I want to serialize an into into a Byte array or array buffer.
I realise that I can use 'java.nio.ByteBuffer' but I am experimenting for fun and trying to do it myself.
The following code works for positive Int but goes wrong when I serialize a negative Int.
Can anyone explain why or show me a correction?
import scala.collection.mutable.ArrayBuffer
object b {
val INTBYTES:Int = 4 // int is 4 bytes
def toArrayBuf(x:Int): ArrayBuffer[Byte] = {
val buf = new ArrayBuffer[Byte](INTBYTES)
for(i <- 0 until INTBYTES) {
buf += ((x >>> (INTBYTES - i - 1 << 3)) & 0xFF).toByte
}
buf
}
}
the following test works as expected:-
int the REPL it prints:-
scala> val test:Int = 0x4f0f0f0f
test: Int = 1326386959
scala> println(test.toBinaryString)
1001111000011110000111100001111
scala> val t1 = b.toArrayBuf(test)
t1: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer(79, 15, 15, 15)
scala> t1.foreach( it => printf("%s ",it.toInt.toBinaryString))
1001111 1111 1111 1111
but this with a negative int does something wierd:-
scala> val test2:Int = 0x8f0f0f0f
test2: Int = -1894838513
scala> println(test2.toBinaryString)
10001111000011110000111100001111
scala> val t2 = b.toArrayBuf(test2)
t2: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer(-113, 15, 15, 15)
scala> t2.foreach( it => printf("%s ",it.toInt.toBinaryString))
11111111111111111111111110001111 1111 1111 1111
notice that the first byte has been 1 filled for the whole int it shoild be '10001111'
Any ideas?
FYI
Im using :-
scala -version
Scala code runner version 2.10.1 -- Copyright 2002-2013, LAMP/EPFL
java -fullversion
java full version "1.7.0_40-b31"
with OpenJDK
Thanks
Scala's toBinaryString method defers to the Java one on Integer. From those documents:
public static String toBinaryString(int i)
Returns a string representation of the integer argument as an unsigned
integer in base 2. The unsigned integer value is the argument plus
2^32 if the argument is negative; otherwise it is equal to the
argument. This value is converted to a string of ASCII digits in
binary (base 2) with no extra leading 0s.
In other words it's working as specified. Your bit-twiddling seems to be OK, but when you're printing the numbers out, you need to realise that the number of characters is dependent on the length of the data type. (E.g. -1: Int in binary is 11111111111111111111111111111111 while -1: Byte is 11111111.) You get away with it for positive numbers only because the leading zeros are not displayed, as specified above.
Solution: make your own toBinaryString for bytes, or just taking the rightmost 8 digits from the Int version should work (though less efficient) i.e.
it.toInt.toBinaryString.takeRight(8)
Taking Luigi's advice I hacked up a pimp for Byte that provides a toBinaryString that works properly, in case anyone else is struggling with similar problems here is what I did.
object b {
val INTBYTES:Int = 4 // int is 4 bytes
val SIZEBYTE:Short = 8
def toArrayBuf(x:Int): ArrayBuffer[Byte] = {
val buf = new ArrayBuffer[Byte](INTBYTES)
for(i <- 0 until INTBYTES) {
buf += ((x >>> (INTBYTES - i - 1 << 3)) & 0xFF).toByte
}
buf
}
def toBinaryString(x: Byte): String = {
val buf = new StringBuilder(SIZEBYTE)
for(i <- 0 until SIZEBYTE) {
buf.append((x >>> (SIZEBYTE - i - 1)) & 0x01)
}
buf.toString()
}
}
//pimp Byte
implicit def fooBar(byte: Byte) = new {def toBinaryString = b.toBinaryString(byte)}
Now when I run the previous experiment it works properly
scala> val test:Int = 0x4f0f0f0f
test: Int = 1326386959
scala> println(test.toBinaryString)
1001111000011110000111100001111
scala> val t1 = toArrayBuf(test)
t1: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer(79, 15, 15, 15)
scala> t1.foreach( it => printf("%s ",it.toBinaryString))
01001111 00001111 00001111 00001111
and
scala> val test2:Int = 0x8f0f0f0f
test2: Int = -1894838513
scala> println(test2.toBinaryString)
10001111000011110000111100001111
scala> val t2 = toArrayBuf(test2)
t2: scala.collection.mutable.ArrayBuffer[Byte] = ArrayBuffer(-113, 15, 15, 15)
scala> t2.foreach( it => printf("%s ",it.toBinaryString))
10001111 00001111 00001111 00001111
Thanks Luigi
Related
I have written a function in Scala that should calculate the sum of the squares of the digits of a number. Eg: 44 -> 32 (4^2 + 4^2 = 16 + 16 = 32)
Here it is:
def digitSum(x:BigInt) : BigInt = {
var sum = 0
val leng = x.toString.toList.length
var y = x.toString.toList
for (i<-0 until leng ) {
sum += y(i).toInt * y(i).toInt
}
return sum
}
However when I call the function let's say with digitSum(44) instead of 32 I get 5408.
