How to interpolate between to vectors using R?

Question

My data look like:

 c1=c(2,5,5,5,8,4,5,4)
 c16=c(40,50,65,75,38,14,45,64)
 c30=c(20,41,35,15,18,14,15,14)

I would like to linearly interpolate c1,c16,c30 to find the values in between i.e. c2,c3,c4,......c15 then c17,c18,c19,..........c29.

IS this possible using R?

Answer 1

You align your vector in a matrix with rbind and use apply on each column to do the interpolation (done by approxfun):

funcs = apply(rbind(c1,c16,c30), 2, approxfun, x=c(1, 16, 30))

Here is the interpolation on points (1, 2), (16, 40), (30, 20):

#> funcs[[1]](1)
#[1] 2

#> funcs[[1]](16)
#[1] 40

#> funcs[[1]](30)
#[1] 20

#funcs[[1]](1:30)
# [1]  2.000000  4.533333  7.066667  9.600000 12.133333 14.666667 17.200000 19.733333 22.266667 24.800000 27.333333 29.866667 32.400000 34.933333 37.466667
#[16] 40.000000 38.571429 37.142857 35.714286 34.285714 32.857143 31.428571 30.000000 28.571429 27.142857 25.714286 24.285714 22.857143 21.428571 20.000000

Here are all interpolations plotted with ggplot:

dfs = Map(function(f, i) data.frame(x=1:30,y=f(1:30),interp=i), funcs, seq(length(funcs)))

library(ggplot2)

df = do.call(rbind, dfs)
ggplot(df, aes(x=x,y=y, color=as.character(interp))) + geom_line(size=1.2)

Edit:

If you want to add output as text files:

m = do.call(rbind, lapply(funcs, function(f) f(1:30)))
lapply(1:ncol(m), function(i) write.table(m[,i], file=paste0('c',i,'.txt')))