Add column to data frame that shows frequency of variable

Question

In R, it's always the small things that confound me.

Say I have a data frame like this:

  location   species
1  seattle   A
2  buffalo   C
3  seattle   D
4  newark    J
5  boston    Q

I would like to append a column to this frame that shows the number of times a location appears in the data set, with a result like this:

  location   species    freq-loc
1  seattle   A          2           #there are 2 entries with location=seattle
2  buffalo   C          1           #there is 1 entry with location=buffalo
3  seattle   D          2
4  newark    J          1
5  boston    Q          1

I know using table(data$location) can give me a contingency table. But I don't know how to map each value in the table to a corresponding entry in the dataframe. Can somebody help?

Update

Thank you so much for all the help! Just for interest, I ran a benchmark test to see how the merge, plyr and ave solutions ran compared to each other. The testing set is a 10,000 rows subset of my original 10 by ~7mil data set.:

Unit: milliseconds
expr        min         lq     median        uq       max neval
MERGE 110.877337 111.989406 112.585420 113.51679 120.23588   100
PLYR  26.305645  27.080403  27.576580  27.87157  68.40763   100
AVE   2.994528   3.117255   3.179898   3.35834  10.02955   100

Answer 1

Here's a base R way with ave.

transform(d, freq.loc = ave(seq(nrow(d)), location, FUN=length))