Homework Problem 2, Due Wednesday September 9, 2009.
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## (a) 
> Polynes = read.table(file=
"http://www-users.math.umd.edu/~evs/s430.old/Data/nature02801-s2.dat",
               header=2, skip=2, nrow=80)[,c(1:5,7,9:11,15)]

> unlist(lapply(Polynes,class))
   Archip     Islnd  ForstRep  Deforstn      Area   Isol_25      Elev    LatitS 
 "factor"  "factor" "numeric" "numeric" "numeric" "numeric" "integer" "numeric" 
     Rain    Tephra 
"integer" "integer" 

## (b)
> Poly2 = is.na(Polynes[,3:10])
> dim(Poly2)
[1] 80  8
> apply(Poly2,2,sum)
ForstRep Deforstn     Area  Isol_25     Elev   LatitS     Rain   Tephra 
       3        1        0        0        0        0        1        0 

## (c)
> apply(Polynes[,3:10],2, function(col) round(c(mean=
           mean(col, na.rm=T), sd=sd(col, na.rm=T), 
           IQR=IQR(col,na.rm=T),
           rng=c(range(col,na.rm=T) %*% c(-1,1))),3) )
     ForstRep Deforstn       Area  Isol_25     Elev LatitS     Rain Tephra
mean    3.266    3.152   5914.239  188.963 1128.737  9.592 2165.329  1.837
sd      0.833    0.856  21525.631  501.988  961.030 15.956 1295.661  0.961
IQR     1.000    1.500   2104.075  130.000  950.000 12.158 1679.500  2.000
rng     4.000    3.000 150717.800 2998.000 4190.000 66.600 6600.000  2.000

## (d) 
> charIsl = levels(Polynes$Islnd)[as.numeric(Polynes$Islnd)]
      ### as suggested by a student in the class: 
###   as.character(Polynes$Islnd)    also works
  chars = nchar(charIsl)
  newnam = substr(charIsl,1,chars-1)
  lastchar = substr(charIsl,chars,chars)
  rowfrst = (newnam[1:79]==newnam[2:80]) & (lastchar[1:79]=="L") &
            (lastchar[2:80]=="W") 
> inds = sort(c((1:79)[rowfrst],1+(1:79)[rowfrst]))
 [1]  1  2  6  7 15 16 21 22 23 24 25 26 37 38 39 40 42 43 44 45 49 50 79 80
> length(inds)/2
[1] 12              #### 12 pairs

> Polynes$Rain[inds[seq(2,24,by=2)]]-Polynes$Rain[inds[seq(1,23,by=2)]]
 [1] 1920 2500 1082 3278  -41 1236 2500  900 6600  600  900  583