将具有不完整观察的行添加到data.frame

Question

Suppose I have the following data.frame :

    foo <- data.frame(ocean=c('Atlantic', 'Pacific'), 
        city=c('Boston', 'San Francisco'), 
        state=c('MA', 'CA'), stringsAsFactors=F)

I would like to add a third row, but in this case, only specify the city and leave the other fields blank.

Some naive ways, which do not work include:

foo$city[3] <- 'Providence'
# generates an error message about replacement has 3 rows, data has 2

This also generates an error message

rbind(foo, data.frame(city='Providence'))
# generates an error message, number of columns of arguments do not match

I'm interested in how other users would approach this.

Answer 1

foo[3,] <- NA
foo$city[3] <- 'Providence'

OR

foo[3,] <- c(NA,'Providence',NA)

Answer 2

Another approach might be to use rbind.fill in the plyr package.

library(plyr)
bar <- data.frame(city="Providence", stringsAsFactors=F)
str(bar)

foobar <- rbind.fill(foo, bar)
str(foobar)

head(foobar)