In Clojure how can I convert a String to a number?

Question

I have various strings, some like \"45\", some like \"45px\". How how I convert both of these to the number 45?

Answer 1

For simple cases you can just use a regex to pull out the first string of digits as mentioned above.

If you have a more complicated situation you may wish to use the InstaParse library:

(ns tst.parse.demo
  (:use tupelo.test)
  (:require
    [clojure.string :as str]
    [instaparse.core :as insta]
    [tupelo.core :as t] ))
(t/refer-tupelo)

(dotest
  (let [abnf-src            "
size-val      = int / int-px
int           = digits          ; ex '123'
int-px        = digits <'px'>   ; ex '123px'
      = 1*digit         ; 1 or more digits
       = %x30-39         ; 0-9
"
    tx-map        {:int      (fn fn-int [& args]
                               [:int (Integer/parseInt (str/join args))])
                   :int-px   (fn fn-int-px [& args]
                               [:int-px (Integer/parseInt (str/join args))])
                   :size-val identity
                  }

    parser              (insta/parser abnf-src :input-format :abnf)
    instaparse-failure? (fn [arg] (= (class arg) instaparse.gll.Failure))
    parse-and-transform (fn [text]
                          (let [result (insta/transform tx-map
                                         (parser text))]
                            (if (instaparse-failure? result)
                              (throw (IllegalArgumentException. (str result)))
                              result)))  ]
  (is= [:int 123]     (parse-and-transform "123"))
  (is= [:int-px 123]  (parse-and-transform "123px"))
  (throws?            (parse-and-transform "123xyz"))))