A few comments:
Your two pieces of code aren't really equivalent:
a) The Python code is just calculating a fibonacci number in a brute force
iterative loop.
b) The Clojure code is creating a big (infinite, lazy) data structure of
all fibonacci numbers, lazily realising the structure in memory and running
along the list until it finds the one that you want. That is a lot more
"work"
The equivalent in Clojure to the python code would use loop / recur and
look something like:
(defn fib
([n] (fib 0 1 n))
([a b n]
(if (<= n 0) a
(recur b (+' a b) (dec n)))))
Note the +' (with the tick) for arbitrary precision addition.
But mostly, I think you aren't benchmarking lazy seq performance at all:
the dominant runtime cost of this code is almost certainly the BigInt
addition with large numbers, not the iteration / realisation of sequences.
On Thursday, 1 January 2015 04:03:03 UTC+8, Sakis K wrote:
>
> Hi,
>
> Clojure newbie here :) I'm reading "Programming Clojure" by Halloway and
> Bedra. In the book there is a lazy-seq example of the Fibonacci sequence:
>
> (defn lazy-seq-fibo
> ([]
> (concat [0 1] (lazy-seq-fibo 0N 1N)))
> ([a b]
> (let [n (+ a b)]
> (lazy-seq
> (cons n (lazy-seq-fibo b n))))))
>
>
> I like the flexibility of this implementation but I am a bit sceptical
> about its performance:
>
> user=> (time (rem (nth (lazy-seq-fibo) 1000000) 1000))
> "Elapsed time: 53552.014713 msecs"
> 875N
>
>
>
> Here's a Python implementation taken from
> http://en.literateprograms.org/Fibonacci_numbers_%28Python%29
>
> def fib(n):
> a, b = 0, 1
> for i in range(n):
> a, b = b, a + b
> return a
>
> if __name__ == '__main__':
> print(fib(1000000) % 1000)
>
>
> time python fib.py
> 875
>
> real 0m16.609s
> user 0m16.475s
> sys 0m0.115s
>
>
> 53 vs 17 seconds is a big gap. Is there a way to achieve better
> performance in Clojure? Maybe the fact that I executed the code in the REPL
> or the Java version that I'm using matters:
> java -version
> java version "1.7.0_65"
> OpenJDK Runtime Environment (IcedTea 2.5.3) (7u71-2.5.3-0ubuntu1)
> OpenJDK 64-Bit Server VM (build 24.65-b04, mixed mode)
>
>
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