site  sp1 sp2 sp3 elev
a      3   4   9   100
b      1   8   10  210
c      4   8   15  165

and reorganize into something like this (perhaps so we can do an ANOVA using species as a factor):

site  elev  sp  abundance
a     100  sp1  3
a     100  sp2  4
a     100  sp3  9
b     210  sp1  1
b     210  sp2  8
b     210  sp3  10
c     165  sp1  4
c     165  sp2  8
c     165  sp3  15

Assuming the first dataframe above is called d, the second dataframe can be obtained using the following code:

> library(ggplot2)
> m = melt(d, id=c('site','elev'))

melt works like this: You specify the ID variables, which are those variables that will REMAIN as dataframe variables. Any others will be considered measured variables. If it’s easier for your data, you can do it the other way: specify the measured variables and the others will be considered ID variables.

Melting results in two new variables, variable and value. variable contains the names of the original columns of the dataframe as factors, and value contains the corresponding values.

Another example

Here’s another example using the built-in dataset, airquality. First, unmelted:

# make all the variable names lowercase
names(airquality) <- tolower(names(airquality))
head(airquality)
  ozone solar.r wind temp month day
1    41     190  7.4   67     5   1
2    36     118  8.0   72     5   2
3    12     149 12.6   74     5   3
4    18     313 11.5   62     5   4
5    NA      NA 14.3   56     5   5
6    28      NA 14.9   66     5   6

and melted:

> head(melt(airquality,id=c('month','day')))
  month day variable value
1     5   1    ozone    41
2     5   2    ozone    36
3     5   3    ozone    12
4     5   4    ozone    18
5     5   5    ozone    NA
6     5   6    ozone    28

You can also use rbind, but it will only be useful for simple cases.

See http://www.statmethods.net/management/merging.html for more details.