I'm new on this site but I've been struggling for several days about this issue I found. I wrote this code in order to solve a challenge of the site Codewars; the challenge consists in calculate the mean and the variance from some data about some fictional rainfalls (I attach the complete page on the bottom). In order to end this challenge I created a function to convert the data from this useless string into an array of Doubles. The weird thing is that the function if called outside the main one works properly but inside returns an empty array. I have no idea why is happening this. Thank you very much for every effort you'll put trying to explain me this.
This is the first part of the Codewars page that explain the callenge
This is the second one
//
// main.swift
// Prova
//
// Created by Lorenzo Santini on 13/06/18.
// Copyright © 2018 Lorenzo Santini. All rights reserved.
//
import Foundation
func mean(_ d: String,_ town: String) -> Double {
let arrayOfValues = obtainArrayOfMeasures(d, town)
var sum: Double = 0
for element in arrayOfValues {
sum += element
}
return sum / Double(arrayOfValues.count)
}
func variance(_ d: String,_ town: String) -> Double {
let meanValue: Double = mean(d, town)
//Here is the problem: when this function is called instead of returning the array containg all the measures for the selected city it returns an empty array
var arrayOfValues = obtainArrayOfMeasures(d, town)
var sum: Double = 0
for element in arrayOfValues {
sum += pow((element - meanValue), 2)
}
return sum / Double(arrayOfValues.count)
}
func isInt(_ char: Character) -> Bool {
switch char {
case "1","2","3","4","5","6","7","8","9":
return true
default:
return false
}
}
func obtainArrayOfMeasures(_ d: String,_ town: String) -> [Double]{
//The first array stores the Data string divided for city
var arrayOfString: [String] = []
//The second array stores the measures of rainfall of the town passed as argument for the function
var arrayOfMeasures: [Double] = []
//Split the d variable containg the data string in separated strings for each town and add it to the arrayOfString array
repeat {
let finalIndex = (data.index(of:"\n")) ?? data.endIndex
arrayOfString.append(String(data[data.startIndex..<finalIndex]))
let finalIndexToRemove = (data.endIndex == finalIndex) ? finalIndex : data.index(finalIndex, offsetBy: 1)
data.removeSubrange(data.startIndex..<finalIndexToRemove)
} while data.count != 0
//Find the string of the town passed as argument
var stringContainingTown: String? = nil
for string in arrayOfString {
if string.contains(town) {stringContainingTown = string; print("true")}
}
if stringContainingTown != nil {
var stringNumber = ""
var index = 0
//Add to arrayOfMeasures the measures of the selected town
for char in stringContainingTown! {
index += 1
if isInt(char) || char == "." {
stringNumber += String(char)
print(stringNumber)
}
if char == "," || index == stringContainingTown!.count {
arrayOfMeasures.append((stringNumber as NSString).doubleValue)
stringNumber = ""
}
}
}
return arrayOfMeasures
}
var data = "Rome:Jan 81.2,Feb 63.2,Mar 70.3,Apr 55.7,May 53.0,Jun 36.4,Jul 17.5,Aug 27.5,Sep 60.9,Oct 117.7,Nov 111.0,Dec 97.9" + "\n" +
"London:Jan 48.0,Feb 38.9,Mar 39.9,Apr 42.2,May 47.3,Jun 52.1,Jul 59.5,Aug 57.2,Sep 55.4,Oct 62.0,Nov 59.0,Dec 52.9" + "\n" +
