clojure - function let variable not mutable - clojure

Question

Hello I am getting a runtime error message while trying to run my clojure program.
Caused by: java.lang.IllegalArgumentException: Cannot assign to non-mutable: x
Please let me know what I am not understanding about the mutable variable so I can solve this code issue. Thanks in advance.
(defn gcd [a b]
(if (zero? b)
a
(recur b (mod a b))
)
)
(defn euler_phi [n]
(let [x 0]
(loop [idx_i 1]
(when (= 1 (gcd idx_i n))
(set! x (inc x))
(print " * ")
)
(when (< idx_i n)
(recur (inc idx_i))
)
)
x
)
)

Answer 1

set! Doc
you cannot assign to function params
or local bindings. Only Java fields, Vars, Refs and Agents are mutable in
Clojure. See http://clojure.org/special_forms for more information.

Answer 2

If you want to write imperative-style code in Clojure, just use an atom any place you would use a Java variable:
(ns tst.demo.core
(:use demo.core ) )
(defn gcd [a b]
(if (zero? b)
a
(recur b (mod a b))))
(defn euler-phi [n]
(let [x (atom 0)]
(loop [idx 1]
(when (= 1 (gcd idx n))
(swap! x inc)
(print " * "))
(when (< idx n)
(recur (inc idx))))
#x))
(dotimes [n 30]
(print "n:" n " ")
(let [result (euler-phi n)]
(println " result=" result)))
With results:
n: 0 * result= 1
n: 1 * result= 1
n: 2 * result= 1
n: 3 * * result= 2
n: 4 * * result= 2
n: 5 * * * * result= 4
n: 6 * * result= 2
n: 7 * * * * * * result= 6
n: 8 * * * * result= 4
n: 9 * * * * * * result= 6
n: 10 * * * * result= 4
n: 11 * * * * * * * * * * result= 10
n: 12 * * * * result= 4
n: 13 * * * * * * * * * * * * result= 12
n: 14 * * * * * * result= 6
n: 15 * * * * * * * * result= 8
n: 16 * * * * * * * * result= 8
n: 17 * * * * * * * * * * * * * * * * result= 16
n: 18 * * * * * * result= 6
n: 19 * * * * * * * * * * * * * * * * * * result= 18
n: 20 * * * * * * * * result= 8
n: 21 * * * * * * * * * * * * result= 12
n: 22 * * * * * * * * * * result= 10
n: 23 * * * * * * * * * * * * * * * * * * * * * * result= 22
n: 24 * * * * * * * * result= 8
n: 25 * * * * * * * * * * * * * * * * * * * * result= 20
n: 26 * * * * * * * * * * * * result= 12
n: 27 * * * * * * * * * * * * * * * * * * result= 18
n: 28 * * * * * * * * * * * * result= 12
n: 29 * * * * * * * * * * * * * * * * * * * * * * * * * * * * result= 28
Here is a more functional version of your code:
(ns tst.demo.core
(:use demo.core tupelo.test )
(:require [tupelo.core :as t] ))
(t/refer-tupelo)
(defn gcd [a b]
(if (zero? b)
a
(recur b (mod a b))))
(defn euler-phi [n]
(apply +
(t/forv [idx (t/thru 1 n)]
(if (= 1 (gcd idx n))
1
0))))
(dotimes [n 30]
(println "n:" n " result=" (euler-phi n)))
with results:
n: 0 result= 0
n: 1 result= 1
n: 2 result= 1
n: 3 result= 2
n: 4 result= 2
n: 5 result= 4
n: 6 result= 2
n: 7 result= 6
n: 8 result= 4
n: 9 result= 6
n: 10 result= 4
n: 11 result= 10
n: 12 result= 4
n: 13 result= 12
n: 14 result= 6
n: 15 result= 8
n: 16 result= 8
n: 17 result= 16
n: 18 result= 6
n: 19 result= 18
n: 20 result= 8
n: 21 result= 12
n: 22 result= 10
n: 23 result= 22
n: 24 result= 8
n: 25 result= 20
n: 26 result= 12
n: 27 result= 18
n: 28 result= 12
n: 29 result= 28
Note that the result for n=0 is different..... I didn't try to debug it.