On Jul 12, 2010, at 6:46 PM, David Winsemius wrote:
On Jul 12, 2010, at 6:03 PM, harsh yadav wrote:
Hi,
I have a function in R that compares two very large strings for
about 1
million records.
The strings are very large URLs like:-
http://query.nytimes.com/gst/sitesearch_selector.html?query=US+Visa+Laws&type=nyt&x=25&y=8
.
..
or of larger lengths.
The data-frame looks like:-
id url
1
http://query.nytimes.com/gst/sitesearch_selector.html?query=US+Visa+Laws&type=nyt&x=25&y=8
.
..
2 http://query.nytimes.com/search/sitesearch?query=US+Visa+Laws&srchst=cse
3
http://www.google.com/search?hl=en&q=us+student+visa+changes+9/11+washington+post&start=10&sa=N
.
..
4
http://www.google.com/search?hl=en&q=us+student+visa+changes+9/11+washington+post&start=10&sa=N
5
http://www.google.com/url?sa=U&start=11&q=http://app1.chinadaily.com.cn/star/2004/0610/fo4-1.html&ei=uUKwSe7XN9CCt
and so on for about 1 million records.
Here is the function that I am using to compare the two strings:-
stringCompare <- function(currentURL, currentId){
j <- currentId - 1
while(j>=1)
previousURL <-
previousURLLength <- nchar(previousURL)
#Compare smaller with bigger
if(nchar(currentURL) <= previousURLLength){
matchPhrase <- substr(previousURL,1,nchar(currentURL))
if(matchPhrase == currentURL){
return(TRUE)
}
}else{
matchPhrase <- substr(currentURL,1,previousURLLength)
if(matchPhrase == previousURL){
return(TRUE)
}
}
j <- j -1
}
return(FALSE)
}
Couldn't you just store the "url" vector after running through
nchar and then do the comparison in a vectorized manner?
test <- rd.txt('id url
1 "http://query.nytimes.com/gst/sitesearch_selector.html?query=US+Visa+Laws&type=nyt&x=25&y=8
"
2 "http://query.nytimes.com/search/sitesearch?query=US+Visa+Laws&srchst=cse
"
3 "http://www.google.com/search?hl=en&q=us+student+visa+changes+9/11+washington+post&start=10&sa=N
"
4 "http://www.google.com/search?hl=en&q=us+student+visa+changes+9/11+washington+post&start=10&sa=N
"
5 "http://www.google.com/url?sa=U&start=11&q=http://app1.chinadaily.com.cn/star/2004/0610/fo4-1.html&ei=uUKwSe7XN9CCt
"', stringsAsFactors=FALSE)
copyUrls <- test[,"url"]
sizeUrls <- nchar(copyUrls)
lengU <- length(sizeUrls)
sizidx <- pmax(sizeUrls[1:(lengU-1)], sizeUrls[2:(lengU)])
substr(copyUrls[2:lengU], 1, sizidx) == substr(copyUrls[1:
(lengU-1)], 1, sizidx)
#[1] FALSE FALSE TRUE FALSE
Let me hasten to admit that when I tried to fix what I thought was an
error in that program, I go the same result. It seemed as though I
should have been getting errors by choosing the maximum string
length. Changing the pmax to pmin did not alter the results ... to my
puzzlement ... until I further noticed that urls #3 and #4 were of the
same length. When I extend the lengths, then only the version using
pmin works properly.
--
David.
Here, I compare the URL at a given row with all the previous URLs
in the
data-frame. I compare the smaller of the two given URls with the
larger one
(upto the length of the smaller).
When I run the above function for about 1 million records, the
execution
becomes really slow, which otherwise is fast if I remove the
string comparison step.
Any ideas how it can be implemented in a fast and efficient way.
Thanks and Regards,
Harsh Yadav
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
