Name:
Directions: Work only on this sheet (on both sides, if
needed); do not turn in any supplementary sheets of pa-
per. There is actually plenty of room for your answers, as
long as you organize yourself BEFORE starting writing.
In order to get full credit, SHOW YOUR WORK.
1. (5) Fill in the blanks: The members of the as() family
are functions similar to
in C/C++.
2. (10) State three approaches from our course to speed
up R code. Be terse, using at most three or four words in
each of your three answers.
3. (15) In the discrete-event simulation code, suppose
we wish to form a matrix of all pending events whose
scheduled times fall into a given time period [a,b]. Fill in
the blank:
timeab <- function(a,b) return(_________________________________)
4. (20) The class v10 will consist of vectors whose ele-
ments are only 1s and 0s. Fill in the blanks:
# constructor; vector has n 1s and 0s, with 1s at the given indices
v10 <- function(indices,n) {
v <- rep(0,n)
v[indices] <- ______________________________
vo <- list(vec = v)
class(vo) <- "v10"
return(vo)
}
# returns the indices of the 1s in v, an object of class v10
getindices <- function(v) {
return(____________________________________)
}
# prints the given object, in terms of indices of the 1s
print.v10 <- function(v) {
print(___________________________________)
}
# returns a v10 object consisting of 1s everywhere va has a 1
# but vb doesn’t; the two input vecs assumed equal length
"%-%" <- function(va,vb) {
a <- getindices(va)
b <- getindices(vb)
i <- ___________________________________
return(__________________________________)
}
Here is an example of use:
> x <- v10(c(1,4),4)
> y <- v10(c(3,4),4)
> x
[1] 1 4
> x$vec
[1] 1 0 0 1
> getindices(x)
[1] 1 4
> z <- x %-% y
> z
[1] 1
> z$vec
[1] 1 0 0 0
5. (10) Consider the code at the top of p.5 of the
Mertz handout. Replace the assign() call by a statement
achieving the same result.
6. (10) Consider this interactive R session:
> u <- c(3,4,5,5,12,13)
> ____________(u,1)
[1] 3
> ____________(u,5)
[1] 12
> ____________(u,6)
[1] 13
Fill in the blanks, with the same answer in all three cases.
7. (10) Fill in the blanks in the folllowing R equivalent
of Python’s reduce() function:
reduce <- function(f,v) {
rslt <- _______________________
for (_____________________) _________________________
return(rslt)
}
8. (20) The following code searches a matrix for the first
row whose sum exceeds a given threshhold. It returns the
index of the row, or 0 if no such row exists. Fill in the
blanks.
# in matrix m, find index of first row having total >= rowtot; m
# is processed in blocks of blksz rows, sent one block at a time to
# cluster cls; return 0 if no such row exists; assumes blksz evenly
# divides nrow(m)
parfindfirst <- function(m,rowtot,cls,blksz) {
for (i in 1:(nrow(m)/blksz)) {
startrow <- 1 + (i-1) * blksz
endrow <- i * blksz
mblk <- m[startrow:endrow,,drop=F]
rslt <- parApply(_________________________________________)
rgt <- ________________________________________
if (any(rgt)) return(_______________________________________)
}
return(0)
}
Solutions:
1. generic, casts
2. vectorization; writing parts of the code in C; parallel
process
3.
sim$evnts[sim$evnts[,1] >= a & sim$evnts[,1] <= b]
4.
1
which(v$vec==1)
getindices(v)
setdiff(a,b)
v10(i,length(va))
5.
inf_vector <<- v
6.
"["
7.
v[1]
for (vi in v[-1]) rslt <- f(rslt,vi)
1
