How do I generate a mapping from numbers to colors in R?

Question

In R, I can easily generate a color ramp, for example colorRampPalette. The following produces a sequence of five colors, from blue to red:

> mypal <- colorRampPalette( c( "blue", "red" ) )( 5 )
> mypal
[1] "#0000FF" "#3F00BF" "#7F007F" "#BF003F" "#FF0000"

How can I easily map a vector of numbers on this palette? Say, I have a vector of numbers between 0 and 10, like this:

x <- c( 1, 9, 8.5, 3, 3.4, 6.2 )

I would like a function, say map2color, such that when I run map2color( mypal, x ), I get

#0000FF, #FF0000, #FF0000, #3F00BF, #3F00BF, #BF003F

I usually do something like the following

mypalette[ findInterval( x, seq( 0, 10, length.out= 6 ), rightmost.closed= T ) ]

but maybe there is a better solution OOB, automatically generating a color scale for a numeric vector.

Answer 1

Here's a map2color function:

map2color<-function(x,pal,limits=NULL){
    if(is.null(limits)) limits=range(x)
    pal[findInterval(x,seq(limits[1],limits[2],length.out=length(pal)+1), all.inside=TRUE)]
}


 map2color(0:11,rainbow(200),limits=c(1,10))
[1] "#FF0000FF" "#FF0000FF" "#FFA800FF" "#ADFF00FF" "#05FF00FF" "#00FFA3FF"
[7] "#00ABFFFF" "#0003FFFF" "#A600FFFF" "#FF00B0FF" "#FF0008FF" "#FF0008FF"
 map2color(0:11,rainbow(200))
[1] "#FF0000FF" "#FF8A00FF" "#EBFF00FF" "#61FF00FF" "#00FF29FF" "#00FFB3FF"
[7] "#00BAFFFF" "#0030FFFF" "#5900FFFF" "#E300FFFF" "#FF0091FF" "#FF0008FF"


mypal <- colorRampPalette( c( "blue", "red" ) )( 5 )
x <- c( 1, 9, 8.5, 3, 3.4, 6.2 )
map2color(x,mypal)
"#0000FF" "#FF0000" "#FF0000" "#3F00BF" "#3F00BF" "#BF003F"

I needed something like this to create a colorramp that would be consistent across different samples of data, so I added an xlim parameter. Thanks for the reminder of the findInterval function in your question.