Ahh. I see. Thanks.
With the sum function, and by extension any function that deals with
iterables, is it safe to assume they work without allocating storage for
the results of the iterable? Maybe a better question would be, are
functions like filter and sum behaving in a lazy fashion?
On Saturday, December 6, 2014 12:37:44 PM UTC-5, Stefan Karpinski wrote:
>
> The issue is that comprehensions pre-allocate their storage and therefore
> only work when the thing being iterated over has a length method that can
> tell it how big an array to allocate. You can use the collect function
> instead:
>
> julia> collect(@task fib(4000000))
> 34-element Array{Any,1}:
> 0
> 1
> 1
> 2
> 3
> 5
> 8
> 13
> 21
> 34
> 55
> 89
> 144
> 233
> 377
> 610
> 987
> 1597
> 2584
> 4181
> 6765
> 10946
> 17711
> 28657
> 46368
> 75025
> 121393
> 196418
> 317811
> 514229
> 832040
> 1346269
> 2178309
> 3524578
>
> julia> filter(iseven, ans)
> 12-element Array{Any,1}:
> 0
> 2
> 8
> 34
> 144
> 610
> 2584
> 10946
> 46368
> 196418
> 832040
> 3524578
>
> julia> sum(ans)
> 4613732
>
>
> But you can also just use the sum function, which will operate on anything
> iterable:
>
> julia> sum(filter(iseven, @task fib(4000000)))
> 4613732
>
>
> A while ago I added a bunch of fairly efficient solutions to Project Euler
> problems to the test suite, including #2, but don't click if you don't want
> spoilers:
>
> https://github.com/JuliaLang/julia/blob/7dd805feb0/test/euler.jl#L9-L20
>
>
>
> On Sat, Dec 6, 2014 at 12:04 PM, Bill Allen <[email protected] <javascript:>
> > wrote:
>
>> To learn julia, I'm working my way through Project Euler
>> <https://projecteuler.net/>. For question #2,
>> <https://projecteuler.net/problem=2> I ran into an issue that I haven't
>> found a solution to.
>>
>> Given a function that computes fibonacci numbers:
>>
>> function fib(n)
>> a,b = 0,1
>> while(a < n)
>> produce(a)
>> a,b=b,a+b
>> end
>> end
>>
>> I can write a loop that computes the value:
>>
>> function fibsum(n)
>> s = 0
>> for x in Task(() -> fib(n))
>> if x % 2 == 0
>> s = s + x
>> end
>> end
>> s
>> end
>>
>> fibsum(4000000)
>> 4613732
>>
>> Analogously, one can write:
>>
>> s = 0
>> for x in [1,2,3]
>> s = s + x
>> end
>> println(s)
>> 6
>>
>> or more succinctly,
>>
>> sum([x for x in [1,2,3]])
>>
>> But, when I try that with the fib Task:
>>
>> sum([x for x in Task(()->fib(10))])
>>
>> I get
>>
>> ERROR: `length` has no method matching length(::Task)
>> in anonymous at no file
>>
>> Which confuses me sense the task works nicely in an explicit loop.
>>
>> Regards,
>> Bill
>>
>>
>
