Laurent... Bill is suggesting building your own indexed database... but this has been done before, so re-inventing the wheel seems inefficient and risky. It is actually impossible to create such a beast without reading the entire file into memory at least temporarily anyway, so you are better off looking at ways to process the entire file efficiently.
For example, you could load the data into a sqlite database in a couple of lines of code and use SQL directly or use the sqldf data frame interface, or use dplyr to query the database.
Or you could look at read_csv_chunked from readr package.
On May 18, 2020 11:37:46 AM PDT, William Michels via R-help <r-help at r-project.org> wrote:
Hi Laurent,
Thank you for explaining your size limitations. Below is an example
using the read.fwf() function to grab the first column of your input
file (in 2000 row chunks). This column is converted to an index, and
the index is used to create an iterator useful for skipping lines when
reading input with scan(). (You could try processing your large file
in successive 2000 line chunks, or whatever number of lines fits into
memory). Maybe not as elegant as the approach you were going for, but
read.fwf() should be pretty efficient:
sensors <- c("N053", "N163")
read.fwf("test2.txt", widths=c(4), as.is=TRUE, flush=TRUE, n=2000,
skip=0)
V1
1 Time
2 N023
3 N053
4 N123
5 N163
6 N193
first_col <- read.fwf("test2.txt", widths=c(4), as.is=TRUE,
flush=TRUE, n=2000, skip=0)
which(first_col$V1 %in% sensors)
index1 <- which(first_col$V1 %in% sensors)
iter_index1 <- iter(1:2000, checkFunc= function(n) {n %in% index1})
unlist(scan(file="test2.txt",
what=list("","","","","","","","","",""), flush=TRUE, multi.line=FALSE,
skip=nextElem(iter_index1)-1, nlines=1, quiet=TRUE))
[1] "N053" "-0.014083" "-0.004741" "0.001443" "-0.010152"
"-0.012996" "-0.005337" "-0.008738" "-0.015094" "-0.012104"
unlist(scan(file="test2.txt",
what=list("","","","","","","","","",""), flush=TRUE, multi.line=FALSE,
skip=nextElem(iter_index1)-1, nlines=1, quiet=TRUE))
[1] "N163" "-0.054023" "-0.049345" "-0.037158" "-0.04112"
"-0.044612" "-0.036953" "-0.036061" "-0.044516" "-0.046436"
(Note for this email and the previous one, I've deleted the first
"hash" character from each line of your test file for clarity).
HTH, Bill.
W. Michels, Ph.D.
On Mon, May 18, 2020 at 3:35 AM Laurent Rhelp <LaurentRHelp at free.fr>
wrote:
Dear William,
Thank you for your answer
My file is very large so I cannot read it in my memory (I cannot use
read.table). So I want to put in memory only the line I need to
With readLines, as I did, it works but I would like to use an
and a foreach loop to understand this way to do because I thought
it was a better solution to write a nice code.
Le 18/05/2020 ? 04:54, William Michels a ?crit :
Apologies, Laurent, for this two-part answer. I misunderstood your
post where you stated you wanted to "filter(ing) some
selected lines according to the line name... ." I thought that
you had a separate index (like a series of primes) that you wanted
use to only read-in selected line numbers from a file (test file
with numbers 1:1000 each on a separate line):
library(gmp)
library(iterators)
iprime <- iter(1:100, checkFunc = function(n) isprime(n))
scan(file="one_thou_lines.txt", skip=nextElem(iprime)-1, nlines=1)
scan(file="one_thou_lines.txt", skip=nextElem(iprime)-1, nlines=1)
scan(file="one_thou_lines.txt", skip=nextElem(iprime)-1, nlines=1)
scan(file="one_thou_lines.txt", skip=nextElem(iprime)-1, nlines=1)
Read 1 item
[1] 7
However, what it really seems that you want to do is read each line
a (possibly enormous) file, test each line "string-wise" to keep or
discard, and if you're keeping it, append the line to a list. I can
certainly see the advantage of this strategy for reading in very,
large files, but it's not clear to me how the "ireadLines" function
in the "iterators" package) will help you, since it doesn't seem to
generate anything but a sequential index.
