How can I identify which factor group a value belongs to?

I'm using the cut function to split my data into groups using the max/min range. here is an example of the code that I am using:

# sample data frame - used to identify intial groups
testdf <- data.frame(a = c(1:100), b = rnorm(100))

# split into groups based on ranges 
k <- 20开发者_开发问答 # number of groups
# split into groups, keep code
testdf$groupCode <- cut(testdf$b, breaks = k, labels = FALSE)
# store factor information 
testdf$group <- cut(testdf$b, breaks = k)                     
head(testdf)

I want to use the factor groupings identified to split another data frame up, but I'm not sure how to use factors to deal with this. I think my code structure should be roughly as follows:

# this is the data I want to categorize based on previous groupings
datadf <- data.frame(a = c(1:100), b = rnorm(100))
datadf$groupCode <- function(x){return(groupCode)}

I see that the factor data is structure as follows, but I don't know how to use it properly:

testdf$group[0]
factor(0)
20 Levels: (-2.15,-1.91] (-1.91,-1.67] (-1.67,-1.44] (-1.44,-1.2]  ... (2.34,2.58]

Two functions that I have been experimenting with (but which do not work) are as follows:

# get group code 
nearestCode <- function( number, groups ){
  return( which( abs( groups-number )== min( abs(groups-number) ) ) )  
}
nearestCode(7, testdf$group[0])

And also experimenting with the which function.

which(7, testdf$group[0])

What is the best way of identifying groupings and applying them to another dataframe?

I would have used:

testdf$groupCode <- cut(testdf$b, breaks = 
                           quantile(testdf$b, seq(0,1, by=0.05), na.rm=TRUE))
grpbrks <- quantile(testdf$b, seq(0,1, by=0.05), na.rm=TRUE)

Then you can use:

 findInterval(newdat$newvar, grpbrks)   # to group new data

And you then won't need to screw around with recovering the breaks from the labels or the data.

Thinking about, I guess you could also use:

 cut(newdat$newvar, grpbrks)  # more isomorphic to original categorization I suppose

Screwing around with some regular expressions seems to be the only way of actually returning the value of an object resulting from cut.

The following code does the necessary screwing:

cut_breaks <- function(x){
  first <- as.numeric(gsub(".{1}(.+),.*", "\\1", levels(x))[1])
  other <- as.numeric(gsub(".+,(.*).{1}", "\\1", levels(x)))
  c(first, other)
}

set.seed(1)
x <- rnorm(100)

cut1 <- cut(x, breaks=20)
cut_breaks(cut1)
 [1] -2.2200 -1.9900 -1.7600 -1.5300 -1.2900 -1.0600 -0.8320 -0.6000 -0.3690
[10] -0.1380  0.0935  0.3250  0.5560  0.7870  1.0200  1.2500  1.4800  1.7100
[19]  1.9400  2.1700  2.4100

levels(cut1)
 [1] "(-2.22,-1.99]"   "(-1.99,-1.76]"   "(-1.76,-1.53]"   "(-1.53,-1.29]"  
 [5] "(-1.29,-1.06]"   "(-1.06,-0.832]"  "(-0.832,-0.6]"   "(-0.6,-0.369]"  
 [9] "(-0.369,-0.138]" "(-0.138,0.0935]" "(0.0935,0.325]"  "(0.325,0.556]"  
[13] "(0.556,0.787]"   "(0.787,1.02]"    "(1.02,1.25]"     "(1.25,1.48]"    
[17] "(1.48,1.71]"     "(1.71,1.94]"     "(1.94,2.17]"     "(2.17,2.41]"    

You can then pass these break values to cut using the breaks= parameter to make your second cut.