R help - Aug 2007 - How to parse a string into the symbol for a data frame object

I have several data frames, eg:
> df1 <- data.frame(x=seq(0,10), y=seq(10,20))
> df2 <- data.frame(a=seq(0,10), b=seq(10,20))
It is common to create loops in R like this:
> for(df in list(df1, df2)){ #etc. }
This works fine when you know the name of the objects to put into the
list.  I assume that the order of the objects in the list is respected
through the loop.  Inside the loop, the objects of the list are
'dereferenced' using 'df' but, to my knowledge, there is no way
to
tell whether 'df' is a current representation of 'df1' or
'df2'
without some additional book keeping.

In addition, I really want to use 'paste' within the loop to create a
new string value that will have the symbol name of a data frame to be
"dereferenced," e.g.:
> for(n in c(1, 2)){ dfString <- paste('df', n, sep="");
print(eval(dfString)) }
[1] "df1"
[1] "df2"

This is not what I want.  I have read through the documentation on
eval and similar commands like substitute and quote.  I program
regularly, but I do not understand these constructs in R.  I do not
understand the R framework for parsing and evaluation and I don't have
a lot of time right now to get lost in this detail.  I could really
use some help to get the string values in my loop to be parsed into
symbols that refer to the data frame objects df1 and df2.  How is this
done?

Best, Darren

One way to do it is to pass in the character name of the dataframe you
want to reference and then use 'get' to access the value: e.g.,

df1 <- data.frame(x=seq(0,10), y=seq(10,20))
df2 <- data.frame(a=seq(0,10), b=seq(10,20))
# use the character names for referencing
for (df in c('df1', 'df2')){
    # get the data to operate on (read-only)
    .val <- get(df)
    # now you can reference the object
    print(names(.val))
    # or construct new objects to store the value in
    # or you can use "assign' to store back in the original object
    assign(paste('temp.', df, sep=''), .val)
}


On 8/19/07, Darren Weber <darren.weber.lists at gmail.com>
wrote:
>  I have several data frames, eg:
>
> > df1 <- data.frame(x=seq(0,10), y=seq(10,20))
> > df2 <- data.frame(a=seq(0,10), b=seq(10,20))
>
> It is common to create loops in R like this:
>
> > for(df in list(df1, df2)){ #etc. }
>
> This works fine when you know the name of the objects to put into the
> list.  I assume that the order of the objects in the list is respected
> through the loop.  Inside the loop, the objects of the list are
> 'dereferenced' using 'df' but, to my knowledge, there is no
way to
> tell whether 'df' is a current representation of 'df1' or
'df2'
> without some additional book keeping.
>
> In addition, I really want to use 'paste' within the loop to create
a
> new string value that will have the symbol name of a data frame to be
> "dereferenced," e.g.:
>
> > for(n in c(1, 2)){ dfString <- paste('df', n,
sep=""); print(eval(dfString)) }
>
> [1] "df1"
> [1] "df2"
>
> This is not what I want.  I have read through the documentation on
> eval and similar commands like substitute and quote.  I program
> regularly, but I do not understand these constructs in R.  I do not
> understand the R framework for parsing and evaluation and I don't have
> a lot of time right now to get lost in this detail.  I could really
> use some help to get the string values in my loop to be parsed into
> symbols that refer to the data frame objects df1 and df2.  How is this
> done?
>
> Best, Darren
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?

You might want to store the data frames in a list to eliminate this
problem and make it more convenient to iterate over them:

L <- list(df1 = df1, df2 = df2)
rm(df1, df2)

# reduce each data frame to its first few rows
for(nm in names(L))   L[[nm]] <- head(L[[nm])

or if you don't need to modify them or know their names:

# print first few lines of each
for(df in L) print(head(df))


On 8/19/07, Darren Weber <darren.weber.lists at gmail.com>
wrote:
>  I have several data frames, eg:
>
> > df1 <- data.frame(x=seq(0,10), y=seq(10,20))
> > df2 <- data.frame(a=seq(0,10), b=seq(10,20))
>
> It is common to create loops in R like this:
>
> > for(df in list(df1, df2)){ #etc. }
>
> This works fine when you know the name of the objects to put into the
> list.  I assume that the order of the objects in the list is respected
> through the loop.  Inside the loop, the objects of the list are
> 'dereferenced' using 'df' but, to my knowledge, there is no
way to
> tell whether 'df' is a current representation of 'df1' or
'df2'
> without some additional book keeping.
>
> In addition, I really want to use 'paste' within the loop to create
a
> new string value that will have the symbol name of a data frame to be
> "dereferenced," e.g.:
>
> > for(n in c(1, 2)){ dfString <- paste('df', n,
sep=""); print(eval(dfString)) }
>
> [1] "df1"
> [1] "df2"
>
> This is not what I want.  I have read through the documentation on
> eval and similar commands like substitute and quote.  I program
> regularly, but I do not understand these constructs in R.  I do not
> understand the R framework for parsing and evaluation and I don't have
> a lot of time right now to get lost in this detail.  I could really
> use some help to get the string values in my loop to be parsed into
> symbols that refer to the data frame objects df1 and df2.  How is this
> done?
>
> Best, Darren
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>