knitr hook to separate 000's, but not for years - r

Question

I define a hook at the top of my rnw to separate '000s with commas:
knit_hooks$set(inline = function(x) {
prettyNum(x, big.mark=",")
})
However, there are some numbers that I don't want to format like this, such as years. Is there a better way to write the hook, or a way to override the hook when I print \Sexpr{nocomma} in the example below?
\documentclass{article}
\begin{document}
<<setup>>=
library(knitr)
options(scipen=999) # turn off scientific notation for numbers
opts_chunk$set(echo=FALSE, warning=FALSE, message=FALSE)
knit_hooks$set(inline = function(x) {
prettyNum(x, big.mark=",")
})
wantcomma <- 1234*5
nocomma <- "September 1, 2014"
#
The hook will separate \Sexpr{wantcomma} and \Sexpr{nocomma}, but I don't want to separate years.
\end{document}
Output:
The hook will separate 6,170 and September 1, 2,014, but I don’t want to separate years.

Answer 1

If the only things your don't want comma-separated are strings that have years in, use:
knit_hooks$set(inline = function(x) {
if(is.numeric(x)){
return(prettyNum(x, big.mark=","))
}else{
return(x)
}
})
That works for your calendar string. But suppose you want to just print a year number on its own? Well, how about using the above hook and converting to character:
What about \Sexpr{2014}? % gets commad
What about \Sexpr{as.character(2014)}? % not commad
or possibly (untested):
What about \Sexpr{paste(2014)}? % not commad
which converts the scalar to character and saves a bit of typing. We're not playing code golf here though...
Alternatively a class-based method:
comma <- function(x){structure(x,class="comma")}
nocomma <- function(x){structure(x,class="nocomma")}
options(scipen=999) # turn off scientific notation for numbers
opts_chunk$set(echo=FALSE, warning=FALSE, message=FALSE)
knit_hooks$set(inline = function(x) {
if(inherits(x,"comma")) return(prettyNum(x, big.mark=","))
if(inherits(x,"nocomma")) return(x)
return(x) # default
})
wantcomma <- 1234*5
nocomma1 <- "September 1, 2014" # note name change here to not clash with function
Then just wrap your Sexpr in either comma or nocomma like:
The hook will separate \Sexpr{comma(wantcomma)} and \Sexpr{nocomma(nocomma1)}, but I don't want to separate years.
If you want the default to commaify then change the line commented "# default" to use prettyNum. Although I'm thinking I've overcomplicated this and the comma and nocomma functions could just compute the string format themselves and then you wouldn't need a hook at all.
Without knowing exactly your cases I don't think we can write a function that infers the comma-sep scheme - for example it would have to know that "1342 cases in 2013" needs its first number commad and not its second...