"Paris:Jan 182.3,Feb 120.6,Mar 158.1,Apr 204.9,May 323.1,Jun 300.5,Jul 236.8,Aug 192.9,Sep 66.3,Oct 63.3,Nov 83.2,Dec 154.7" + "\n" +
"NY:Jan 108.7,Feb 101.8,Mar 131.9,Apr 93.5,May 98.8,Jun 93.6,Jul 102.2,Aug 131.8,Sep 92.0,Oct 82.3,Nov 107.8,Dec 94.2" + "\n" +
"Vancouver:Jan 145.7,Feb 121.4,Mar 102.3,Apr 69.2,May 55.8,Jun 47.1,Jul 31.3,Aug 37.0,Sep 59.6,Oct 116.3,Nov 154.6,Dec 171.5" + "\n" +
"Sydney:Jan 103.4,Feb 111.0,Mar 131.3,Apr 129.7,May 123.0,Jun 129.2,Jul 102.8,Aug 80.3,Sep 69.3,Oct 82.6,Nov 81.4,Dec 78.2" + "\n" +
"Bangkok:Jan 10.6,Feb 28.2,Mar 30.7,Apr 71.8,May 189.4,Jun 151.7,Jul 158.2,Aug 187.0,Sep 319.9,Oct 230.8,Nov 57.3,Dec 9.4" + "\n" +
"Tokyo:Jan 49.9,Feb 71.5,Mar 106.4,Apr 129.2,May 144.0,Jun 176.0,Jul 135.6,Aug 148.5,Sep 216.4,Oct 194.1,Nov 95.6,Dec 54.4" + "\n" +
"Beijing:Jan 3.9,Feb 4.7,Mar 8.2,Apr 18.4,May 33.0,Jun 78.1,Jul 224.3,Aug 170.0,Sep 58.4,Oct 18.0,Nov 9.3,Dec 2.7" + "\n" +
"Lima:Jan 1.2,Feb 0.9,Mar 0.7,Apr 0.4,May 0.6,Jun 1.8,Jul 4.4,Aug 3.1,Sep 3.3,Oct 1.7,Nov 0.5,Dec 0.7"
var prova = variance(data, "London")
The problem is that func obtainArrayOfMeasures modifies the global data
variable. When called the second time, data is an empty string.
An indicator for this problem is also that making the global data variable constant
let data = "Rome:..."
causes a compiler error at
data.removeSubrange(data.startIndex..<finalIndexToRemove)
// Cannot use mutating member on immutable value: 'data' is a 'let' constant
An immediate fix would be to operate on a local mutable copy:
func obtainArrayOfMeasures(_ d: String,_ town: String) -> [Double]{
var data = d
// ...
}
Note however that the function can be simplified to
func obtainArrayOfMeasures(_ d: String,_ town: String) -> [Double] {
let lines = d.components(separatedBy: .newlines)
guard let line = lines.first(where: { $0.hasPrefix(town)}) else {
return [] // No matching line found.
}
let entries = line.components(separatedBy: ",")
let numbers = entries.compactMap { Double($0.filter {".0123456789".contains($0) })}
return numbers
}
without mutating any values. You might also consider to return nil
or abort with fatalError() if no matching entry is found.
Related
I am trying to learn swift by solving interview questions. One of the question that I am trying to solve is as follows.
Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).
Example:
Input: S = "ADOBECODEBANC", T = "ABC"
Output: "BANC"
My implementation is as follows which holds t string characters and its corresponding index retrieved from s.
func minimumWindowSubstring(_ s: String, _ t: String) -> String{
let sChar = [Character](s)
let tChar = [Character](t)
var indexTemp = [[Character:Int]()]
for tc in tChar
{
for (j, sc) in sChar.enumerated()
{
if sc == tc
{
indexTemp.append([tc:j])
}
}
}
return ""
}
what I have in indexTemp array is as follows
Now I wonder how could I able to use this array to find the minimumwindow, I stuck.
I thought it was an interesting problem so I gave it a shot. Instead of using a dictionary I used a simple class to store the range of characters found, as well as a String that stores which characters haven't been found.
It only goes through the main string once, so it should be O(n).
You can run this in the playground.
