Comparing elements for equality

-----Original Message-----
From: Carlos J. Gil Bellosta [mailto:cgb at datanalytics.com] 
Sent: Tuesday, January 13, 2009 2:55 PM
To: Doran, Harold
Cc: r-help at r-project.org
Subject: Re: [R] Comparing elements for equality

Hello,

You could build your output dataframe along the following lines:

foo <- function(x) length( unique(x) ) == 1

results <- data.frame(
	freq = tapply( dat$id,   dat$id, length ),
	var1 = tapply( dat$var1, dat$id, foo ),
	var2 = tapply( dat$var2, dat$id, foo )
)

Best regards,

Carlos J. Gil Bellosta
http://www.datanalytics.com


On Tue, 2009-01-13 at 14:17 -0500, Doran, Harold wrote:

Suppose I have a dataframe as follows:

dat <- data.frame(id = c(1,1,2,2,2), var1 = 

c(10,10,20,20,25), var2 = 

c('foo', 'foo', 'foo', 'foobar', 'foo'))

Now, if I were to subset by id, such as:

subset(dat, id==1)

  id var1 var2
1  1   10  foo
2  1   10  foo

I can see that the elements in var1 are exactly the same and the 
elements in var2 are exactly the same. However,

subset(dat, id==2)

  id var1   var2
3  2   20    foo
4  2   20 foobar
5  2   25    foo

Shows the elements are not the same for either variable in this 
instance. So, what I am looking to create is a data frame 

that would 

be like this

id	freq	var1	var2
1	2	TRUE	TRUE	
2	3	FALSE	FALSE

Where freq is the number of times the ID is repeated in the 

dataframe. 

A TRUE appears in the cell if all elements in the column 

are the same 

for the ID and FALSE otherwise. It is insignificant which values 
differ for my problem.

The way I am thinking about tackling this is to loop through the ID 
variable and compare the values in the various columns of 

the dataframe.

The problem I am encountering is that I don't think all.equal or 
identical are the right functions in this case.

So, say I was wanting to compare the elements of var1 for id ==1. I 
would have

x <- c(10,10)

Of course, the following works

all.equal(x[1], x[2])

[1] TRUE

As would a similar call to identical. However, what if I 

only have a 

vector of values (or if the column consists of names) that 

I want to 

assess for equality when I am trying to automate a process over 
thousands of cases? As in the example above, the vector may contain 
only two values or it may contain many more. The number of 

values in 

the vector differ by id.

Any thoughts?

Harold