Swift - Parse a String to extract and edit numbers in it

Question

Given a String taken from the internet, such as:

"A dusting of snow giving way to moderate rain (total 10mm) heaviest on Thu night. Freeze-thaw conditions (max 8°C on Fri morning, min -2°C on Wed night). Mainly strong winds."

Using Swift 3, I want to convert the temperatures to Fahrenheit. So I need to find any numbers that have °C after them (including negative numbers); convert them to Fahrenheit, and then replace the integer back into the string.

I was originally trying to use: components(separatedBy: String). I did get it to work with this method. Although I think there is probably a better way.

func convertStringToFahrenheit (_ message: String) -> String{
    var stringBuilder = String()
    let stringArray = message.components(separatedBy: "°C")
    for subString in stringArray {
        if subString != stringArray.last {
            if subString.contains("(max "){
                let subStringArray = subString.components(separatedBy: "(max ")
                stringBuilder.append(subStringArray[0])
                stringBuilder.append("(max ")
                if var tempInt = Int(subStringArray[1]){
                    tempInt = convertCelsiusToFahrenheit(tempInt)
                    stringBuilder.append(String(tempInt))
                    stringBuilder.append("°F")
                }
            }
            else if subString.contains(", min "){
                let subStringArray = subString.components(separatedBy: ", min ")
                stringBuilder.append(subStringArray[0])
                stringBuilder.append(", min ")
                if var tempInt = Int(subStringArray[1]){
                    tempInt = convertCelsiusToFahrenheit(tempInt)
                    stringBuilder.append(String(tempInt))
                    stringBuilder.append("°F")
                }
            }
        }
        else {
            stringBuilder.append(subString)
        }
    }
    return stringBuilder
}

BTW you should post your code. What you have tried and what it is not working. — Leo Dabus, Apr 19 '17 at 18:30
I am not using an API because this app is just a lesson for my students on grabbing content from a web page. We will be using API's in a future lesson. — Justin Lewis, Apr 19 '17 at 18:48
Tip: Get the range of "max " and the range of "°C" searching only the substring starting from the "max " range upperbound. The same approach searching the rest of the string for "min " — Leo Dabus, Apr 19 '17 at 18:52
You should try to solve it yourself and if it doesn't work edit your question and post what you have tried. — Leo Dabus, Apr 19 '17 at 18:54
I think substringWithRange is what I needed. Thank you. I will edit my post when done. — Justin Lewis, Apr 19 '17 at 18:59

vadian · Answer 1 · 2017-04-19T19:48:45.860

A job for regular expression.

The pattern "(-?\\d+)°C" searches for

an optional minus sign -?
followed by one or more digits \\d+
followed by °C

The group – within the parentheses – captures the degrees value.

var string = "A dusting of snow giving way to moderate rain (total 10mm) heaviest on Thu night. Freeze-thaw conditions (max 8°C on Fri morning, min -2°C on Wed night). Mainly strong winds."

let pattern = "(-?\\d+)°C"

do {
    let regex = try NSRegularExpression(pattern: pattern)
    let matches = regex.matches(in: string, range: NSRange(location: 0, length: string.utf16.count))
    for match in matches.reversed() { // reversed() is crucial to preserve the indexes.
        let celsius = (string as NSString).substring(with: match.rangeAt(1))
        let fahrenheit = Double(celsius)! * 1.8 + 32
        let range = match.range
        let start = string.index(string.startIndex, offsetBy: range.location)
        let end = string.index(start, offsetBy: range.length)
        string = string.replacingCharacters(in: start..<end, with: String(format: "%.0f°F", fahrenheit))
    }
} catch {
    print("Regex Error:", error)
}

print(string)

The most complicated part of the code is the conversion NSRange -> Range<String.Index>

Swift - Parse a String to extract and edit numbers in it

1 Answers1