Anyway, below is an absolutely standard read-in of your data using
read.table(). Hopefully some of the code I've posted has been
sensors <- c("N053", "N163")
read.table("test2.txt")
V1 V2 V3 V4 V5 V6 V7
V8 V9 V10
1 Time 0.000000 0.000999 0.001999 0.002998 0.003998 0.004997
0.005997 0.006996 0.007996
2 N023 -0.031323 -0.035026 -0.029759 -0.024886 -0.024464 -0.026816
-0.033690 -0.041067 -0.038747
3 N053 -0.014083 -0.004741 0.001443 -0.010152 -0.012996 -0.005337
-0.008738 -0.015094 -0.012104
4 N123 -0.019008 -0.013494 -0.013180 -0.029208 -0.032748 -0.020243
-0.015089 -0.014439 -0.011681
5 N163 -0.054023 -0.049345 -0.037158 -0.041120 -0.044612 -0.036953
-0.036061 -0.044516 -0.046436
6 N193 -0.022171 -0.022384 -0.022338 -0.023304 -0.022569 -0.021827
-0.021996 -0.021755 -0.021846
Laurent_data <- read.table("test2.txt")
Laurent_data[Laurent_data$V1 %in% sensors, ]
V1 V2 V3 V4 V5 V6 V7
V8 V9 V10
3 N053 -0.014083 -0.004741 0.001443 -0.010152 -0.012996 -0.005337
-0.008738 -0.015094 -0.012104
5 N163 -0.054023 -0.049345 -0.037158 -0.041120 -0.044612 -0.036953
-0.036061 -0.044516 -0.046436
Best, Bill.
W. Michels, Ph.D.
On Sun, May 17, 2020 at 5:43 PM Laurent Rhelp
<LaurentRHelp at free.fr> wrote:
Dear R-Help List,
I would like to use an iterator to read a file filtering some
selected lines according to the line name in order to use after a
foreach loop. I wanted to use the checkFunc argument as the
example found on internet to select only prime numbers :
| iprime <- ||iter||(1:100,
ireadLines (package iterators). So, I did the code below to solve
problem but I am sure that I miss something to use iterators with
Since I found nothing on the web about ireadLines and the
argument, could somebody help me to understand how we have to use
iterator (and foreach loop) on files keeping only selected lines ?
Thank you very much
Laurent
Presently here is my code:
## mock file to read: test.txt
##
# Time 0 0.000999 0.001999 0.002998 0.003998
0.005997 0.006996 0.007996
# N023 -0.031323 -0.035026 -0.029759 -0.024886
-0.026816 -0.03369 -0.041067 -0.038747
# N053 -0.014083 -0.004741 0.001443 -0.010152
-0.005337 -0.008738 -0.015094 -0.012104
# N123 -0.019008 -0.013494 -0.01318 -0.029208
-0.020243 -0.015089 -0.014439 -0.011681
# N163 -0.054023 -0.049345 -0.037158 -0.04112
-0.036953 -0.036061 -0.044516 -0.046436
# N193 -0.022171 -0.022384 -0.022338 -0.023304
-0.021827 -0.021996 -0.021755 -0.021846
# sensors to keep
sensors <- c("N053", "N163")
library(iterators)
library(rlist)
file_name <- "test.txt"
con_obj <- file( file_name , "r")
ifile <- ireadLines( con_obj , n = 1 )
## I do not do a loop for the example
res <- list()
r <- get_Lines_iter( ifile , sensors)
res <- list.append( res , r )
res
r <- get_Lines_iter( ifile , sensors)
res <- list.append( res , r )
res
r <- get_Lines_iter( ifile , sensors)
do.call("cbind",res)
## the function get_Lines_iter to select and process the line
get_Lines_iter <- function( iter , sensors, sep = '\t', quiet =
## read the next record in the iterator
r = try( nextElem(iter) )
while( TRUE ){
if( class(r) == "try-error") {
return( stop("The iterator is empty") )
} else {
## split the read line according to the separator
r_txt <- textConnection(r)
fields <- scan(file = r_txt, what = "character", sep = sep,
quiet)
## test if we have to keep the line
if( fields[1] %in% sensors){
## data processing for the selected line (for the example
transformation in dataframe)
n <- length(fields)
x <- data.frame( as.numeric(fields[2:n]) )
names(x) <- fields[1]
## We return the values
print(paste0("sensor ",fields[1]," ok"))
return( x )
}else{
print(paste0("Sensor ", fields[1] ," not selected"))
r = try(nextElem(iter) )}
}
}# end while loop
}
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