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

This will work on 10px or px10

(defn parse-int [s]
   (Integer. (re-find  #"\d+" s )))

it will parse the first continuous digit only so

user=> (parse-int "10not123")
10
user=> (parse-int "abc10def11")
10

Answer 2

This isn't perfect, but here's something with filter, Character/isDigit and Integer/parseInt. It won't work for floating point numbers and it fails if there is no digit in the input, so you should probably clean it up. I hope there's a nicer way of doing this that doesn't involve so much Java.

user=> (defn strToInt [x] (Integer/parseInt (apply str (filter #(Character/isDigit %) x))))
#'user/strToInt
user=> (strToInt "45px")
45
user=> (strToInt "45")
45
user=> (strToInt "a")
java.lang.NumberFormatException: For input string: "" (NO_SOURCE_FILE:0)

Answer 3

The question asks about parsing a string into a number.

(number? 0.5)
;;=> true

So from the above decimals ought to be parsed as well.

Perhaps not exactly answering the question now, but for general use I think you would want to be strict about whether it is a number or not (so "px" not allowed) and let the caller handle non-numbers by returning nil:

(defn str->number [x]
  (when-let [num (re-matches #"-?\d+\.?\d*" x)]
    (try
      (Float/parseFloat num)
      (catch Exception _
        nil))))

And if Floats are problematic for your domain instead of Float/parseFloat put bigdec or something else.

Answer 4

How about this one to avoid an exception on certain strings ?

(defn string-to-number [in]
  (let [s (strip-whitespace in)      ;; trim
        f (re-find #"\d+" s)]        ;; search digit else nil
    (if f (Integer/parseInt f) 0)))  ;; if not-nil do cast

(string-to-number "-")
(string-to-number "10")
(string-to-number "px10")
(string-to-number "1200 xr")

Answer 5

(defn parse-int [s]
  (Integer. (re-find #"[0-9]*" s)))

user> (parse-int "10px")
10
user> (parse-int "10")
10

Answer 6

New answer

I like snrobot's answer better. Using the Java method is simpler and more robust than using read-string for this simple use case. I did make a couple of small changes. Since the author didn't rule out negative numbers, I adjusted it to allow negative numbers. I also made it so it requires the number to start at the beginning of the string.

(defn parse-int [s]
  (Integer/parseInt (re-find #"\A-?\d+" s)))

Additionally I found that Integer/parseInt parses as decimal when no radix is given, even if there are leading zeroes.

Old answer

First, to parse just an integer (since this is a hit on google and it's good background information):

You could use the reader:

(read-string "9") ; => 9

You could check that it's a number after it's read:

(defn str->int [str] (if (number? (read-string str))))

I'm not sure if user input can be trusted by the clojure reader so you could check before it's read as well:

(defn str->int [str] (if (re-matches (re-pattern "\\d+") str) (read-string str)))

I think I prefer the last solution.

And now, to your specific question. To parse something that starts with an integer, like 29px:

(read-string (second (re-matches (re-pattern "(\\d+).*") "29px"))) ; => 29