[R] using eval-parse-paste in a loop

[Tony Plate]

Using eval-parse for this looks like overkill.  You should just be able to 
do something straightforward like:

for (i in 1:120)
   assign(paste("book", i, sep=""), read.xls(paste("Book", i, ".xls", 
sep=""), sheet=1, from=4, colClasses="numeric"))

which would put your spreadsheets in variables book1, book2, etc.

Or, if you want to put everything in a list:

books <- lapply(1:120, function(i) read.xls(paste("Book", i, ".xls", 
sep=""), sheet=1, from=4, colClasses="numeric"))

BTW, the reason your attempts with parse() were failing is that default 
argument of parse() is a filename (do args(parse) to see this quickly). 
You wanted parse(text=...).

-- Tony Plate

Michael Anyadike-Danes wrote:
> R-helpers
> 
>  
> 
> I have 120 small Excel sheets to read and I am using
> library(xlsReadWrite): one example below.
> 
>  
> 
> I had hoped to read sheets by looping over a list of numbers in their
> name (eg Book1.xls, Book2.xls, etc).
> 
>  
> 
> I thought I had seen examples which used eval-parse-paste in this way. 
> 
>  
> 
> However, I have not been able to get it to work..
> 
>  
> 
> 1. is this a feasible approach?
> 
>  
> 
> 2. if not advice would be welcome.
> 
>  
> 
> 3. Equally, advice about a better approach would also be v. welcome.
> 
>  
> 
> I haven't included the data because my failed attempt is
> data-independent (and probably more basic).  
> 
>  
> 
>  
> 
> Michael Anyadike-Danes
> 
>  
> 
> ############# show that read.xls works
> 
>  
> 
>> test <- read.xls("Book1.xls",sheet=1,from=4,colClasses="numeric")
> 
>> str(test)
> 
> 'data.frame':   23 obs. of  13 variables:
> 
>  $ Off.Flows..Thousands.: num  117.19    NaN     NA   1.43   2.26 ...
> 
>  $ Off.Flows..Thousands.: num  8.42 NaN NA 0.08 0.11 0.01 0.11 1.59 0.16
> 0.04 ...
> 
>  $ Off.Flows..Thousands.: num  20 NaN NA 0.2 0.3 0.02 0.32 4.39 0.41
> 0.11 ...
> 
>  $ Off.Flows..Thousands.: num  12.36   NaN    NA  0.14  0.27 ...
> 
>  $ Off.Flows..Thousands.: num  7.59 NaN NA 0.11 0.18 0.01 0.14 1.46 0.23
> 0.06 ...
> 
>  $ Off.Flows..Thousands.: num  10.31   NaN    NA  0.12  0.23 ...
> 
>  $ Off.Flows..Thousands.: num  7.55 NaN NA 0.08 0.2 0.01 0.11 1.6 0.23
> 0.05 ...
> 
>  $ Off.Flows..Thousands.: num  10.57   NaN    NA  0.19  0.21 ...
> 
>  $ Off.Flows..Thousands.: num  9.36 NaN NA 0.13 0.26 0.02 0.13 2.12 0.27
> 0.07 ...
> 
>  $ Off.Flows..Thousands.: num  8.21 NaN NA 0.1 0.19 0.01 0.1 1.9 0.23
> 0.06 ...
> 
>  $ Off.Flows..Thousands.: num  9.04 NaN NA 0.13 0.11 0.01 0.17 1.54 0.17
> 0.05 ...
> 
>  $ Off.Flows..Thousands.: num  13.42   NaN    NA  0.14  0.19 ...
> 
>  $ Off.Flows..Thousands.: num  0.37 NaN NA NaN 0.01 NaN 0.01 0.05 0.02
> NaN ...
> 
>  
> 
> ############### simple minded attempt substituting eval-parse-paste
> 
>  
> 
>> nam <- 1
> 
>  
> 
>> test <-
> eval(parse(paste('read.xls("Book',nam,'.xls",sheet=1,from=4,colClasses="
> numeric")',sep='')))
> 
> Error in file(file, "r") : unable to open connection
> 
> In addition: Warning message:
> 
> In file(file, "r") :
> 
>   cannot open file
> 'read.xls("Book1.xls",sheet=1,from=4,colClasses="numeric")', reason
> 'Invalid argument'
> 
>  
> 
> ############### stripping off eval, looking for clues
> 
>  
> 
> parse(paste('read.xls("Book',nam,'.xls",sheet=1,from=4,colClasses="numer
> ic")',sep=''))
> 
> Error in file(file, "r") : unable to open connection
> 
> In addition: Warning message:
> 
> In file(file, "r") :
> 
>   cannot open file
> 'read.xls("Book1.xls",sheet=1,from=4,colClasses="numeric")', reason
> 'Invalid argument'
> 
>  
> 
> ############### stripping off parse, still looking for clues
> 
>  
> 
> paste('read.xls("Book',nam,'.xls",sheet=1,from=4,colClasses="numeric")',
> sep='')
> 
> [1] "read.xls(\"Book1.xls\",sheet=1,from=4,colClasses=\"numeric\")"
> 
>  
> 
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> 
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