Why is this happening? Does it have to do with the fact that in the list there are Strings? If so why does the .toInt method do not work?
Thanks!
The answer to your questions has been already covered here Scala int value of String characters, have a good read through and you will have more information than required ;)
Also looking at your code, it can benefit more from Scala expressiveness and functional features. The same function can be written in the following manner:
def digitSum(x: BigInt) = x.toString
.map(_.asDigit)
.map(a => a * a)
.sum
In the future try to avoid using mutable variables and standard looping techniques if you could.
When you do toString you're mapping the String to Chars not Ints and then to Ints later. This is what it looks like in the repl:
scala> "1".toList.map(_.toInt)
res0: List[Int] = List(49)
What you want is probably something like this:
def digitSum(x:BigInt) : BigInt = {
var sum = 0
val leng = x.toString.toList.length
var y = x.toString.toList
for (i<-0 until leng ) {
sum += (y(i).toInt - 48) * (y(i).toInt - 48) //Subtract out char base
}
sum
}
I am new to Scala and functional programming.
I was solving problem where you have to read number, and then that number of integers. After that you should calculate sum of all digits in all the integers.
Here is my code
def sumDigits(line: String) =
line.foldLeft(0)(_ + _.toInt - '0'.toInt)
def main(args: Array[String]) {
val numberOfLines = Console.readInt
val lines = for (i <- 1 to numberOfLines) yield Console.readLine
println(lines.foldLeft(0)( _ + sumDigits(_)))
}
Is there more elegant or efficient way?
sumDigits() can be implemented easier with sum:
def sumDigits(line: String) = line.map(_.asDigit).sum
Second foldLeft() can also be replaced with sum:
lines.map(sumDigits).sum
Which brings us to the final version (notice there is no main, instead with extend App):
object Main extends App {
def sumDigits(line: String) = line.map(_.asDigit).sum
val lines = for (_ <- 1 to Console.readInt) yield Console.readLine
println(lines.map(sumDigits).sum)
}
Or if you really want to squeeze as much as possible in one line, inline sumDigits (not recommended):
lines.map(_.map(_.asDigit).sum).sum
I like compact code, so I might (if I was really going for brevity)
object Reads extends App {
import Console._
println( Seq.fill(readInt){readLine.map(_ - '0').sum}.sum )
}
which sets the number of lines inline and does the processing as you go. No error checking, though. You could throw in a .filter(_.isDigit) right after the readLine to at least discard non-digits. You might also def p[A](a: A) = { println(a); a } and wrap the reads in p so you can see what had been typed (by default on some platforms at least there's no echo to screen).
One-liner Answer:
Iterator.continually(Console.readLine).take(Console.readInt).toList.flatten.map(_.asDigit).sum
To start with, you have to do some kind of parsing on line to break apart the existing decimal integers sub-strings:
val numbers = "5 1 4 9 16 25"
val ints = numbers.split("\\s+").toList.map(_.toInt)
Then you want to pull off the first one as the count and keep the rest to decode and sum:
val count :: numbers = ints
Then use the built-in sum method:
val sum = numbers.sum
Altogether in the REPL:
scala> val numbers = "5 1 4 9 16 25"
numbers: String = 5 1 4 9 16 25
scala> val ints = numbers.split("\\s+").toList.map(_.toInt)
ints: List[Int] = List(5, 1, 4, 9, 16, 25)
scala> val count :: numbers = ints
count: Int = 5
numbers: List[Int] = List(1, 4, 9, 16, 25)
scala> val sum = numbers.sum
sum: Int = 55
If you want to do something with the leading number count, you could verify that it's correct:
scala> assert(count == numbers.length)
Which produces no output, since the assertion passes.
I want to convert a char to an int value.
I am a bit puzzled by the way toInt works.
println(("123").toList) //List(1, 2, 3)
("123").toList.head // res0: Char = 1
("123").toList.head.toInt // res1: Int = 49 WTF??????
49 pops up randomly for no reason.
How do you convert a char to int the right way?
For simple digit to int conversions there is asDigit:
scala> "123" map (_.asDigit)
res5: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3)
Use Integer.parseInt("1", 10). Note that the 10 here is the radix.
val x = "1234"
val y = x.slice(0,1)
val z = Integer.parseInt(y)
val z2 = y.toInt //equivalent to the line above, see #Rogach answer
val z3 = Integer.parseInt(y, 8) //This would give you the representation in base 8 (radix of 8)
49 does not pop up randomly. It's the ascii representation of "1". See http://www.asciitable.com/
.toInt will give you the ascii value. It's probably easiest to write
"123".head - '0'
If you want to handle non-numeric characters, you can do
c match {
case c if '0' <= c && c <= '9' => Some(c - '0')
case _ => None
}
You can also use
"123".head.toString.toInt
If there is any way to call a def from a block
def factor (n: Int) : Int = if (n == 0 ) 1 else n * factor(n-1)
val i = 1000
i.toString.foreach ( x => sum += factor(x.toInt) )
at the end I want to get the sum of factorial of every digit
But it seems like def doesn't return a value, everytime is 0
How to fix it?
