Swift iOS programing - How to handle error (i.e. prevent the app from crashing/exiting)

Question

I am building a Calculator app which uses a parser that is called DDMathParser. It can evaluate a string such as "2+3-1" and outputs 4 as the answer. However, if I put something like "3+++3" or "log5" instead of log(5), the app crashes which is expected (the error message is like "invalid format: blah blah blah... fatal error: unexpectedly found nil while unwrapping an Optional value." I am wondering if there is a way to handle these kind of error to prevent the app from crashing? I mean if the user input "3++++3", the app will output an error message on a textview box or nothing, as long as it doesn't crash the app... Thanks in advance!

Edit: The code is "str.numberByEvaluatingString().doubleValue" where str is a string like "3+(2-4)" or "ln(3)" etc... and numberByEvaluatingString() is a function from DDMathParser. If I input the correct format (i.e. 3+2, ln(5)), it will give me the correct result, however, if I input some string like 3+++3, or ln5 without the bracket, it will crash...

Please post some code, e.g. how you input the `3+++3` expression, and at which point it gives you the error. — Khanh Nguyen, Sep 06 '15 at 05:49
See [the accepted answer here](http://stackoverflow.com/questions/30720497/swift-do-try-catch-syntax) for explanations and examples of `do catch` examples. — Eric Aya, Sep 06 '15 at 14:46

score 0 · Answer 1 · answered Sep 07 '15 at 08:44

0

Try

if let num = str.numberByEvaluatingString() {
    let value = num.doubleValue
} else {
    // Handle error here
}

answered Sep 07 '15 at 08:44

Khanh Nguyen

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Swift iOS programing - How to handle error (i.e. prevent the app from crashing/exiting)

1 Answers1