Re: question on compare functions for the vecsort command

[Loïc Grenié <loic.grenie@gmail.com>]

On Mon 12 May, 2025, at 05:50, American Citizen wrote:

Hello:

Suppose we have a small vector composed of 4 component vectors, say

V = [ [1,7,5,2], [3,7,1,4], [2,0,6,9], [8,0,5,1], etc...]

What is the compare function for vecsort(V, compare_function, flag) so
that I can

1. sort the 2nd column by ascending size, then sort the 3rd column by
descending size?

V = [ [1,7,5,2], [3,7,1,4], [2,0,6,9], [8,0,5,1] ];

vcmp(a,b)=if(a[2]!=b[2],a[2]-b[2],b[3]-a[3]);

vecsort(V,vcmp)

\\ [[2, 0, 6, 9], [8, 0, 5, 1], [1, 7, 5, 2], [3, 7, 1, 4]

    I interpreted "then" as "if the 2nd components are equal, then

  consider the 3rd"

    You can inline the function as well:

vecsort(V,(a,b)->if(a[2]!=b[2],a[2]-b[2],b[3]-a[3]))
\\ [[2, 0, 6, 9], [8, 0, 5, 1], [1, 7, 5, 2], [3, 7, 1, 4]]

 

2. add the 3rd column to the 4th column, if the 2nd column > 1st column?

     You want to change the elements of the vector? This is not a work

  for vecsort, but for apply

apply(a->if(a[2]>a[1],a[4]+=a[3]);a,V)

  (or vecsor(V,vcmp), or you can vecsort afterwards)

       Hope this helps,

             Loïc