I'm working the exercises in SICP using Clojure and learning Clojure as I
go. I've stumbled across a mystifying NullPointerException. I'm implementing
exercise 2.23, where you're supposed to implement a "for-each" method that
takes a function and a list of items. It applies the function to teach item.
Here's what I tried.
(defn for-each [f items]
(if (not (empty? items))
((f (first items)) (for-each f (rest items))))) ; line #3
(for-each (fn [x] (println (* x x))) (list 1 2 3 4 5))
Here's the output:
1
4
9
16
25
Exception in thread "main" java.lang.NullPointerException (ex2.23.clj:0)
at clojure.lang.Compiler.eval(Compiler.java:4620)
at clojure.lang.Compiler.load(Compiler.java:4934)
at clojure.lang.Compiler.loadFile(Compiler.java:4901)
at clojure.main$load_script__7261.invoke(main.clj:210)
at clojure.main$script_opt__7298.invoke(main.clj:262)
at clojure.main$main__7322.doInvoke(main.clj:337)
at clojure.lang.RestFn.invoke(RestFn.java:413)
at clojure.lang.Var.invoke(Var.java:359)
at clojure.lang.AFn.applyToHelper(AFn.java:173)
at clojure.lang.Var.applyTo(Var.java:476)
at clojure.main.main(main.java:37)
Caused by: java.lang.NullPointerException
at user$for_each__1.invoke(ex2.23.clj:3)
at user$for_each__1.invoke(ex2.23.clj:3)
at user$for_each__1.invoke(ex2.23.clj:3)
at user$for_each__1.invoke(ex2.23.clj:3)
at user$for_each__1.invoke(ex2.23.clj:3)
at user$eval__4.invoke(ex2.23.clj:5)
at clojure.lang.Compiler.eval(Compiler.java:4604)
... 10 more
I tried replacing the list in the last line with a vector. Same result.
Suggestions welcome!
dean
--
Dean Wampler
coauthor of "Programming Scala" (O'Reilly)
- http://programmingscala.com
twitter: @deanwampler, @chicagoscala
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http://www.linkedin.com/in/deanwampler
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