R help - Feb 2009 - understanding how R determines numbers and characters when creating a data frame

Hello R Users and Developers,

I have a basic question about how R works.  Over the past few years I have
struggled when I try to generate a new data frame that I believe should
contain numeric data in some columns and character data in others only to
find everything converted to character data. Is there a general method to
create data frames that contain the data in the desired format:  numbers as
numeric and character as a factor etc?  I often have this problem and in the
worst case I have to export the file and read it back it in.    I have
emulated a simple example of the problem.  It often happens while using
"for" loops.  Could someone explain how to avoid this problem by
properly
creating data frames in for loops that can contain both numeric and
character data.

 

********Question for example 1.

Why does the cbind command convert the numeric data to character data?  Why
can't the character data be converted to numeric data using the fix command?


### Example 1  #############

data(iris)

obsnum<-NULL

results<-NULL

for(s in unique(as.character(iris$Species))){

temp1<-iris[iris$Species==s,]

obsnum<-length(unique(temp1$Sepal.Length))  # a number

out1<-cbind(species=as.character(paste(s)),obsnum)  # number converted to
character

results<-rbind(out1,results)

}

results

#fix(results)  # cannot convert obsnum to numeric using fix

####################################

 

******Question for example 2

Why does adding the data.frame command allow the character data to be
converted to numeric data using fix command?

### Example 2  #############

data(iris)

obsnum<-NULL

results<-NULL

for(s in unique(as.character(iris$Species))){

temp1<-iris[iris$Species==s,]

obsnum<-length(unique(temp1$Sepal.Length))

out1<-data.frame(cbind(species=as.character(paste(s)),obsnum)) # number
converted to character

results<-rbind(out1,results)

}

results

#fix(results)  # can now convert obsnum to numeric using fix

 

######

 

 

Thank you,

Alan Smith


	[[alternative HTML version deleted]]

The culprit is the cbind function.  When given 2 vectors (not already something
else), cbind will create a matrix, not a data frame.  A matrix can only have 1
type, so the numbers get converted to character.  In your first example you
never do create a data frame, you just build a matrix (try str(results)) so fix
cannot change a single column to numeric in something that is a matrix.  In the
second example you do create a data frame so fix will allow changing of columns,
but the cbind inside the call to data.frame is still creating a matrix (and
converting numeric to character) before it is included in the data frame. 
Remove the cbind and just do:

out1 <- data.frame(species=as.character(paste(s)),obsnum=obsnum)

and then out1 will be a data frame without ever converting the number obsnum to
a character.

Hope this helps,

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Alan Smith
> Sent: Wednesday, February 18, 2009 2:01 PM
> To: r-help at r-project.org
> Subject: [R] understanding how R determines numbers and characters when
> creating a data frame
> 
> Hello R Users and Developers,
> 
> I have a basic question about how R works.  Over the past few years I
> have
> struggled when I try to generate a new data frame that I believe should
> contain numeric data in some columns and character data in others only
> to
> find everything converted to character data. Is there a general method
> to
> create data frames that contain the data in the desired format:
> numbers as
> numeric and character as a factor etc?  I often have this problem and
> in the
> worst case I have to export the file and read it back it in.    I have
> emulated a simple example of the problem.  It often happens while using
> "for" loops.  Could someone explain how to avoid this problem by
> properly
> creating data frames in for loops that can contain both numeric and
> character data.
> 
> 
> 
> ********Question for example 1.
> 
> Why does the cbind command convert the numeric data to character data?
> Why
> can't the character data be converted to numeric data using the fix
> command?
> 
> 
> ### Example 1  #############
> 
> data(iris)
> 
> obsnum<-NULL
> 
> results<-NULL
> 
> for(s in unique(as.character(iris$Species))){
> 
> temp1<-iris[iris$Species==s,]
> 
> obsnum<-length(unique(temp1$Sepal.Length))  # a number
> 
> out1<-cbind(species=as.character(paste(s)),obsnum)  # number converted
> to
> character
> 
> results<-rbind(out1,results)
> 
> }
> 
> results
> 
> #fix(results)  # cannot convert obsnum to numeric using fix
> 
> ####################################
> 
> 
> 
> ******Question for example 2
> 
> Why does adding the data.frame command allow the character data to be
> converted to numeric data using fix command?
> 
> ### Example 2  #############
> 
> data(iris)
> 
> obsnum<-NULL
> 
> results<-NULL
> 
> for(s in unique(as.character(iris$Species))){
> 
> temp1<-iris[iris$Species==s,]
> 
> obsnum<-length(unique(temp1$Sepal.Length))
> 
> out1<-data.frame(cbind(species=as.character(paste(s)),obsnum)) # number
> converted to character
> 
> results<-rbind(out1,results)
> 
> }
> 
> results
> 
> #fix(results)  # can now convert obsnum to numeric using fix
> 
> 
> 
> ######
> 
> 
> 
> 
> 
> Thank you,
> 
> Alan Smith
> 
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org 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.

