How to convert a numeric string to number (decimal) and number to string

Question

How do I go about writing a function to convert a decimal number string to a decimal and a decimal number to a string?

Solution 1

This example converts a numeric string into a number.

(defn String->Number [str]
  (let [n (read-string str)]
       (if (number? n) n nil)))

sample:

user=> (String->Number "4.5")
4.5
user=> (str 4.5)
"4.5"
user=> (String->Number "abc")
nil

Solution 2

There is very convenient clojure functions to convert from anything to string and from something resembling a number to BigDecimal:

user=> (bigdec "1234")
1234M
user=> (str 1234M)
"1234"

I guess this is clojure canonical way.

Solution 3

Note that read-string gives you a floating-point number, not a decimal:

user=> (.getClass (read-string "1.01"))
java.lang.Double

The number you get back prints like what you want but it isn't, exactly.

user=> (new BigDecimal (read-string "1.01"))
1.0100000000000000088817841970012523233890533447265625M

You can use java.math.BigDecimal instead and avoid floating-point complications:

user=> (new BigDecimal "1.01")
1.01M
user=> (.toString (new BigDecimal "1.01"))
"1.01"

Solution 4

From your question, you seem to want a toggle function, that is one that can read in a number and convert to a string and also read in a string and return a number, if the string contains numeric digits, like 123.0 or "123.0".

Here is an example:

(defn cvt-str-num [val]
    (if (try 
            (number? val)
            (catch Exception e (str "Invalid number: " (.getMessage e))))
        (str val)
        (let [n-val (read-string val)]
            (if (number? n-val)
                n-val
                nil))))

I could not see a way around the let binding of n-val, because an interim place was needed to store read-string return value, so it could be tested as a number. If it is a number it is returned; else nil is returned.

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Comments

[BLUEPIXY]

@octopusgrabbus -Thanks for your help.

[Matt Luongo]

You probably shouldn't use this in real life. From the docs, it can execute arbitrary code clojuredocs.org/clojure.core/read-string which is a huge security vulnerability. I think bigdec is a better solution.