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Thursday, March 2nd, 2006

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Why I like Fortran (reason # 324)
Consider the following simplified piece of a program that I was recently writing:
type vector
  real :: x, y, z
end type

type field
  type(vector) :: data(5,5,5)
end type

type(field) my_field

my_field%data(:,:,2)%z = 3
Most of that's just setup of user-defined variable types, and is nothing different than one would do in C++. It sets up a type called vector with real-number variables x, y, and z; sets up a field type containing a component data, which is a three-dimensional array of vectors (I've used fixed values for the dimensions to make things simple); and defines a variable my_field of this type. Also, Fortran has a slightly different notation for accessing components; one writes my_field%data rather than the my_field.data that most other languages use.

The part that's particularly nice about Fortran is the last line, which illustrates what I can do with this -- and, in particular, how array indexing works in combination with data structures of this sort. The index notation (:,:,2) represents the portion of the array where the first two indices can take on any value, but the third index is limited to 2 -- that is, a two-dimensional slice through the overall array. Thus, my_field%data(:,:,2)%z is exactly what it looks like -- a reference to all of the z components of the vectors that make up that slice. Fortran treats this as equivalent to any other two-dimensional array; in this case, I've assigned it to a constant value of 3, which simply sets the z components of all the elements in the slice to 3. That's a rather trivial example; there are far more interesting ones, like passing it as an argument to a function that expects a two-dimensional array of real numbers.

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