[R] Exceptional slowness with read.csv

[Dave Dixon]

Thanks, yeah, I think scan is more promising. I'll check it out.

On 4/8/24 11:49, Bert Gunter wrote:
> No idea, but have you tried using ?scan to read those next 5 rows? It 
> might give you a better idea of the pathologies that are causing 
> problems. For example, an unmatched quote might result in some huge 
> number of characters trying to be read into a single element of a 
> character variable. As your previous respondent said, resolving such 
> problems can be a challenge.
>
> Cheers,
> Bert
>
>
>
> On Mon, Apr 8, 2024 at 8:06 AM Dave Dixon <ddixon using swcp.com> wrote:
>
>     Greetings,
>
>     I have a csv file of 76 fields and about 4 million records. I know
>     that
>     some of the records have errors - unmatched quotes, specifically.
>     Reading the file with readLines and parsing the lines with
>     read.csv(text
>     = ...) is really slow. I know that the first 2459465 records are
>     good.
>     So I try this:
>
>      > startTime <- Sys.time()
>      > first_records <- read.csv(file_name, nrows = 2459465)
>      > endTime <- Sys.time()
>      > cat("elapsed time = ", endTime - startTime, "\n")
>
>     elapsed time =   24.12598
>
>      > startTime <- Sys.time()
>      > second_records <- read.csv(file_name, skip = 2459465, nrows = 5)
>      > endTime <- Sys.time()
>      > cat("elapsed time = ", endTime - startTime, "\n")
>
>     This appears to never finish. I have been waiting over 20 minutes.
>
>     So why would (skip = 2459465, nrows = 5) take orders of magnitude
>     longer
>     than (nrows = 2459465) ?
>
>     Thanks!
>
>     -dave
>
>     PS: readLines(n=2459470) takes 10.42731 seconds.
>
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