Hi I am writing an app in kotlin and need to decompose a number into powers of 2.
I have already done this in c#, PHP and swift but kotlin works differently somehow.
having researched this I believe it is something to do with the numbers in my code going negative somewhere and that the solution lies in declaring one or more of the variable as "Long" to prevent this from happening but i have not been able to figure out how to do this.
here is my code:
var salads = StringBuilder()
var value = 127
var j=0
while (j < 256) {
var mask = 1 shl j
if(value != 0 && mask != 0) {
salads.append(mask)
salads.append(",")
}
j += 1
}
// salads = (salads.dropLast()) // removes the final ","
println("Salads = $salads")
This shoud output the following:
1,2,4,8,16,32,64
What I actually get is:
1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384,32768,65536,131072,262144,524288,1048576,2097152,4194304,8388608,16777216,33554432,67108864,134217728,268435456,536870912,1073741824,-2147483648,
Any ideas?
This works for the one input that you specified, at the very least:
fun powersOfTwo(value :Long): String {
val result = ArrayList<String>()
var i = 0
var lastMask = 0
while (lastMask < value) {
val mask = 1 shl i
if (value != 0.toLong() && mask < value) {
result.add(mask.toString())
}
lastMask = mask
i += 1
}
return result.joinToString(",")
}
Ran it in a unit test:
#Test
fun addition_isCorrect() {
val result = powersOfTwo(127)
assertEquals("1,2,4,8,16,32,64", result)
}
Test passed.
You can get a list of all powers of two that fit in Int and test each of them for whether the value contains it with the infix function and:
val value = 126
val powersOfTwo = (0 until Int.SIZE_BITS).map { n -> 1 shl n }
println(powersOfTwo.filter { p -> value and p != 0}.joinToString(","))
// prints: 2,4,8,16,32,64
See the entire code in Kotlin playground: https://pl.kotl.in/f4CZtmCyI
Hi I finally managed to get this working properly:
fun decomposeByTwo(value :Int): String {
val result = ArrayList<String>()
var value = value
var j = 0
while (j < 256) {
var mask = 1 shl j
if ((value and mask) != 0) {
value -= mask
result.add(mask.toString())
}
j += 1
}
return result.toString()
}
I hope this helps someone trying to get a handle on bitwise options!
Somehow you want to do the "bitwise AND" of "value" and "mask" to determine if the j-th bit of "value" is set. I think you just forgot that test in your kotlin implementation.
How to change 12345 to 54321?
With a string, you can change the string to a rune, and reverse it, but you cannot do the same for an integer. I have searched and found no one talking about this. Examples
131415 >>> 514131
1357 >>> 7531
123a >>> ERROR
-EDIT-
I was thinking, why not create a slice and index that?
Then I realized that you can't index int
(http://play.golang.org/p/SUSg04tZsc)
MY NEW QUESTION IS
How do you index an int?
OR
How do you reverse a int?
Here is a solution that does not use indexing an int
package main
import (
"fmt"
)
func reverse_int(n int) int {
new_int := 0
for n > 0 {
remainder := n % 10
new_int *= 10
new_int += remainder
n /= 10
}
return new_int
}
func main() {
fmt.Println(reverse_int(123456))
fmt.Println(reverse_int(100))
fmt.Println(reverse_int(1001))
fmt.Println(reverse_int(131415))
fmt.Println(reverse_int(1357))
}
Result:
654321
1
1001
514131
7531
Go playground
I converted the integer to a string, reverse the string, and convert the result back to a string.
package main
import (
"fmt"
"strconv"
)
func main() {
fmt.Println(reverse_int(123456))
fmt.Println(reverse_int(100))
fmt.Println(reverse_int(1001))
fmt.Println(reverse_int(131415))
fmt.Println(reverse_int(1357))
}
func reverse_int(value int) int {
intString := strconv.Itoa(value)
newString := ""
for x := len(intString); x > 0; x-- {
newString += string(intString[x - 1])
}
newInt, err := strconv.Atoi(newString)
if(err != nil){
fmt.Println("Error converting string to int")
}
return newInt
}
Very similar to the first answer but this checks to make sure you don't go out of bounds on the type.