Alan Smith wrote:
> Hello R Users and Developers,
>
> I have a basic question about how R works.  Over the past few years I have
> struggled when I try to generate a new data frame that I believe should
> contain numeric data in some columns and character data in others only to
> find everything converted to character data. Is there a general method to
> create data frames that contain the data in the desired format:  numbers as
> numeric and character as a factor etc?  I often have this problem and in
the
> worst case I have to export the file and read it back it in.    I have
> emulated a simple example of the problem.  It often happens while using
> "for" loops.  Could someone explain how to avoid this problem by
properly
> creating data frames in for loops that can contain both numeric and
> character data.
>
>  
>
> ********Question for example 1.
>
> Why does the cbind command convert the numeric data to character data?  Why
> can't the character data be converted to numeric data using the fix
command?
>   
See ?cbind for a detailed explanation.
Anyway, when cbind/rbind is used on vector / matrix it returns matrix. 
Matrix are necessarily composed of the same type of data (see 
Introduction to R): combining character and numeric data you are 
implicitly converting the "short" type (numeric) to the
"long" type
(character).
>
> ### Example 1  #############
>
> data(iris)
>
> obsnum<-NULL
>
> results<-NULL
>
> for(s in unique(as.character(iris$Species))){
>
> temp1<-iris[iris$Species==s,]
>
> obsnum<-length(unique(temp1$Sepal.Length))  # a number
>
>   
Instead of using cbind here:
> out1<-cbind(species=as.character(paste(s)),obsnum)  # number converted
to
> character
>   
using data.frame:
out1 <- data.frame(species=as.character(paste(s)),obsnum)

you are telling R to convert character in factor and to preserve the 
numeric:
c(class(results$species),mode(results$species))
c(class(results$obsnum),mode(results$obsnum))

You can keep the character using the stringsAsFactors argument of the 
data.frame() function:
out1 <- data.frame(species=as.character(paste(s)),obsnum, 
stringsAsFactors=FALSE)

And then:
class(results$species)

The message is: if you want to mix up different data type you need lists 
(and data.frame are a special type of list where each component has the 
same number of elements).

Ciao,
domenico
> results<-rbind(out1,results)
>
> }
>
> results
>
> #fix(results)  # cannot convert obsnum to numeric using fix
>
> ####################################
>
>  
>
> ******Question for example 2
>
> Why does adding the data.frame command allow the character data to be
> converted to numeric data using fix command?
>
> ### Example 2  #############
>
> data(iris)
>
> obsnum<-NULL
>
> results<-NULL
>
> for(s in unique(as.character(iris$Species))){
>
> temp1<-iris[iris$Species==s,]
>
> obsnum<-length(unique(temp1$Sepal.Length))
>
> out1<-data.frame(cbind(species=as.character(paste(s)),obsnum)) # number
> converted to character
>
> results<-rbind(out1,results)
>
> }
>
> results
>
> #fix(results)  # can now convert obsnum to numeric using fix
>
>  
>
> ######
>
>  
>
>  
>
> Thank you,
>
> Alan Smith
>
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org 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.
>
>