Problem

I’ve solved this question. Wasn’t sure if I was suppose to use a dictionary in this case, as the question was to use a dynamic array, but I didnt know how else to do it. Was wondering if I can get feedback on my code or if there’s a better way to approach it:

```
import Foundation
// reads input for N(number of sequences) and Q(number of queries)
let input = readLine()!.componentsSeparatedByString(" ").map { Int($0)! }
let numOfSequences = input[0]
let numOfQueries = input[1]
var lastAns = 0
var seqDictionary = [Int: [Int]]()
// loop through numOfQueries
for _ in 0..<numOfQueries {
// each query is given in format [1, x, y] or [2, x, y]
let query = readLine()!.componentsSeparatedByString(" ").map { Int($0)! }
// check to see if query[0] is 1 or 2
switch query[0] {
case 1:
// formula for seqDictionaryIndex
// ^ is the XOR operator
let seqDictionaryIndex = ((query[1] ^ lastAns) % numOfSequences)
// since we initalized an empty dictionary, we gotta set the first key to a value to start the array so we can start appending
guard seqDictionary[seqDictionaryIndex] != nil else { seqDictionary[seqDictionaryIndex] = [query[2]] ; continue }
seqDictionary[seqDictionaryIndex]?.append(query[2])
case 2:
let seqDictionaryIndex = ((query[1] ^ lastAns) % numOfSequences)
// calculate the size of the particular sequence in seqDictionary
let size = seqDictionary[seqDictionaryIndex]?.count
// formula for finding index in particular sequence in seqDictionary
let index = query[2] % size!
// set last answer to that element at the index calculated above
lastAns = seqDictionary[seqDictionaryIndex]![index]
print(lastAns)
default: break
}
}
```

Solution

The task is about dynamic arrays, and using an array (instead of a

dictionary, as you did) simplifies things considerably.

If you start with an array of `numOfSequences`

empty arrays:

```
var seqList = [[Int]](count: numOfSequences, repeatedValue: [])
```

then the `1 x y`

query simplifies to

```
let seqIndex = ((x ^ lastAns) % numOfSequences)
seqList[seqIndex].append(y)
```

without the need to check for an empty dictionary value.

Some more suggestions:

Reading a list of integers occurs at two places in the program, that

justifies a separate function:

```
func readIntegers() -> [Int] {
return readLine()!.componentsSeparatedByString(" ").map { Int($0)! }
}
```

(*Remark:* Usually, forced unwrapping with `!`

should be avoided,

but here we have a programming challenge with well-defined input data.)

To increase the legibility of the code, I would assign the three

parameters of a query to variables `type`

, `x`

, `y`

.

The code then becomes almost self-explaining.

The entire code then looks like this:

```
func readIntegers() -> [Int] {
return readLine()!.componentsSeparatedByString(" ").map { Int($0)! }
}
// Read N(number of sequences) and Q(number of queries):
let input = readIntegers()
let (numOfSequences, numOfQueries) = (input[0], input[1])
var lastAns = 0
// Array of `numOfSequences` empty arrays:
var seqList = [[Int]](count: numOfSequences, repeatedValue: [])
for _ in 0..<numOfQueries {
let input = readIntegers()
let (type, x, y) = (input[0], input[1], input[2])
switch type {
case 1:
let seqIndex = ((x ^ lastAns) % numOfSequences)
seqList[seqIndex].append(y)
case 2:
let seqIndex = ((x ^ lastAns) % numOfSequences)
let index = y % seqList[seqIndex].count
lastAns = seqList[seqIndex][index]
print(lastAns)
default:
fatalError("Unexpected query")
}
}
```