[R] Arrange values on a timeline

[Ralf B]

I have a set of labels arranged along a timeframe in a. Each label has
a timestamp and marks a state until the next label. The dataframe a
contains 5 such timestamps and 5 associated labels. This means, on a
continious scale between 1-100, there are 5 markers. E.g. 'abc' marks
the timestampls between 10 and 19, 'def' marks the timestamps between
20 and 32, and so on.

a <- data.frame(timestamp=c(3,5,8), mylabel=c("abc","def","ghi"))
b <- data.frame(timestamp=c(1:10))

I would like to assign these labels as an extra collumn 'label' to the
data.frame b which currently only consists of a the timestamp. The
output would then look like this:

       timestamp      label
1     1                    NA
2     2                    NA
3     3                    "abc"
4     4                    "abc"
5     5                    "def"
6     6                    "def"
7     7                    "def"
8     8                    "ghi"
9     9                    "ghi"
10  10                    "ghi"

What is the simplest way to assign these labels based on timestamps to
get this output. The real dataset is several millions of rows...

Thanks,
Ralf