return index from a vector of the value closest to a given element

one way:

# as mnel points out in his answer, the difference,
# using `which` here gives all indices that match
which(abs(x-0.4) == min(abs(x-0.4)))

where x is your vector.

Alternately,

# this one returns the first index, but is SLOW
sort(abs(x-0.4), index.return=T)$ix[1]

You can also use base::findInterval(0.4, x)

I would use which.min

which.min(abs(x-0.4))

This will return the first index of the closest number to 0.4.