Select the name of a certain value in a table (e.g

问题:

I have a vector of numbers. Let's call it mydata:

str(mydata)
# num [1:236] 2 1 1 2 2 1 2 1 2 2 ...

I can then count each value using table:

table(mydata)
# mydata
#  1    2    9  10 
# 20  200   14   2

Now, I want to select the value with the highest count (in this case, "2").

I can find the highest count (e.g. 200 in this case) by using the max function: max(table(mydata)). But how to get the name associated with the max count in the table, i.e. "2"?

回答1:

A table is very much like a list or a data frame, in that it has values and names (attributes) that are accessible through vector subsetting.

> mydata <- c(rep(1, 20), rep(2, 200), rep(9, 14), rep(10, 2))
> tab <- table(mydata)
> tab
## mydata
##   1   2   9  10 
##  20 200  14   2
> names(tab)
## [1] "1"  "2"  "9"  "10" 
> c(val = names(tab)[tab == max(tab)], freq = max(tab))
##  val  freq 
##  "2" "200"

The following are equivalent

> tab[ names(tab)[tab == max(tab)] ]
##   2 
## 200
> tab["2"]
##   2
## 200

Other useful things to know about an object is described in its attributes

> attributes(tab)
$dim
[1] 4

$dimnames
$dimnames$mydata
[1] "1"  "2"  "9"  "10"

$class
[1] "table"

回答2:

I'd probably do this

tab<-table(mydata)
names(tab)[which.max(tab)]

That will return "2" as a string. You can do as.numeric() if you want to get it back to a number. This one-liner is a bit more ugly and probably less efficient, but hey, it's one line.

sapply(list(table(mydata)), function(x) names(x[which.max(x)]))

or maybe

with(as.data.frame(table(data)), data[which.max(Freq)])

which will actually return a factor with a value of "2". If you want to make that numeric, you need to do as.numeric(as.character(x)). I was just trying to find ways to avoid having a table variable lying around if i really didn't need it. I wish there were an easier way to convert a table to a named vector.