4 ms·

What are the highlights of Fortran's array manipulation capabilities?

myrryr 7y ago
Well, things like... you have 2, 2 dimensional arrays of the same size, A and B. You want to make a new array, where each cell in A is added to the same cell in B. So you are going to do a loop over 2 variables, right? in FORTRAN the code is... C = A + B in Julia the code is C = A .+ B
- the_af 7y ago
  Interesting. Does it result in a compile error if you try arrays of different sizes? (If it's just a run time error, I don't see how it's different to implementing this with operator overloading in any language that supports it).
  - palae 7y ago
    Sure, if you try with non conformable arrays, gfortran for example would tell you something like this: Different shape for array assignment at (1) on dimension 1 Note that a scalar is conformable to any array, so that A = 2 for example would work for any shape of A, and would fill A with 2's.
- nwallin 7y ago
  That's what I do in C++ with Eigen or boost::ublas or any other library. Operator overloading is not a feature limited to Fortran. Fortran gets its performance from prohibition of aliasing, which is something that C++ admits as per the standard. One could enable the same performance boosts with a non standard compiler flag, but this breaks a lot of codebases, including the Linux kernel AFAIK. Aliasing is a pox upon the world. There's a lot of work gone into stuff like UBSan for C++. I think aliasing should be added to stuff that it checks for. Dunno how much overhead that would be though.
  - sampo 7y ago
    Taking rows and columns is still nice in Eigen. For example taking the second column of a matrix is m.col(1) (Eigen, 0-based indexing) and m(:,2) (Fortran, 1-based indexing unless specified something else). But more generic array slicing with strides seems to get nasty in Eigen. For example if v2 = v(1:2:20) is Map<RowVectorXf,0,InnerStride<2> > v2(v.data(), v.size()/2); in Eigen, I don't want to read the docs for how to do the same for 2- or 3-dimensional arrays. https://eigen.tuxfamily.org/dox/group__TutorialReshapeSlicing.html https://eigen.tuxfamily.org/dox/group__TutorialReshapeSlicin...
- torrance 7y ago
  In Julia, it is C = A + B too. (You don't need the elementwise operator ('.') if the operation is a well defined expression in linear algebra.)

Well, things like... you have 2, 2 dimensional arrays of the same size, A and B. You want to make a new array, where each cell in A is added to the same cell in B. So you are going to do a loop over 2 variables, right? in FORTRAN the code is... C = A + B in Julia the code is C = A .+ B

the_af 7y ago
Interesting. Does it result in a compile error if you try arrays of different sizes? (If it's just a run time error, I don't see how it's different to implementing this with operator overloading in any language that supports it).
- palae 7y ago
  Sure, if you try with non conformable arrays, gfortran for example would tell you something like this: Different shape for array assignment at (1) on dimension 1 Note that a scalar is conformable to any array, so that A = 2 for example would work for any shape of A, and would fill A with 2's.
nwallin 7y ago
That's what I do in C++ with Eigen or boost::ublas or any other library. Operator overloading is not a feature limited to Fortran. Fortran gets its performance from prohibition of aliasing, which is something that C++ admits as per the standard. One could enable the same performance boosts with a non standard compiler flag, but this breaks a lot of codebases, including the Linux kernel AFAIK. Aliasing is a pox upon the world. There's a lot of work gone into stuff like UBSan for C++. I think aliasing should be added to stuff that it checks for. Dunno how much overhead that would be though.
- sampo 7y ago
  Taking rows and columns is still nice in Eigen. For example taking the second column of a matrix is m.col(1) (Eigen, 0-based indexing) and m(:,2) (Fortran, 1-based indexing unless specified something else). But more generic array slicing with strides seems to get nasty in Eigen. For example if v2 = v(1:2:20) is Map<RowVectorXf,0,InnerStride<2> > v2(v.data(), v.size()/2); in Eigen, I don't want to read the docs for how to do the same for 2- or 3-dimensional arrays. https://eigen.tuxfamily.org/dox/group__TutorialReshapeSlicing.html https://eigen.tuxfamily.org/dox/group__TutorialReshapeSlicin...
torrance 7y ago
In Julia, it is C = A + B too. (You don't need the elementwise operator ('.') if the operation is a well defined expression in linear algebra.)