Swift: How to get the last integer from a NSString array? - ios

Question

var LabelAQI = "-"
var LabelExtraInfo = "-"
var arrAQI = [NSString]()
var AQI1:NSString = "55a"
var AQI2:NSString = "95a"
var AQI3:NSString = "66"
var AQI4:NSString = "25"
var AQI5:NSString = "88b"
var AQI6:NSString = "#"
arrAQI[0...5] = [AQI1, AQI2, AQI3, AQI4, AQI5, AQI6]
Using Swift, I'm getting AQI (Air Quality Index) data from a website in the form of NSString. AQI1 to AQI6 is essentially AQI data for every hour (e.g. 12am to 5am, I'm having up to AQI24 in the actual code though). "#" means data is not yet available. The integer is the AQI, and sometimes there may be associated extra information represented by "a"/"b" with the integers which means "Carbon Monoxide present"/"Sulphur Dioxide present". What I'm trying to do is to:
1) Display the last integer from arrAQI (excluding "#", and not displaying "a"/"b", which is "88") on LabelAQI
2) Display "Carbon Monoxide present"/"Sulphur Dioxide present" on LabelExtraInfo if there is "a"/"b" with the last integer from arrAQI, if not, leave LabelExtraInfo.text = "-"
I'm only a few weeks into programming. Can anybody help with this? Is there a better way to do what I want to do? Thank you so much in advance.

Answer 1

It usually helps to break the problem down into multiple functions. Here, you need to do two things – process the string, extracting a number and possible extra info. Then, loop over the array of strings until you find a suitable entry.
First the string processing. Given a string, you want to return either a pair (the reading and the extra info), or “not found”. Whenever you need to return something or not found, an optional is a good choice. So you want a function that takes a string and returns an optional (Int,String) pair:
func extractData(input: String) -> (Int,String)?
But you could go one step further and define an enum to represent the different bits of extra info:
enum AirQualityInfo {
case CarbonMonoxide,
SulphurDioxide,
NoInfo
}
func extractData(input: String) -> (Int,AirQualityInfo)?
This way, all your nasty string processing is contained within extractData.
Within that function, you want to check for a known trailing character, then strip that off, and if what remains is a number, return the two values. But if what remains isn’t a number, return nil:
func extractData(input: String) -> (Int,AirQualityInfo)? {
let stripped: String
let extraInfo: AirQualityInfo
switch last(input) {
case .Some("a"):
stripped = dropLast(input)
extraInfo = .CarbonMonoxide
case .Some("b"):
stripped = dropLast(input)
extraInfo = .SulphurDioxide
default:
stripped = input
extraInfo = .NoInfo
}
return stripped.toInt().map { ($0,extraInfo) }
}
You could, as others have suggested, use regular expressions for this, but personally I think this is overkill given your data parsing needs are so specific.
Once you have this function, you can loop over the array in reverse order, checking each value using the function, until you find a valid value:
for idx in reverse(indices(arrAQI)) {
if let (index, extraInfo) = extractData(arrAQI[idx]) {
LabelAQI = toString(index)
switch extraInfo {
case .CarbonMonoxide:
LabelExtraInfo = "Carbon Monoxide Present"
case .SulphurDioxide:
LabelExtraInfo = "SulphurDioxide Present"
case .NoInfo:
LabelExtraInfo = "-"
}
// stop going around the loop
break
}
}
You could also factor out that conversion of the string out further as well:
extension AirQualityInfo {
var displayString: String {
switch self {
case .CarbonMonoxide:
return "Carbon Monoxide Present"
case .SulphurDioxide:
return "SulphurDioxide Present"
case .NoInfo:
return "-"
}
}
}
for idx in reverse(indices(arrAQI)) {
if let (index, extraInfo) = extractData(arrAQI[idx]) {
LabelAQI = toString(index)
LabelExtraInfo = extraInfo.displayString
break
}
}
Finally, if you’re feeling super-adventurous, you could write a function that does that finding and mapping operation in one shot:
func findSome<C: CollectionType, T>
(source: C, match: C.Generator.Element -> T?)
-> T? {
for element in source {
if let x = match(element) {
return x
}
}
return nil
}
if let (index, extraInfo) = findSome(reverse(arrAQI),extractData) {
LabelAQI = toString(index)
LabelExtraInfo = extraInfo.displayString
}
By the way, a few other Swift tips: it’s generally better to use String rather than NSString unless you have a specific need for something to be an NSString (which it doesn’t look like here); you don’t have to name the types when declaring them – you can write let str = "hello" rather than let str: String = "hello" which tends to make code look a little cleaner and easier to read; and it’s best to use let rather than var unless you explicitly need to change (“mutate”) the value later in the code… so given all that, here’s how you could declare your original array:
let AQI1 = "55a"
let AQI2 = "95a"
let AQI3 = "66"
let AQI4 = "25"
let AQI5 = "88b"
let AQI6 = "#"
let arrAQI = [AQI1,AQI2,AQI3,AQI4,AQI5,AQI6,]

