VisibleList _ {...} -- get some entries of a list

VisibleList _ {...} -- get some entries of a list

Synopsis:

Usage: v = w_{i,j,...}

Operator: _

Input:

w, an instance of class VisibleList: a list

{i,j,...}, an instance of class List: the list of subscripts

Output:

an instance of class VisibleList: .

i1 : w = {a,b,c,d,e,f,g,h};

i2 : w_{1,3,4}



o2 = {b, d, e}



o2 : List

We can use this operation to compute composition of permutations represented as lists.

i3 : {4,2,3,1,0} _ {2,1,3,4,0}



o3 = {3, 2, 1, 0, 4}



o3 : List

Remark: any subscripts which are sequences will have their elements spliced into the rest of the list.

i4 : {a,b,c,d,e}_{2..4}



o4 = {c, d, e}



o4 : List

Code:

     -- ../../../Macaulay2/m2/lists.m2:17
     VisibleList _ List := VisibleList => (x,y) -> apply(splice y, i -> x#i)