Maybe point 3 on the noteworthy differences list could be expanded. If Stephan doesn't do so, I can try to give it a shot later
Op maandag 13 oktober 2014 19:53:58 UTC+2 schreef Ivar Nesje: > > This behaviour is already bullet 2 and 3 on the list you suggested. > Emphasizing that Matlab implicitly inserts a copy(x), when you mutate > arrays after some boundaries might be a good idea. I also find it somewhat > hard to grasp the consequences of the current wording, but I'm not the > target audience for that part of the documentation. > > As always when you want to change something in Julia, the prefered way is > to submit a pull request on Github. For documentation, you can even use the > github > web ui > <https://github.com/JuliaLang/julia/edit/master/doc/manual/noteworthy-differences.rst> > to > edit. If you open a pull request you should also link to this thread as > previous discussion. > > Regards Ivar > > kl. 16:35:52 UTC+2 mandag 13. oktober 2014 skrev Stephan Buchert følgende: >> >> Thanks for the explanations. >> >> To me this seems now reasonable, after realizing that in Julia not only >> reshape etc, but already the normal assignment b=a (for a an array) opens >> for the same kind of "surprises" as I had noticed them with reshape. Coming >> from Matlab this is a surprise, but for someone being familiar with certain >> other languages, I guess, it is not? >> >> julia> a=[1, 2, 3] >> 3-element Array{Int64,1}: >> 1 >> 2 >> 3 >> >> julia> b=a; >> >> julia> b[2]=0 >> 0 >> >> julia> a >> 3-element Array{Int64,1}: >> 1 >> 0 >> 3 >> >> But then "+-" is different again: >> >> julia> b+=1; >> >> julia> a >> 3-element Array{Int64,1}: >> 1 >> 0 >> 3 >> >> >> Perhaps one could add in >> http://julia.readthedocs.org/en/latest/manual/noteworthy-differences/ >> under Matlab near the top something like: >> >> * Arrays are mutable. Assignments like b=a and functions b=f(a) behave in >> Matlab always equivalent to b=copy(a) and b=f(copy(a)), but in Julia b >> becomes conceptually a pointer or different view of the contents of a. The >> values of a can be altered through b. >> >> PS: Yes, linear indices work directly also in Julia, I had overlooked this >> >> Am Montag, 13. Oktober 2014 10:33:26 UTC+2 schrieb Jutho: >>> >>> Some more comments / answers: >>> >>> I too was confused by the precise use of ! when I first started using >>> Julia in cases like reshape. However, it is very clear that reshape should >>> not have an exclamation mark. >>> >>> The use of ! in Julia functions is whenever the function modifies one of >>> its input arguments. This can be for several reasons: >>> >>> 1. The function performs some operation in-place: e.g. >>> scale!(A,lambda) multiplies all elements of the array A with the scalar >>> lambda in place. >>> 2. The operation stores the result of the computation in one >>> (typically the first) argument: e.g. copy!(B,A), permutedims!(B,A,perm), >>> scale!(B,A,lambda), Base.transpose!(B,A): do something with the data of >>> A >>> and store the result in the pre-allocated but not necessarily >>> initialised >>> array B >>> 3. The function overwrites the input arguments for use as temporary >>> workspace, but the actual result is just a normal return value as in >>> typical functions: e.g. the different [eig,svd,qr]fact!(A) methods >>> >>> The Base.LinAlg.BLAS methods do something in between 1. and 2.; they >>> store the result of the computation in a dedicated 'output' argument as in >>> 2., but they can also just add the result to that output argument, in which >>> case it needs to be correctly initialised. >>> >>> In summary, the exclamation mark means that if you need the original >>> values of all of the arguments after calling he function, you should better >>> make a copy, since calling the function allows it to destroy the original >>> values of its arguments (only possible with arrays or other mutable types). >>> Also note that most of these functions do not change the size of array >>> arguments, resize! is one of the notable few exceptions and only works with >>> one-dimensional arrays (vectors). >>> >>> reshape doesn't fit in any of these categories. B=reshape(A,newsize) >>> does not destroy the values of a. It is however true that the output B >>> shares data with A, just like if you do B=slice(A,...) or B=sub(A,...). >>> Note that none of these functions use an exclamation mark, nor is there any >>> other stylistic convention to indicate that this happens. That's just a new >>> aspect of the language that you have to take into account when coming from >>> e.g. Matlab, but that is very natural when coming from e.g. C. >>> >>> Finally, note that linear indexing indeed works on an array without >>> reshaping, but that if you want the 'vectorized' version a multidimensional >>> array, there is the build in function B=vec(A), which is indeed equivalent >>> to B=reshape(A,prod(size(A))) >>> >>> >>> >>> >>> Op zaterdag 11 oktober 2014 22:20:13 UTC+2 schreef Stephan Buchert: >>>> >>>> julia> a=[1 2; 3 4; 5 6] >>>> 3x2 Array{Int64,2}: >>>> 1 2 >>>> 3 4 >>>> 5 6 >>>> >>>> julia> b=reshape(a, prod(size(a))); >>>> >>>> julia> b[3]=0 >>>> 0 >>>> >>>> julia> a >>>> 3x2 Array{Int64,2}: >>>> 1 2 >>>> 3 4 >>>> 0 6 >>>> >>>>
