Dereferencing an atom will give you the value of the atom at the time you
dereferenced it. So at the start of the doseq loop, you deref the atom and
get back an immutable vector of values. It's the same as writing:
(let [data @a-data]
(doseq [element data]
...))
You dereference first, and then you iterate over that fixed sequence. The
value of "a-data" might change, but the dereferenced "data" does not.
- James
On 5 April 2015 at 18:00, Shahrdad Shadab <shahrd...@gmail.com> wrote:
> Hi folks
>
> It may seem silly question but why when I doseq over a vector that is
> wrapped in an atom and change the vector using swap! while I am inside
> doseq, the doseq sets to the beginning of the vector intermittently:
>
>
> (def a-data (atom [15 9 8 1 4 11 7 12 13 14 5 3 16 2 10 6]))
>
> (defn switch-two-elements [the-vector] ... ) ;; swaping two elements in
> given vector
>
> (doseq [element @a-data]
> (println @a-data) (println element)
> (swap! a-data switch-two-elements))
>
> [15 9 8 1 4 11 7 12 13 14 5 3 16 2 10 6]
> 15
>
> [1 9 8 15 4 11 7 12 13 14 5 3 16 2 10 6]
> 9
>
> [1 4 8 15 9 11 7 12 13 14 5 3 16 2 10 6]
> 8
>
> [1 4 5 15 9 11 7 12 13 14 8 3 16 2 10 6]
> 1 ==> I expect this to be 15
>
> I suspect the reason would be the same as the one behind not changing a
> collection in java while iterating over it.
>
> I appreciate any insight on this.
>
> Thanks a lot
> Best regards
>
>
>
>
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