在Clojure中循环

Question

    n = (count somevector)        
   (foo [w] ( for [i (range n) 
                   j (range w)
                   :let [n (* i j)] 
                   :while ( < i j)
                  ] 
              (println n)))

When i run it.

=> (foo 10) ClassCastException clojure . lang . LazySeq cannot be cast to clojure . lang . IFn HelloWorld.core/foo

What I am really looking to do is have a couple of nested for loops and have some ifs and else inside it do some checking.

Edit ::

As it turns out the following also causes error

(defn foo [arg]
 (      
      for [ i (range (count vector)) ]
      (
        for [j arg ]
        [i j]
        ) 

     ) ) 

IllegalArgumentException Don't know how to create ISeq from: java.lang.Long clojure.lang.RT.seqFrom

Edit::

the following works

(defn foo [arg]
  (def w arg) 
(for [ i (range (count vector)) ]
      (
        for [j (range v)  ]
        [i j]
        )
  )   )

Now can someone help me understand why would it not work in the first place ?

Answer 1

if you use :while , for-loop will return null, because first value is false, :when is better.

user=> (for [i (range 3) j (range 5)] [i j])
([0 0] [0 1] [0 2] [0 3] [0 4] [1 0] [1 1] [1 2] [1 3] [1 4] [2 0] [2 1] [2 2] [2 3] [2 4])
user=> (for [i (range 3) j (range 5) :while (< i j)] [i j])
()
user=> (for [i (range 3) j (range 5) :when (< i j)] [i j])
([0 1] [0 2] [0 3] [0 4] [1 2] [1 3] [1 4] [2 3] [2 4])

Answer 2

Don't know how to create ISeq from: java.lang.Long clojure.lang.RT.seqFrom

This error message states the cause of exception accurately. I guess you passed a long value where a seq is expected. I could reproduce it:

user> (def v [1 2 3])
#'user/v
user> (defn foo [arg]
        (for [ i (range (count v)) ]
             (for [j arg ]
                  [i j]))) 
#'user/foo
user> (foo (range 3))
(([0 0] [0 1] [0 2]) ([1 0] [1 1] [1 2]) ([2 0] [2 1] [2 2]))
user> (foo 3)
; Evaluation aborted.
; Don't know how to create ISeq from: java.lang.Long
;  [Thrown class java.lang.IllegalArgumentException]

Your foo function works. However, the arg argument must be a seq , because arg is binded in the nested for to j

Answer 3

The first thing you should know is, that clojure's for is not a for loop, it's a list comprehension .

This means, that it is used to build lists (lazy sequence, actually) with the specified properties.

Your original for looks like this:

(for [i (range n) 
      j (range w)
      :let [n (* i j)] 
      :while ( < i j)] 
  (println n))

(I don't get a class cast exception with your original example).

What it says is: take pairs of i and j while i < j do something. The first pair of i and j is i = 0 and j = 0 . Is 0 < 0? No. Stop. That is, you're constructing an empty sequence.

Now, if we change the :while with :when , it'll change the meaning to: for each pair of i and j , for which i < j , and do something with them. That is, you won't stop constructing the sequence when you encounter an i >= j, you'll just skip this pair.

The next thing to note is the println . This function prints its arguments, but always returns nil . This return value is what you'll get in the result sequence and the printing is called a side effect. Generally, you don't want side effect with lazy sequences, because they (the side effects) will happen when needed. That is, if you assign the returned sequence to some variable, none of the println s will be called. They'll start popping up, when the sequence gets realized .

The bottom line is, don't think of for as a loop, think of it as a sequence constructor.