hmisc - How to assign "cut" range midpoints in R? -

i using cut divide data bins, gives resulting bin (x1,x2]. can tell me how might make new column expresses these bins midpoint of bin? example, following dataframe:

structure(list(x = c(1l, 4l, 6l, 7l, 8l, 9l, 12l, 18l, 19l),      y = 1:9), .names = c("x", "y"), class = "data.frame", row.names = c(na,  -9l)) 

i can use

test$xrange <- cut(test$x, breaks=seq(0, 20, 5)) 

to give

    x   y   xrange 1   1   1   (0,5] 2   4   2   (0,5] 3   6   3   (5,10] 4   7   4   (5,10] 5   8   5   (5,10] 6   9   6   (5,10] 7   12  7   (10,15] 8   18  8   (15,20] 9   19  9   (15,20] 

but result need should instead like:

    x   y   xrange        xmidpoint 1   1   1   (0,5]         2.5 2   4   2   (0,5]         2.5 3   6   3   (5,10]        7.5 4   7   4   (5,10]        7.5 5   8   5   (5,10]        7.5 6   9   6   (5,10]        7.5 7   12  7   (10,15]       12.5 8   18  8   (15,20]       17.5 9   19  9   (15,20]       17.5 

i've done searching, , came upon similar question @ divide range of values in bins of equal length: cut vs cut2, gives solution as

cut2 <- function(x, breaks) {   r <- range(x)   b <- seq(r[1], r[2], length=2*breaks+1)   brk <- b[0:breaks*2+1]   mid <- b[1:breaks*2]   brk[1] <- brk[1]-0.01   k <- cut(x, breaks=brk, labels=false)   mid[k] } 

but when try on case, using

test$xmidpoint <- cut2(test$x, 5) 

it not return correct midpoint. perhaps entering breaks incorrectly in cut2? can tell me i'm doing incorrectly?

unless miss something, looks valid:

brks = seq(0, 20, 5) ints = findinterval(test$x, brks, all.inside = t) #mapply(function(x, y) (x + y) / 2, brks[ints], brks[ints + 1])  #which ridiculous #[1]  2.5  2.5  7.5  7.5  7.5  7.5 12.5 17.5 17.5 (brks[ints] + brks[ints + 1]) / 2  #as sgibb noted #[1]  2.5  2.5  7.5  7.5  7.5  7.5 12.5 17.5 17.5 (head(brks, -1) + diff(brks) / 2)[ints] #or using thelatemail's idea comments #[1]  2.5  2.5  7.5  7.5  7.5  7.5 12.5 17.5 17.5