How to make a string exercise into int in kotlin?

Question

Everything I tried gives an Error, it can handle the numbers but not the "+",what I want is the result to be "10". I'm attempting to make a calculator in android studio... This is what I tried:

var sum = "5+5" val num = sum.toInt() exercise.text = "$num" //the text displayed on the screen

I mean technically you're hoping there's already a calculator built into the toInt call, which there isn't! If you want to parse a string as a set of numbers and operations, that's kinda complicated, but doable if you keep it simple (you'd be better finding a tutorial on it though if you're a beginner). If you want to actually make a typical calculator app, you'd be storing button presses instead, not holding the whole thing as a string and trying to work out what it means at the end — cactustictacs
– cactustictacs, Commented Aug 9, 2021 at 16:51
cactustictacs, you're right! thank you for fixing my thinking process! I might try that, but I'm attempting to start coding with no tutorials because I often get into a tutorial loop or "hell" if you will and can't find my own way of doing things. — omoor
– omoor, Commented Aug 9, 2021 at 19:26
Mathematical expressions would need to be parsed. If you want to learn how to write a parser, they're probably a very good place to start, as the language is fairly simple and well-understood! Otherwise, you'd need to find a library you can use. — gidds
– gidds, Commented Aug 9, 2021 at 20:22

Anshul · Accepted Answer · 2021-08-09 16:52:46Z

1

Use eval function insted of toInt() , just a little research See

*Above is only available for Kotlin/Js.

answered Aug 9, 2021 at 16:52

Anshul

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3 Comments

omoor Over a year ago

Thank you, I'll be sure to try that!

gidds Over a year ago

Note the big scary warning in the linked docs about how that “is an enormous security risk”. A hand-written or library parser would be far safer — and probably a lot faster too.

Anshul Over a year ago

but op wants to build a calculator!

omoor · Accepted Answer · 2021-08-11 12:08:06Z

I followed a tutorial and I can now answer my own question: You can use implementation 'net.objecthunter:exp4j:0.4.8' in your build gradle, then use .append(string) to add a string into your text and then use

val expression = ExpressionBuilder(Text.text.toString()).build() val result = expression.evaluate() val longResult = result.toLong() if(result == longResult.toDouble()) Text.text = "=$longResult" else Text.text = "=$result"

to evaluate and paste the result in the text you have

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