to do a null check in this case.
object LPrimeFactor {
def main(arg:Array[String]):Unit = {
start(13195)
start(600851475143)
}
def start(until:Long){
var all_prime_fac:Array[Int] = Array()
var i = 2
(compile:compileIncremental) Compilation failed
integer number too large
Even though I changed the arg type to Long, it's still not fixed.
Pass the argument as a Long (notice the L at the end of the number):
start(600851475143L)
// ^
To create a Long literal you must add L to the end of it.
start(600851475143L)
Please remember literals values, if you has not any type direct suffix, the compiler try to get your numeric type values, such as 600851475143 as type Int, which is 32-bit length, two complement representation
MIN_VALUE = -2147483648(- 2 ^ 31)
MAX_VALUE = 2147483647(2 ^ 31 - 1)
So please add right suffix on the literal value, as 600851475143L
Still trying to understand how to use class. I have now written the following:
`import random
class Grid():
def __init__(self, grid_row, grid_column):
self.__row = grid_row
self.__col = grid_column
self.__board=[]
def make_board(self):
for row in range(self.__row):
self.__board.append([])
for col in range(self.__col):
self.__board[row].append('0')
return self.__board
def change_tile(self):
choices = (0,1,2)
x = random.choice(choices)
y= random.choice(choices)
self.__board[x][y] = str(2)
def __repr__(self):
for row in self.__board:
print( " ".join(row))
g = Grid(3,3)
g.make_board()
g.change_tile()
print(g)
Firstly when I run this I get a grid printed followed by:
TypeError: __str__ returned non-string (type NoneType)
I don't understand why this happens. Second question. If I want to return the self.board, __str only returns the last row (0,0,0).With 'print' all three rows and columns are printed. Is there a way around the issue with 'return'?Is it an issue ( apart from the fact that I want to 'see' what I am doing)?
How would one call Grid(3,3) and get a grid with a randomly placed 2 without having to call each function separately as I have done in my example? Lastly why can I not use the integers 0 or 2, but have to convert everything to a string?. I hope that I have not exceeded the goodwill that exists on this forum by asking so many dumb questions!
The special methods __repr__ and __str__ are required to return a string. If there is no __str__ implementation given, the __repr__ will be used for the string conversion too.
Now in your case, __repr__ prints something instead of returning a string. It actually returns nothing, so None is implicitely returned. You have to change it to return a string. For example like this:
def __repr__(self):
return '\n'.join([' '.join(row) for row in self.__board])