[R] how to bread while loop reading from connection with read.csv

[Collins, Stephen]

Hello,

I'm trying to read a file rows at a time, so as to not read the entire file into memory.  When reading the "connections" and "readLines" help, and "R help archive," it seems this should be possible with read.csv and a file connection, making use of the "nrows" argument, and checking where the "nrow()" of the new batch is zero rows. 

>From certain posts, it seemed that read.csv should return "character(0)" when the end of file is reached, and there are no more rows to read.  Instead, I get an error there are "no lines available for input."  Have I made a mistake with the file, or calling read.csv?  

What is the proper way to check the end-of-file condition with read.csv, such that I could break a while loop reading the data in?  

#example, make a test file
con <- file("test.csv","wt")
cat("a,b,c\n", "1,2,3\n", "4,5,6\n", "7,6,5\n", "4,3,2\n", "3,2,1\n",file=con)
unlink(con)

#show the file is valid
con <- file("test.csv","rt")
read.csv(con,header=T)
unlink(con)

#show that readLines ends with "character(0)", like expected
con <- file("test.csv","rt")
readLines(con,n=10)
readLines(con,n=10)
unlink(con)

#show that read.csv end with error
con <- file("test.csv","rt")
read.csv(con,header=T,nrows=10)
read.csv(con,header=F,nrows=10)
unlink(con)



Sincerely,

Stephen Collins
Predictive Modeler
Allstate Insurance Company


> sessionInfo()
R version 2.15.0 (2012-03-30)
Platform: i386-pc-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=English_United States.1252 
[2] LC_CTYPE=English_United States.1252   
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] tools_2.15.0






Stephen Collins
Predictive Modeler
Quantitative Research & Analytics

Allstate Insurance Company
2775 Sanders Road, Suite D2W
Northbrook, IL 60062
t: 1+ 847 402 1465
e: stephen.collins at allstate.com