(I know you wanted help in fixing your code and my answer doesn't do that, but I'm hoping it will provide enough insight for you to adjust your own code)
import Foundation
let string = "ADOBECODEBANC"
let sub = "ABC"
// Create a class to hold the start and end index of the current search range, as well as a remaining variable
// that will store which characters from sub haven't been found
class Container {
var start: Int
var end: Int?
var remaining: String
// Consume will attempt to find a given character within our remaining string, if it has found all of them,
// it will store the end index
func consume(character: Character, at index: Int) {
// If remaining is empty, we're done
guard remaining != "" else { return }
// We're assuming that sub won't have repeating characters. If it does we'll have to chage this
remaining = remaining.replacingOccurrences(of: String(character), with: "")
if remaining == "" {
end = index
}
}
init(start: Int, remaining: String) {
self.start = start
self.remaining = remaining
}
}
// ClosedContainer is similar to Container, but it can only be initialized by an existing container. If the existing
// container doesn't have an end value, the initialization will fail and return nil. This way we can weed out containers
// for ranges where we didn't find all characters.
class ClosedContainer {
let start: Int
let end: Int
init?(container: Container) {
guard let end = container.end else { return nil }
self.start = container.start
self.end = end
}
var length: Int {
return end - start
}
}
var containers = [Container]()
// Go through each character of the string
string.enumerated().forEach { index, character in
// Look for matches in sub
if sub.contains(character) {
// Allow each existing container to attempt to consume the character
containers.forEach { container in
container.consume(character: character, at: index)
}
// Create a new container starting on this index. It's remaining value will be the sub string without the
// character we just found
let container = Container(start: index, remaining: sub.replacingOccurrences(of: String(character), with: ""))
containers.append(container)
}
}
// Convert Containers into ClosedContainers using compactMap, then find the one with the shortest length
let closedContainers = containers.compactMap(ClosedContainer.init)
let maybeShortest = closedContainers.min { $0.length < $1.length }
if let shortest = maybeShortest {
// Convert int to String indices
let start = string.index(string.startIndex, offsetBy: shortest.start)
let end = string.index(string.startIndex, offsetBy: shortest.end)
// Get the result string
let result = string[start...end]
print("Shortest substring of", string, "that contains", sub, "is", result)
} else {
// No range was found that had all characters in sub
print(string, "doesn't contain all characters in", sub)
}
I have a string of numbers (each number is separated by ,) that looks like this:
"12,3,5,75,584,364,57,88,94,4,79,333,7465,867,56,6,748,546,573,466"
I want to split the string to an array of strings, that each element is a string that has maximum 10 number in it.
For the example I've added I want to achieve something like this:
stringsArray:
Element 0: "12,3,5,75,584,364,57,88,94,4"
Element 1: "79,333,7465,867,56,6,748,546,573,466"
And so on...
I've been thinking a lot about a way to do this with Swift, but couldn't find anything...
Does anybody has an idea?
Thank you!
Step 1 - get fully separated array:
let numbers = "12,3,5".components(separatedBy: ",")
Step 2 - chunk your result to parts with ext:
extension Array {
func chunked(by chunkSize: Int) -> [[Element]] {
return stride(from: 0, to: self.count, by: chunkSize).map {
Array(self[$0..<Swift.min($0 + chunkSize, self.count)])
}
}
}
let chunkedNumbers = numbers.chunked(by: 10)
Step 3:
let stringsArray = chunkedNumbers.map { $0.joined(separator: ",") }
Result: ["12,3,5,75,584,364,57,88,94,4", "79,333,7465,867,56,6,748,546,573,466"]
Link to gist playground.
I would look at the position of 10th comma in your original string, get the prefix up to this position, remove this prefix and repeat until remaining string is empty.
This is a bit brute force, but works.
