CODEWITHSUNDEEP
stringswift
jdlace
jdlace

Reputation: 341

How to compare upper and lowercase letters in a conditional in Swift

Apologies if this is a duplicate. I have a helper function called inputString() that takes user input and returns a String. I want to proceed based on whether an upper or lowercase character was entered. Here is my code: 

 print("What do you want to do today? Enter 'D' for Deposit or 'W' for Withdrawl.")

    operation = inputString()

    if  operation == "D" || operation == "d" {

        print("Enter the amount to deposit.")

My program quits after the first print function, but gives no compiler errors. I don't know what I'm doing wrong.

Upvotes: 1

Views: 354

Answers (2)

William Ku
William Ku

Reputation: 798

the cause of the error that you experienced is explained at Swift how to compare string which come from NSString. Essentially, we need to remove any whitespace or non-printing characters such as newline etc.

I also used .uppercaseString to simplify the comparison

the amended code is as follows:

func inputString() -> String {
    var keyboard = NSFileHandle.fileHandleWithStandardInput()
    var inputData = keyboard.availableData
    let str: String = (NSString(data: inputData, encoding: NSUTF8StringEncoding)?.stringByTrimmingCharactersInSet(
    NSCharacterSet.whitespaceAndNewlineCharacterSet()))!
    return str
}

print("What do you want to do today? Enter 'D' for Deposit or 'W' for Withdrawl.")

let operation = inputString()

if operation.uppercaseString == "D"  {
    print("Enter the amount to deposit.")
}

Upvotes: -1

nhgrif
nhgrif

Reputation: 62052

It's important to keep in mind that there is a whole slew of purely whitespace characters that show up in strings, and sometimes, those whitespace characters can lead to problems just like this.

So, whenever you are certain that two strings should be equal, it can be useful to print them with some sort of non-whitespace character on either end of them.

For example:

print("Your input was <\(operation)>")

That should print the user input with angle brackets on either side of the input.

And if you stick that line into your program, you'll see it prints something like this:

Your input was <D
>

So it turns out that your inputString() method is capturing the newline character (\n) that the user presses to submit their input. You should improve your inputString() method to go ahead and trim that newline character before returning its value.

I feel it's really important to mention here that your inputString method is really clunky and requires importing modules. But there's a way simpler pure Swift approach: readLine().

Swift's readLine() method does exactly what your inputString() method is supposed to be doing, and by default, it strips the newline character off the end for you (there's an optional parameter you can pass to prevent the method from stripping the newline).

My version of your code looks like this:

func fetchInput(prompt: String? = nil) -> String? {
    if let prompt = prompt {
        print(prompt, terminator: "")
    }
    return readLine()
}

if let input = fetchInput("Enter some input: ") {
    if input == "X" {
        print("it matches X")
    }
}

Upvotes: 2

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