How to get substring from user input? - ios

Question

i wrote code to get character when user enter in text field and do math with them
this :
#IBOutlet weak internal var textMeli: UITextField!
var myChar = textMeli.text
var numb = [myChar[0]*3 , myChar[1]*7]
but one is wrong

Answer 1

textMeli.text is a String.
myChar is a String.
You can't access a Character from a String using bracket notation.
Take a look at the documentation for the String structure.
You'll see that you can access the string's characters through the characters property. This will return a collection of Characters. Initalize a new array with the collection and you can then use bracket notation.
let string = "Foo"
let character = Array(string.characters)[0]
character will be of type Character.
You'll then need to convert the Character to some sort of number type (Float, Int, Double, etc.) to use multiplication.
Type is important in programming. Make sure you are keeping track so you know what function and properties you can use.
Off the soap box. It looks like your trying to take a string and convert it into a number. I would skip the steps of using characters. Have two text fields, one to accept the first number (as a String) and the other to accept the second number (as a String). Use a number formatter to convert your string to a number. A number formatter will return you an NSNumber. Checking out the documentation and you'll see that you can "convert" the NSNumber to any number type you want. Then you can use multiplication.
Something like this:
let firstNumberTextField: UITextField!
let secondNumberTextField: UITextField!
let numberFormatter = NumberFormatter()
let firstNumber = numberFormatter.number(from: firstNumberTextField.text!)
let secondNumber = numberFormatter.number(from: secondNumberTextField.text!)
let firstInt = firstNumber.integerValue //or whatever type of number you need
let secondInt = secondNumber.integerValue
let product = firstInt * secondInt

Answer 2

Dealing with Swift strings is kind of tricky because of the way they deal with Unicode and "grapheme clusters". You can't index into String objects using array syntax like that.
Swift also doesn't treat characters as interchangeable with 8 bit ints like C does, so you can't do math on characters like you're trying to do. You have to take a String and cast it to an Int type.
You could create an extension to the String class that WOULD let you use integer subscripts of strings:
extension String {
subscript (index: Int) -> String {
let first = self.startIndex
let startIndex = self.index(first, offsetBy: index)
let nextIndex = self.index(first, offsetBy: index + 1)
return self[startIndex ..< nextIndex]
}
}
And then:
let inputString = textMeli.text
let firstVal = Int(inputString[0])
let secondVal = Int(inputString[2])
and
let result = firstVal * 3 + secondVal * 7
Note that the subscript extension above is inefficient and would be a bad way to do any sort of "heavy lifting" string parsing. Each use of square bracket indexing has as bad as O(n) performance, meaning that traversing an entire string would give nearly O(n^2) performance, which is very bad.
The code above also lacks range checking or error handling. It will crash if you pass it a subscript out of range.
Note that its very strange to take multiple characters as input, then do math on the individual characters as if they are separate values. This seems like really bad user interface.
Why don't you step back from the details and tell us what you are trying to do at a higher level?