[R] What am I doing wrong with sapply ?

[Joshua Wiley]

Hi Eric,

Hopefully the following addresses your question:

library(XML)
## please do not include rm(list=ls()) in code other's might copy and paste
## it could remove things precious to them
url <-
"http://webapp.montcopa.org/sherreal/salelist.asp?saledate=05/25/2011"
tbl <-data.frame(readHTMLTable(url))[2:404, c(3,5,6,8,9)]
## the ";" you had was unnecessary since the code is on one line
names(tbl) <- c("Address", "Township", "Parcel", "SaleDate", "Costs")
## a better way to reset row names
rownames(tbl) <- NULL
## not sure why you made x, I did not
v <- gregexpr("( aka )|( AKA )", tbl$Address)
## having simplified how v is referenced
## it is easy to see that you are doing something fundamentally different
## with sapply(), you are operating on unlist()ed v
## with the for loop, you are unlisting each element of v
## Note the slight differences between s, s2, v1, & v2
s <- sapply(unlist(v), max)
s2 <- sapply(v, function(x) max(unlist(x)))
v1 <- numeric(length(v))
for(i in 1 :length(v)) v1[i] <- max(unlist(v[i]))
v2 <- numeric(length(unlist(v)))
for(i in 1:length(unlist(v))) v2[i] <- max(unlist(v)[i])

all.equal(s, v1)
all.equal(s, v2)
all.equal(s2, v1)

## For your edification note the following
all.equal(c(1:length(tbl[,1])), 1:length(tbl[,1]))
all.equal(v[c(1:length(v))], v)
## given that both are the same, it is generally best
## to choose the simpler of the two, so
## 1:length(tbl[, 1]) and v, NOT
## c(1:length(tbl[, 1])) and v[c(1:length(v))]

HTH,

Josh


On Wed, May 25, 2011 at 9:49 PM, eric <ericstrom at aol.com> wrote:
> Statement 9 using sapply does not seem to give the correct answer (or at
> least to me). Yet I do what I think is the same thing with statement 11 and
> I get the answer I'm looking for.
>
> 9 : s <-sapply(unlist(v[c(1:length(v))]), max)
> 11: for(i in 1 :length(v)) v1[i] <- max(unlist(v[i]))
>
> Shouldn't I get the same answer ?
>
>
> library(XML)
> rm(list=ls())
> url <-
> "http://webapp.montcopa.org/sherreal/salelist.asp?saledate=05/25/2011"
> tbl <-data.frame(readHTMLTable(url))[2:404, c(3,5,6,8,9)]
> names(tbl) <- c("Address", "Township", "Parcel", "SaleDate", "Costs");
> rownames(tbl) <- c(1:length(tbl[,1]))
> x <-tbl
> v <- gregexpr("( aka )|( AKA )",x$Address)
> s <-sapply(unlist(v[c(1:length(v))]), max)
> v1 <-numeric(length(v))
> for(i in 1 :length(v)) v1[i] <- max(unlist(v[i]))
>
> --
> View this message in context: http://r.789695.n4.nabble.com/What-am-I-doing-wrong-with-sapply-tp3551598p3551598.html
> Sent from the R help mailing list archive at Nabble.com.
>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/