I first add extension to String for convenience.
extension String {
func startIndexesOf(_ string: String) -> [Int] {
var result: [Int] = []
var start = startIndex
while let range = range(of: string, options: .literal, range: start..<endIndex) {
result.append(range.lowerBound.encodedOffset)
start = range.upperBound
}
return result
}
subscript (r: Range<Int>) -> String {
let start = index(self.startIndex, offsetBy: r.lowerBound)
let end = self.index(self.startIndex, offsetBy: r.upperBound)
return String(self[Range(start ..< end)])
}
}
let test = "12,3,5,75,584,364,57,88,94,4,79,333,7465,867,56,6,748,546,573,466,999"
var remaining = test
var arrayOf10 : [String] = []
repeat {
let indexes = remaining.startIndexesOf(",")
if indexes.count < 10 {
arrayOf10.append(remaining) // Just add what remains
break
}
let position = indexes[9]
let endBeginning = remaining.index(test.startIndex, offsetBy: position) // Beginning of what remain to parse
let beginningSubstring = remaining[remaining.startIndex ..< endBeginning]
let beginningText = String(beginningSubstring)
arrayOf10.append(beginningText)
let startNext = remaining.index(test.startIndex, offsetBy: position+1) // What will remain to parse after taking out the beginning
let remainingSubString = remaining[startNext ..< remaining.endIndex]
remaining = String(remainingSubString)
} while remaining.count > 0
for (c, s) in arrayOf10.enumerated() { print("Element", c, ": ", s)}
This will print as desired
Element 0 : 12,3,5,75,584,364,57,88,94,4
Element 1 : 79,333,7465,867,56,6,748,546,573,466
Element 2 : 999
I am trying to solve code fights interview practice questions, but I am stuck on how to solve this particular problem in swift. My first thought was to use a dictionary with the counts of each character, but then I would have to iterate over the string again to compare, so that doesn't work per the restrictions. Any help would be good. Thank you. Here is the problem and requirements:
Note: Write a solution that only iterates over the string once and uses O(1) additional memory, since this is what you would be asked to do during a real interview.
Given a string s, find and return the first instance of a non-repeating character in it. If there is no such character, return '_'
Here is the code I started with (borrowed from another post)
func firstNotRepeatingCharacter(s: String) -> Character {
var countHash:[Character:Int] = [:]
for character in s {
countHash[character] = (countHash[character] ?? 0) + 1
}
let nonRepeatingCharacters = s.filter({countHash[$0] == 1})
let firstNonRepeatingCharacter = nonRepeatingCharacters.first!
return firstNonRepeatingCharacter
}
firstNotRepeatingCharacter(s:"abacabad")
You can create a dictionary to store the occurrences and use first(where:) method to return the first occurrence that happens only once:
Swift 4
func firstNotRepeatingCharacter(s: String) -> Character {
var occurrences: [Character: Int] = [:]
s.forEach{ occurrences[$0, default: 0] += 1 }
return s.first{ occurrences[$0] == 1 } ?? "_"
}
Swift 3
func firstNotRepeatingCharacter(s: String) -> Character {
var occurrences: [Character:Int] = [:]
s.characters.forEach{ occurrences[$0] = (occurrences[$0] ?? 0) + 1}
return s.characters.first{ occurrences[$0] == 1 } ?? "_"
}
Another option iterating the string in reversed order and using an array of 26 elements to store the characters occurrences
func firstNotRepeatingCharacter(s: String) -> Character {
var chars = Array(repeating: 0, count: 26)
var characters: [Character] = []
var charIndex = 0
var strIndex = 0
s.characters.reversed().forEach {
let index = Int(String($0).unicodeScalars.first!.value) - 97
chars[index] += 1
if chars[index] == 1 && strIndex >= charIndex {
characters.append($0)
charIndex = strIndex
}
strIndex += 1
}
return characters.reversed().first { chars[Int(String($0).unicodeScalars.first!.value) - 97] == 1 } ?? "_"
}
Use a dictionary to store the character counts as well as where they were first encountered. Then, loop over the dictionary (which is constant in size since there are only so many unique characters in the input string, thus also takes constant time to iterate) and find the earliest occurring character with a count of 1.
