在Clojure中评估数学字符串

Question

I need to implement a function called eval-math-string in Clojure, which takes a math string as input and evaluates it: (eval-math-string "7+8/2") => 11

So I've managed to break apart an expression using re-seq, and now I want to evaluate it using Incanter. However, I have an expression like ("7" "+" "8" "/" "2"), but Incanter needs an expression like ($= 7 + 8 / 2), where $= is the incanter keyword. How can I feed the list of one-character strings into a list including $= so that it executes properly. If the arguments are strings, the function won't work, but I can't convert +, *, / etc. to numbers, so I'm a little stuck.

Does anyone know how I can do this, or if there is a better way to do this?

Answer 1

Incanter's $= macro just calls the infix-to-prefix function , so all you need to do is convert your list of strings to a list of symbols and numbers, then call infix-to-prefix .

I'm going to assume that the input is just a flat list of strings, each of which represents either an integer (eg "7" , "8" , "2" ) or a symbol (eg "+" , "/" ). If that assumption is correct, you could write a conversion function like this:

(defn convert [s]
  (try
    (Long/parseLong s)
    (catch NumberFormatException _
      (symbol s))))

Example:

(infix-to-prefix (map convert ["7" "+" "8" "/" "2"]))

Of course, if you're just writing a macro on top of $= , there's no need to call infix-to-prefix , as you would just be assembling a list with $= as the first item. I'm going to assume that you already have a function called math-split that can transform something like "7+8/2" into something like ["7" "+" "8" "/" "2"] , in which case you can do this:

(defmacro eval-math-string [s]
  `($= ~@(map convert (math-split s))))

Example:

(macroexpand-1 '(eval-math-string "7+8/2"))
;=> (incanter.core/$= 7 + 8 / 2)