R help - May 2011 - unwanted coercion using apply

I need to convert a dataframe to a record-structure, to be able to encode it
later in JSON. Suppose this is the data:

mydata <- data.frame(foo=1:3,
bar=c("M","M","F"));

I would like to convert this to a unnamed list (json array) of key-value
pairs. For example like this:

apply(data.frame(foo=1:3,
bar=c("M","M","F")),1,as.list)

However, when I do this, all the numeric values are converted to strings. I
don't understand why this is, because when I try to convert one record
simulataniously, this does not happen:

as.list(mydata[1,]);

I am not sure if this is indended behaviour or not, but is there an elegant
way to apply 'as.list' to all of the dataframe rows without coercing
everything to strings?


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On 2011-05-21 23:11, Jeroen Ooms wrote:
> I need to convert a dataframe to a record-structure, to be able to encode
it
> later in JSON. Suppose this is the data:
>
> mydata<- data.frame(foo=1:3,
bar=c("M","M","F"));
>
> I would like to convert this to a unnamed list (json array) of key-value
> pairs. For example like this:
>
> apply(data.frame(foo=1:3,
bar=c("M","M","F")),1,as.list)
>
> However, when I do this, all the numeric values are converted to strings. I
> don't understand why this is, because when I try to convert one record
> simulataniously, this does not happen:
But it does; see below;
>
> as.list(mydata[1,]);
This avoids apply. Try:

   apply(mydata[1,], 1, as.list)

and you'll get the conversion of numerics to strings. apply() operates
on arrays and a data frame will be coerced to a matrix (which requires
all elements to be of the same type). This is documented in ?apply.
>
> I am not sure if this is indended behaviour or not, but is there an elegant
> way to apply 'as.list' to all of the dataframe rows without
coercing
> everything to strings?

This may not be elegant, but why not just use a loop:

  L <- vector("list", 3)
  for( i in 1:3 ) L[[i]] <- as.list(mydata[i, ])

Peter Ehlers
>
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/unwanted-coercion-using-apply-tp3541637p3541637.html
> Sent from the R help mailing list archive at Nabble.com.
>
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> and provide commented, minimal, self-contained, reproducible code.

On May 22, 2011, at 2:11 AM, Jeroen Ooms wrote:
> I need to convert a dataframe to a record-structure, to be able to  
> encode it
> later in JSON. Suppose this is the data:
>
> mydata <- data.frame(foo=1:3,
bar=c("M","M","F"));
>
> I would like to convert this to a unnamed list (json array) of key- 
> value
> pairs. For example like this:
>
> apply(data.frame(foo=1:3,
bar=c("M","M","F")),1,as.list)
>
> However, when I do this, all the numeric values are converted to  
> strings. I
> don't understand why this is, because when I try to convert one record
> simulataniously, this does not happen:
>
> as.list(mydata[1,]);
>
> I am not sure if this is indended behaviour or not, but is there an  
> elegant
> way to apply 'as.list' to all of the dataframe rows without
coercing
> everything to strings?
?pairlist

pairlist( letters[1:3], 1:3)

pairlist(mydata$bar, ydata$foo)  # assuming the numbers were values  
for letter names.

-- 

David Winsemius, MD
West Hartford, CT