func firstUniqueCharacter(in s: String) -> Character
{
var characters = [Character: (count: Int, firstIndex: Int)]()
for (i, c) in s.characters.enumerated()
{
if let t = characters[c]
{
characters[c] = (t.count + 1, t.firstIndex)
}
else
{
characters[c] = (1, i)
}
}
var firstUnique = (character: Character("_"), index: Int.max)
for (k, v) in characters
{
if v.count == 1 && v.firstIndex <= firstUnique.index
{
firstUnique = (k, v.firstIndex)
}
}
return firstUnique.character
}
Swift
Use dictionary, uniqueCharacter optional variable with unique characters array to store all uniquely present characters in the string , every time duplication of characters found should delete that character from unique characters array and same time it is the most first character then should update the dictionary with its count incremented , refer following snippet , how end of the iteration through all characters gives a FIRST NON REPEATED CHARACTER in given String. Refer following code to understand it properly
func findFirstNonRepeatingCharacter(string:String) -> Character?{
var uniqueChars:[Character] = []
var uniqueChar:Character?
var chars = string.lowercased().characters
var charWithCount:[Character:Int] = [:]
for char in chars{
if let count = charWithCount[char] { //amazon
charWithCount[char] = count+1
if char == uniqueChar{
uniqueChars.removeFirst()
uniqueChar = uniqueChars.first
}
}else{
charWithCount[char] = 1
uniqueChars.append(char)
if uniqueChar == nil{
uniqueChar = char
}
}
}
return uniqueChar
}
// Use
findFirstNonRepeatingCharacter(string: "eabcdee")
I need to save files in an alphabetical order.
Now my code is saving files in numeric order
1.png
2.png
3.png ...
The problem is when i read this files again I read this files as described here
So I was thinking of changing the code and to save the files not in a numeric order but in an alphabetical order as:
a.png b.png c.png ... z.png aa.png ab.png ...
But in Swift it's difficult to increment even Character type.
How can I start from:
var s: String = "a"
and increment s in that way?
You can keep it numeric, just use the right option when sorting:
let arr = ["1.png", "19.png", "2.png", "10.png"]
let result = arr.sort {
$0.compare($1, options: .NumericSearch) == .OrderedAscending
}
// result: ["1.png", "2.png", "10.png", "19.png"]
If you'd really like to make them alphabetical, try this code to increment the names:
/// Increments a single `UInt32` scalar value
func incrementScalarValue(_ scalarValue: UInt32) -> String {
return String(Character(UnicodeScalar(scalarValue + 1)))
}
/// Recursive function that increments a name
func incrementName(_ name: String) -> String {
var previousName = name
if let lastScalar = previousName.unicodeScalars.last {
let lastChar = previousName.remove(at: previousName.index(before: previousName.endIndex))
if lastChar == "z" {
let newName = incrementName(previousName) + "a"
return newName
} else {
let incrementedChar = incrementScalarValue(lastScalar.value)
return previousName + incrementedChar
}
} else {
return "a"
}
}
var fileNames = ["a.png"]
for _ in 1...77 {
// Strip off ".png" from the file name
let previousFileName = fileNames.last!.components(separatedBy: ".png")[0]
// Increment the name
let incremented = incrementName(previousFileName)
// Append it to the array with ".png" added again
fileNames.append(incremented + ".png")
}
print(fileNames)
// Prints `["a.png", "b.png", "c.png", "d.png", "e.png", "f.png", "g.png", "h.png", "i.png", "j.png", "k.png", "l.png", "m.png", "n.png", "o.png", "p.png", "q.png", "r.png", "s.png", "t.png", "u.png", "v.png", "w.png", "x.png", "y.png", "z.png", "aa.png", "ab.png", "ac.png", "ad.png", "ae.png", "af.png", "ag.png", "ah.png", "ai.png", "aj.png", "ak.png", "al.png", "am.png", "an.png", "ao.png", "ap.png", "aq.png", "ar.png", "as.png", "at.png", "au.png", "av.png", "aw.png", "ax.png", "ay.png", "az.png", "ba.png", "bb.png", "bc.png", "bd.png", "be.png", "bf.png", "bg.png", "bh.png", "bi.png", "bj.png", "bk.png", "bl.png", "bm.png", "bn.png", "bo.png", "bp.png", "bq.png", "br.png", "bs.png", "bt.png", "bu.png", "bv.png", "bw.png", "bx.png", "by.png", "bz.png"]`
You will eventually end up with
a.png
b.png
c.png
...
z.png
aa.png
ab.png
...
zz.png
aaa.png
aab.png
...
