hackerrank Circular Array Rotation Go Runtime Error - go

Question

I did solved a hackerrank problem which is "Circular Array Rotation" using Go
when i run the code it gives me a correct response and when i try to submit the code all the test passes except one and it says Runtime Error
I tried the failing test localy and it passes
my code is
package main
import "fmt"
func CircularArrayRotation() {
var n, k, q int
fmt.Scanf("%d%d%d", &n, &k, &q)
a := make([]int, n)
for i := range a {
fmt.Scanf("%d", &a[i])
}
var i int
for t:=0; t<q; t++ {
fmt.Scanf("%d", &i)
j := (i - k)
if j<0 {
j+=n
}
fmt.Println(a[j])
}
}
func main() {
//Enter your code here. Read input from STDIN. Print output to STDOUT
CircularArrayRotation()
}

Answer 1

For those that want more information you can look here: https://www.hackerrank.com/challenges/circular-array-rotation/problem
The Input for his failing case is this and the Expected Output is this
Your run time error is this:
panic: runtime error: index out of range
goroutine 1 [running]: main.CircularArrayRotation() solution.go:22
+0x349 main.main() solution.go:29 +0x20
So your issue is on line 22, where your index is out of range: fmt.Println(a[j])
This happens because your code currently cannot handle multiple rotations, so in your code you end up executing the following:
fmt.Println(a[-99477])
This happens when i is 8 and k is 100000
Imagine you had this input:
n = 3
k = 10
q = 1
i = 2
When you perform i - k we get -8, we then try to add n which gives us -5 (-8 + 3), then we try to access an index that does not exist. a[-5]
If you wish to fix this issue you can do the below (inside spoiler in case you want to work this out yourself):
put this above your i - k
k = k % n
The reason this fixes your code is:
It works out how many rotations are left after we've fully looped x times. That way we don't have to worry about multiple rotations.
--
As a side note for this challenge there's some interesting stuff you can do with slices for rotating an array using some of the stuff covered here: https://tour.golang.org/moretypes/10

Answer 2

Most of your code is correct, however it fails because you only check if j < 0 once. After adding n to it is still negative, so it fails.
Running your code and printing the values when it fails gives:
n: 515, k:100000, q:500, j:-99477
The fix is simple, change your if to a for. This will keep adding n until your index is postive again.
Fixed:
package main
import "fmt"
func CircularArrayRotation() {
var n, k, q int
fmt.Scanf("%d%d%d", &n, &k, &q)
a := make([]int, n)
for i := range a {
fmt.Scanf("%d", &a[i])
}
var i int
for t := 0; t < q; t++ {
fmt.Scanf("%d", &i)
j := i - k
for j < 0 {
j += n
}
fmt.Println(a[j])
}
}
func main() {
//Enter your code here. Read input from STDIN. Print output to STDOUT
CircularArrayRotation()
}