Keep or drop symbols
keep.RdKeep or drop symbols
Note
Function keep() needs an explicit return() to prevent it
from returning invisibly.
The functions are vectorised in the first argument but not the second.
The second argument—the symbols to keep or discard—is formally a
vector of nonnegative integers, but the functions coerce it to a free
object. The symbols kept or dropped are the union of the symbols in
the elements of the vector. Function discard() was formerly
known as drop() but this conflicted with base::drop().
These functions have nothing in common with APL's take() and
drop().
Examples
(x <- rfree(20,5,8))
#> [1] b^5.c.b.h^-5 f^-4.d^5.e^3.f^3 a^3.g.a^-3.g^-5.c^-1
#> [4] h^3.a^-2.c^-2.f^-2 c^-4.e.b^3.e^4.g^-5 b^-3.a^-2.d^-1.h^-4
#> [7] h^5.a^-2.h^-4 g^3.a^-1.g^-5.d^-5.e^-3 d.b^2.g^2.f.d^4
#> [10] b^4.f^-1.h^5.d^8 g^-3.a^3.c^-2.d^-3 d^4.e^-3.b.e^-3
#> [13] c^-2.h.a^-2.b^-3 b^-2.g^-1.f^4.b^5 b^4.g^3.c^-3.g^2.d^-4
#> [16] h^4.f^-5.h^-5.a^2.f^-2 d^-7.f^5.c^-4 d^-4.c^4.h^5.f^3
#> [19] b^-5.a^-3.f^-1.g^-3.d^2 f^-3.g^5.d^2.h^4
keep(x,abc(4)) # keep only symbols a,b,c,d
#> [1] b^5.c.b d^5 c^-1 a^-2.c^-2 c^-4.b^3
#> [6] b^-3.a^-2.d^-1 a^-2 a^-1.d^-5 d.b^2.d^4 b^4.d^8
#> [11] a^3.c^-2.d^-3 d^4.b c^-2.a^-2.b^-3 b^3 b^4.c^-3.d^-4
#> [16] a^2 d^-7.c^-4 d^-4.c^4 b^-5.a^-3.d^2 d^2
discard(x,as.free('cde')) # drop symbols c,d,e
#> [1] b^6.h^-5 f^-1 a^3.g.a^-3.g^-5
#> [4] h^3.a^-2.f^-2 b^3.g^-5 b^-3.a^-2.h^-4
#> [7] h^5.a^-2.h^-4 g^3.a^-1.g^-5 b^2.g^2.f
#> [10] b^4.f^-1.h^5 g^-3.a^3 b
#> [13] h.a^-2.b^-3 b^-2.g^-1.f^4.b^5 b^4.g^5
#> [16] h^4.f^-5.h^-5.a^2.f^-2 f^5 h^5.f^3
#> [19] b^-5.a^-3.f^-1.g^-3 f^-3.g^5.h^4
keep(x,alpha(3)) # keep only abc
#> [1] c 0 c^-1 c^-2 c^-4 0 0 0 0 0 c^-2 0 c^-2 0 c^-3
#> [16] 0 c^-4 c^4 0 0