Checking if a variable is a number in R

Question

I have following data frame:

> ddf = data.frame(name=c('a','b'), value=c(10,20))
> ddf
  name value
1    a    10
2    b    20

I try to get xx from ddf using following command:

> xx = ddf[ddf$name=='a','value']
> xx
[1] 10
> xx = ddf[ddf$name=='c','value']
> xx
numeric(0)

How can I test if xx is a valid number and not 'numeric(0)'. I tried following:

> is.numeric(xx)
[1] TRUE
> is.na(xx)
logical(0)
> is.null(xx)
[1] FALSE
> is.logical(xx)
[1] FALSE

I have to ask for xx = ddf[ddf$name=='a', 'value'] from different ddf data frames. Sometimes ddf does not contain 'a' and hence no value is returned. I want to detect this.

Answer 1

The easiest way to do this using base R is to check the length of xx.

if(length(xx)>0)
  {
  <do something>
  }

If you want to check that the variable is also numeric use is.numeric

if (length(xx)>0 & is.numeric(xx))

For instance, taking your example:

xx <- ddf[ddf$name=='a','value']

is.numeric(xx) & length(xx)>0
[1] TRUE


xx <- ddf[ddf$name=='c','value']

is.numeric(xx) & length(xx)>0
[1] FALSE


xx <- ddf[ddf$name=='a','name']

is.numeric(xx) & length(xx)>0
[1] FALSE

Answer 2

The following code uses regular expressions to confirm that the character string on contains numeric digits and has at least a single digit. See below:

grepl("^[0-9]{1,}$", xx)

Or if you need to deal with negative numbers and decimal places:

grepl("^[-]{0,1}[0-9]{0,}.{0,1}[0-9]{1,}$", xx)