func reverse(x int) int {
rev := 0
for x != 0 {
pop := x % 10
x /= 10
if rev > math.MaxInt32/10 || (rev == math.MaxInt32 /10 && pop > 7) {
return 0
}
if rev < math.MinInt32/10 || (rev == math.MinInt32/10 && pop < -8) {
return 0
}
rev = rev * 10 + pop
}
return rev
}
Also flips negative numbers int
func Abs(x int) int {
if x < 0 {
return -x
}
return x
}
func reverse_int(n int) int {
newInt := 0
sign := 1
if n < 0 {
sign = -1
}
n = Abs(n)
for n > 0 {
remainder := n % 10
newInt = newInt*10 + remainder
n /= 10
}
return newInt * sign
}
func main() {
fmt.Println(reverse_int(-100))
fmt.Println(reverse_int(-1001))
fmt.Println(reverse_int(131415))
fmt.Println(reverse_int(1357))
}
Similar to Fokiruna's answer but also checks for a 32bit overflow
func reverse(x int) int {
result, sign := 0, 1
if(x < 0) {
sign = -1
x = -x
}
for x > 0 {
remainder := x % 10;
result = result * 10 + remainder
x = x/10
}
var checkInt int = int(int32(result))
if checkInt != result {
return 0
}
return result * sign
}
How would I test against user input from fmt.Scan/Scanf/Scanln?
For example how could I test that the function input will accept "4 5\n" and "1 2 3 4\n" from STDIN and return n == 5 and array == [1, 2, 3, 4].
package main
import (
"fmt"
)
// input gets an array from the user.
func input() (m int, array []int) {
fmt.Print("Enter the size of the array, n, and the difference, m: ")
var n int
_, err := fmt.Scanf("%d %d", &n, &m)
if err != nil {
panic(err)
}
fmt.Print("Enter the array as a space seperated string: ")
array = make([]int, n)
for i := 0; i < n; i++ {
_, _ = fmt.Scan(&array[i])
}
return m, array
}
func main() {
m, array := input()
fmt.Println(m, array)
}
Here's a very rough draft to illustrate the principle.
program.go
package main
import (
"fmt"
"os"
)
// input gets an array from the user.
func input(in *os.File) (m int, array []int) {
if in == nil {
in = os.Stdin
}
fmt.Print("Enter the size of the array, n, and the difference, m: ")
var n int
_, err := fmt.Fscanf(in, "%d %d", &n, &m)
if err != nil {
panic(err)
}
fmt.Print("Enter the array as a space seperated string: ")
array = make([]int, n)
for i := 0; i < n; i++ {
_, _ = fmt.Fscan(in, &array[i])
}
return m, array
}
func main() {
m, array := input(nil)
fmt.Println(m, array)
}
program_test.go
package main
import (
"fmt"
"io"
"io/ioutil"
"os"
"testing"
)
func TestInput(t *testing.T) {
var (
n, m int
array []int
)
in, err := ioutil.TempFile("", "")
if err != nil {
t.Fatal(err)
}
defer in.Close()
_, err = io.WriteString(in, "4 5\n"+"1 2 3 4\n")
if err != nil {
t.Fatal(err)
}
_, err = in.Seek(0, os.SEEK_SET)
if err != nil {
t.Fatal(err)
}
n, array = input(in)
if n != 5 || fmt.Sprintf("%v", array) != fmt.Sprintf("%v", []int{1, 2, 3, 4}) {
t.Error("unexpected results:", n, m, array)
}
}
Output:
$ go test
ok command-line-arguments 0.010s
You can't. At least not so easily, such that, it would be worth the effort.