# [R] Question on the idiom: start <- coef; start[fit\$pivot] <- coef

Barnet Wagman wagman at enteract.com
Fri Apr 30 09:36:59 CEST 1999

```I wonder if someone could  explain how the following R idiom works (it's
used in
glm.fit).

start <- coef
start[fit\$pivot] <- coef

coef is a vector of coefficients, set by  .Fortran("dqrls", ...).
fit\$pivot is a vector of integer indexes (indicating how dqrls permuted
the columns
of x).  If coef has n elements, fit\$pivot is a permutation of seq(1,5).

start[fit\$pivot] is simple enough, but the assignment performs a futher
permuation
that I don't understand.  For example

> pivot <- c(1,3,4,2,5)
> a <- c(100,200,300,400,500)
> b <- a
> b[pivot] <- a
> b
[1] 100 400 200 300 500

I guess there must be a replacement function for [], but I haven't
been able to dig up any documentation on it (and I'm not quite sure
where
to look in the source code).   I'd like to understand what R does when a
vector
is assigned to another vector in this way.

Thanks,
(and thanks for all the other help I've gotten from member of the list
lately)

-------------------
Barnet Wagman

wagman at enteract.com
773-645-8369

1361 N. Hoyne
Chicago, IL 60622
--------------------

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```