Paste this code in the playground and check result. n numbers supported means you can enter any high number such as 99999999999999 enjoy!
you can uncomment for loop code to check code is working fine or not
but don't forget to assign a lesser value to counter variable otherwise Xcode will freeze.
var fileName:String = ""
var counter = 0.0
var alphabets = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
let totalAlphaBets = Double(alphabets.count)
let numFiles = 9999
func getCharacter(counter c:Double) -> String {
var chars:String
var divisionResult = Int(c / totalAlphaBets)
let modResult = Int(c.truncatingRemainder(dividingBy: totalAlphaBets))
chars = getCharFromArr(index: modResult)
if(divisionResult != 0){
divisionResult -= 1
if(divisionResult > alphabets.count-1){
chars = getCharacter(counter: Double(divisionResult)) + chars
}else{
chars = getCharFromArr(index: divisionResult) + chars
}
}
return chars
}
func getCharFromArr(index i:Int) -> String {
if(i < alphabets.count){
return alphabets[i]
}else{
print("wrong index")
return ""
}
}
for _ in 0...numFiles {
fileName = getCharacter(counter: counter)+".png"
print(fileName)
counter += 1
}
fileName = getCharacter(counter: Double(numFiles))+".png"
print(fileName)
I've write a simple code:
extension String {
func trailingSpaces (width: Int) -> String {
var s = "\(self)"
for i in count(s)..<width {
s = s + " "
}
return s
}
func leadingSpaces (width: Int) -> String {
var s = "\(self)"
for i in count(s)..<width {
s = " " + s
}
return s
}
}
class ViewController: UIViewController {
var users = ["Marco", "Gianni", "Antonio", "Giulio", "Franco"]
var ages = [29, 45, 17, 33, 37]
override func viewDidLoad() {
super.viewDidLoad()
var merged = [String: Int] ()
var totalAge = 0.0
for var i = 0; i < ages.count; i++ {
merged[users[i]] = ages[i]
}
for user in sorted(merged.keys) {
let age = merged[user]
totalAge += Double(age!)
let paddedUser = user.trailingSpaces(10)
let paddedAge = "\(age)".leadingSpaces(3)
println("\(paddedUser) \(age!)")
}
println("\n\(merged.count) users")
println("average age: \(totalAge / Double(merged.count))")
}
}
but I can't make it work the leadingSpaces function and I can't understand the reason, it's quite identical to the other extension func that works.
It give the error
fatal error: Can't form Range with end < start
on runtime
in case you run into this kind of problem, always do a println() of the variable you are using
println("\(age)") right before let paddedAge = "\(age!)".leadingSpaces(3)
reveals the problem
age is an optional, meaning that you are trying to do the padding on a String which has this value "Optional(17)"
Thus, your count(s) is higher than 3, and you have an invalid range
Your variable age is not an Int - it's an optional - Int?. You know this already as you are unwrapping it in the lines totalAge += Double(age!) and println("\(paddedUser) \(age!)") - but you are not unwrapping it in the failing line let paddedAge = "\(age)".leadingSpaces(3). The string being passed to leadingSpaces is not "17", it's "Optional(17)", which is why your padding function is failing, as the length is greater than the requested width.
Having said that, as the commentator #milo256 points out, Swift can only iterate upwards, and so unless you put a check on width >= .count in your padding functions they will crash at some point.