Thanks!
The problem actually has nothing to do with Scala per se; your code and your def are fine. The issue is with toInt:
scala> '3'.toInt
res7: Int = 51
toInt doesn't actually convert it as a decimal digit, but as a unicode (ish?) character value. These are producing very large numbers which go beyond what factor can handle:
scala> factor(6)
res8: Int = 720
scala> factor(20)
res9: Int = -2102132736
scala> factor(100)
res10: Int = 0
So instead use (thanks to Luigi)
x.asDigit
I have a dynamically changing input reading from a file. The numbers are either Int or Double. Why does Scala print .0 after every Double number? Is there a way for Scala to print it the same way it reads it?
Example:
var x:Double = 1
println (x) // This prints '1.0', I want it to print '1'
x = 1.0 // This prints '1.0', which is good
I can't use Int because some of the input I get are Doubles. I can't use String or AnyVal because I perform some math operations.
Thank you,
scala> "%1.0f" format 1.0
res3: String = 1
If your input is either Int or Double, you can do it like this:
def fmt(v: Any): String = v match {
case d : Double => "%1.0f" format d
case i : Int => i.toString
case _ => throw new IllegalArgumentException
}
Usage:
scala> fmt(1.0)
res6: String = 1
scala> fmt(1)
res7: String = 1
scala> fmt(1.0f)
java.lang.IllegalArgumentException
at .fmt(<console>:7)
at .<init>(<console>:6)
at .<clinit>(<console>)
at RequestResult$.<init>(<console>:4)
at RequestResult$.<clinit>(<console>)
at RequestResult$result(<console>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.Dele...
Otherwise, you might use BigDecimals. They are slow, but they do come with the scale, so "1", "1.0" and "1.00" are all different:
scala> var x = BigDecimal("1.0")
x: BigDecimal = 1.0
scala> x = 1
x: BigDecimal = 1
scala> x = 1.0
x: BigDecimal = 1.0
scala> x = 1.000
x: BigDecimal = 1.0
scala> x = "1.000"
x: BigDecimal = 1.000
var x:Double = 1
var y:Double = 1.0
print(x) // => 1.0
print(y) // => 1.0
If i understand you question you want scala to print x and y differently? The problem is that x and y are both a variable of the type Double and look the same.
Why do you explicitly define the type of the vars?
var x = 1
var y= 1.0
print(x) // => 1
print(y) // => 1.0
Use printf:
printf("The value is %.0f", x)
For a description of the format string, see this page from the Java SE 6 API documentation.
Note that you can ofcourse also use the Java library from Scala, so other ways to format numbers from Java can also be used from Scala. You can for example use class java.text.DecimalFormat:
val df = new java.text.DecimalFormat("#####")
println(df.format(x))
Starting with Scala 2.10 you can use the f interpolator:
scala> val x: Double = 1
x: Double = 1.0
scala> println(f"$x%.0f")
1
scala> val i = 1
i: Int = 1
scala> println(f"$i%.0f")
1
The use of a "_.0" at the end of floating point numbers is a convention. Just a way to know that the number is actually floating point and not an integer.
If you really need to "to print it the same way it reads it" you may have to rethink the way your code is structured, possibly preserving your input data. If it's just a formatting issue, the easiest way is to convert the values to integers before printing:
val x = 1.0
println(x.toInt)
If some are integers and some are not, you need a bit more code:
def fmt[T <% math.ScalaNumericConversions](n : T) =
if(n.toInt == n) n.toInt.toString else n.toString
val a : Double = 1.0
val b : Double = 1.5
val c : Int = 1
println(fmt(a))
println(fmt(b))
println(fmt(c))
The code above should print:
1
1.5
1
The signature of the fmt method accepts any type that either is a subtype of ScalaNumericConversions or can be converted to one through implicit conversions (so we can use the toInt method).
If you are working with a Double and want to format it as a String without .0 when it's a whole number and with its decimals otherwise, then you could use String::stripSuffix:
x.toString.stripSuffix(".0")
// val x: Double = 1.34 => "1.34"
// val x: Double = 1.0 => "1"
Use type inference, rather than explicit typing.
scala> val xi = 1
xi: Int = 1
scala> val xd = 1.0
xd: Double = 1.0
scala> println(xi)
1
scala> println(xd)
1.0