matchsort

Replicate a sort operation using the returned permutation indices

Description

Replicates a previous sort operation using the permutation matrix from that previous sort. Useful for sorting multiple matrices in parallel. If no dimension argument is supplied, matchsort will operate on the first non-singleton dimension just like sort does. Second output is the permutation matrix transformed into the corresponding matrix of linear indices.

Usage: y=matchsort(x,i)
       y=matchsort(x,i,dim)
       [y,li]=matchsort(x,i,dim)

Example:
     
Sort a and then sort the elements of b,c exactly the same way.
      b=1./a
      c=-a
      [d,i]=sort(a)
      e=matchsort(b,i)
      f=matchsort(c,i)
      isequal(d,1./e,-f)

• Acknowledgements: This submission has inspired the following:; SORTLIND (v2.1)
• MATLAB release: MATLAB 7.5 (R2007b)
• Other requirements: Not tested on pre-7 Matlab.

[John D'Errico]

I'm confused. Exactly what does this function do that the trivial code below does not do?

[x,tags] = sort(x);
y = y(tags);

[Garrett Euler]

Sure its trivial if x is a vector...not as much if x is a n-d matrix sorted down an arbitrary dimension. Note that your example won't work for anything other than a vector. This function is useful for parallel sorting of matrices of 2+ dimensions.

A clearer example:
a=magic(10);
b=1./a
c=-a
[d,i]=sort(a,2)
e=matchsort(b,i,2)
f=matchsort(c,i,2)
isequal(d,1./e,-f)

[John D'Errico]

I''ll concede that as a valid use.