DESCRIPTION

Students:  Please make sure to log in to OMSI using the address of the
form ajones.bsmith@ucdavis.edu, concatenating the official UCD e-mail
addresses as shown.

QUESTION 

(30 pts) A difficulty discussed in class regarding interfacing R to
C/C++ involved "backwards" (or "flipped") matrices.  Explain.

QUESTION -ext .R -run 'Rscript omsi_answer2.R'

(35 pts) Here you will implement iterators in R.  The basic object will
be an R environment, consisting of functions nxt() (get next iterate)
and rst() (reset counter) and whatever data items are needed.  That
function will have access to a variable named 'self', pointing to the
object of which nxt() is a member, similar to C++/Java.  An external
function, nextIter(itrObj) will be called with the environment as
argument, and similarly resetIter(itrObj) will reset the object.  Note:
You will need to use global variables.  

nextIter <- function(itrObj) {


}

resetIter <- function(itrObj) {


}

# test example

# create the iterator object, just gets 'a' element by element
q <- new.env()
q$a <- c(5,12,13)
q$i <- 0
q$nxt <- function() {
   self$i <- self$i + 1
   self$a[self$i]
}
q$rst <- function() {
   self$i <- 0
}

# try it
print(nextIter(q))  # 5
print(nextIter(q))  # 12
print(nextIter(q))  # 13
resetIter(q)
print(nextIter(q))  # 5

QUESTION -ext .R -run 'Rscript omsi_answer3.R'

(35 pts) Here you will use the R walk() function presented in the
supplement.  The goal is to write a function findLtrFile() that 
finds SOME file (there could be more than 1) whose name begins with the
specified letter.  Once such a file is found, the directory and file
name are returned, avoiding unnecessary further computation to the
extent possible.


# run this ONCE
setup <- function() {
   source('http://heather.cs.ucdavis.edu/~matloff/145/SuppMaterials/Walk.R')
   download.file('http://heather.cs.ucdavis.edu/testdir.zip','timp')
   unzip('timp')
}

findLtrFile <- function(startDir,startLtr) {


}