[R] iterators : checkFunc with ireadLines

[Laurent Rhelp]

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 following 
example found on internet to select only prime numbers :

|                                iprime <- ||iter||(1:100, checkFunc = 
||function||(n) ||isprime||(n))|

|(https://datawookie.netlify.app/blog/2013/11/iterators-in-r/) 
<https://datawookie.netlify.app/blog/2013/11/iterators-in-r/>|

but the checkFunc argument seems not to be available with the function 
ireadLines (package iterators). So, I did the code below to solve my 
problem but I am sure that I miss something to use iterators with files. 
Since I found nothing on the web about ireadLines and the checkFunc 
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 0.004997    
0.005997    0.006996    0.007996
# N023    -0.031323    -0.035026    -0.029759    -0.024886 -0.024464    
-0.026816    -0.03369    -0.041067    -0.038747
# N053    -0.014083    -0.004741    0.001443    -0.010152 -0.012996    
-0.005337    -0.008738    -0.015094    -0.012104
# N123    -0.019008    -0.013494    -0.01318    -0.029208 -0.032748    
-0.020243    -0.015089    -0.014439    -0.011681
# N163    -0.054023    -0.049345    -0.037158    -0.04112 -0.044612    
-0.036953    -0.036061    -0.044516    -0.046436
# N193    -0.022171    -0.022384    -0.022338    -0.023304 -0.022569    
-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 = FALSE){
   ## 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 = 
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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