CODEWITHSUNDEEP
clojure
Nick
Nick

Reputation: 9061

How to make sure the input is a number?

I use {:pre [(number? input )]} function to test whether the input is a number; however, this result to an exception when clojure is not sure what type it is. Foe example:

(number? A)

Unable to resolve symbol…

What's the idiomatic way to make sure the input is a number?

Edit: After I read the docs and tried several times, plus the kindly answers in this post, I find that probably a better question is how to handle exception. The purpose is to prevent someone from input something like a (letter a without any punctuation).

Yes, the function (number? some-value) does work well with numbers, strings. But it can't deal with wrong input like this:

A input-without-quotation-marks

Upvotes: 0

Views: 702

Answers (2)

Diego Basch
Diego Basch

Reputation: 13079

Unable to resolve symbol is a compile-time exception. A symbol that's not defined cannot be tested by a function because the expression will not compile.

Do you really want to test against random symbols that don't resolve to anything or are you just experimenting in the repl? I suspect the latter, but if for some reason you wanted the former you'd need a macro.

Upvotes: 1

Chiron
Chiron

Reputation: 20245

Are you sure that it isn't working?

(defn numbers-only [n] {:pre [(number? n)]}
  (println "Yay!"))

(numbers-only "clojure")
AssertionError Assert failed: (number? n)

(numbers-only 27)
Yay!

Maybe you are mistyping the name of the param in the pre condition?

Upvotes: 1

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