sort_rc, a Fortran77 code which sorts a list of any kind of objects, using reverse communication (RC).
The program works by repeatedly asking the user to compare two items on the list. The user is free to carry out the comparison in the calling program, in any desired way.
A typical usage, to sort an array of 100 items, might look something like this:
n = 100
indx = 0 -- Indicates the beginning of a sort
begin loop
sort_rc ( n, indx, i, j, isgn )
if ( indx < 0 ) (Compare items I and J)
if ( a(i) <= a(j) )
isgn = -1
else
isgn = +1
else if ( 0 < indx ) (Swap items I and J)
k = a(i)
a(i) = a(j)
a(j) = k
else
exit loop
end loop
The original version of sort_rc() requires the use of several variables declared internally to the function, whose values must be preserved between calls. The conventions for such computations vary from language to language; moreover, this kind of approach means that a single copy of the function cannot handle multiple requests for computation that might arise, especially in parallel computations. For that reason, a revised function, called sort_safe_rc(), is available, which does not rely on hidden internal variables and may safely be used to manage multiple simultaneous sorts.
The computer code and data files described and made available on this web page are distributed under the MIT license
sort_rc is available in a C version and a C++ version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.
backtrack_binary_rc, a Fortran77 library which carries out a backtrack search for a set of binary decisions, using reverse communication.
bisection_rc, a Fortran77 library which seeks a solution to the equation F(X)=0 using bisection within a user-supplied change of sign interval [A,B]. The procedure is written using reverse communication.
CG_RC, a Fortran77 library which implements the conjugate gradient (CG) method for solving a positive definite sparse linear system A*x=b, using reverse communication (RC).
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