Answer 2

This could by done by exploding string into array of objects, and then check each of them:
var LabelAQI = "-"
var LabelExtraInfo = "-"
var stringFromServer = "55a 95a 66 25 88b #"
var objectsSeparated = stringFromServer.componentsSeparatedByString(" ")
for object in objectsSeparated
{
if object.hasSuffix("a")
{
LabelAQI = object.substringToIndex(object.endIndex.predecessor())
LabelExtraInfo = "Carbon Monoxide present"
}
else if object.hasSuffix("b")
{
LabelAQI = object.substringToIndex(object.endIndex.predecessor())
LabelExtraInfo = "Sulphur Dioxide present"
}
else if object.hasSuffix("#")
{
LabelAQI = object.substringToIndex(object.endIndex.predecessor())
LabelExtraInfo = "-"
}
else
{
LabelAQI = object
LabelExtraInfo = "-"
}
}
UPDATE
To find the last object, that doesn't have '#' suffix you could use following code:
var LabelAQI = "-"
var LabelExtraInfo = "-"
var stringFromServer = "55a 95a 66 25 88b #"
var objectsSeparated = stringFromServer.componentsSeparatedByString(" ")
var index = objectsSeparated.count
while 0 >= index
{
var object = objectsSeparated[index]
index--
if object.hasSuffix("#")
{
//this object has '#' suffix -> skip it
continue
}
else
{
//found non '#' object
if object.hasSuffix("a")
{
LabelAQI = object.substringToIndex(object.endIndex.predecessor())
LabelExtraInfo = "Carbon Monoxide present"
}
else if object.hasSuffix("b")
{
LabelAQI = object.substringToIndex(object.endIndex.predecessor())
LabelExtraInfo = "Sulphur Dioxide present"
}
else
{
LabelAQI = object
LabelExtraInfo = "-"
}
break
}
}

Answer 3

There are a number of ways you could do this, here's one suggestion using NSScanner:
var LabelAQI = "-"
var LabelExtraInfo = "-"
var arrAQI = [NSString]()
let AQI1:NSString = "55a", AQI2 = "95a", AQI3 = "66", AQI4 = "25", AQI5 = "88b", AQI6 = "#"
arrAQI.extend([AQI1, AQI2, AQI3, AQI4, AQI5, AQI6])
for AQI in reverse(arrAQI) {
if AQI != "#" {
var str:NSString?
let numberSet = NSCharacterSet.decimalDigitCharacterSet()
let letterSet = NSCharacterSet(charactersInString: "ab")
let scanner = NSScanner(string: AQI as String)
if scanner.scanCharactersFromSet(numberSet, intoString:&str) {
LabelAQI = str as! String
if scanner.scanCharactersFromSet(letterSet, intoString: &str) {
switch str as! String {
case "a":
LabelExtraInfo = "Carbon Monoxide present"
case "b":
LabelExtraInfo = "Sulphur Dioxide present"
default:
break
}
}
break
}
}
}
LabelAQI // "88"
LabelExtraInfo // "Sulphur Dioxide present"
I first of all reverse the array and loop through it for the last non-hash symbol entry. I then scan the string first for a number and second for the letters a/b using NSCharacterSet searches. Once found we can break out of the loop and all is done.
Note: this line of code arrAQI[0...5] = [AQI1, AQI2, AQI3, AQI4, AQI5, AQI6] didn't work, so I replaced it with arrAQI.extend([AQI1, AQI2, AQI3, AQI4, AQI5, AQI6]).

Answer 4

Using regular expressions, you can easily get the numbers out of a string. You probably want to do something along these lines:
let regex = NSRegularExpression(pattern: "[\a-z\s]", options: nil, error: nil)
let aq1value = regex?.stringByReplacingMatchesInString(arrAQI[0], options: nil, range: NSMakeRange(0, count(arrAQI[0])), withTemplate: "")
This will take your string and remove all alphabetic characters, leaving you with only numbers. You can then cast the string as an integer as you wish!