I was looking at the help page for the aggregate function in R. I had never used this convenience function but I have a process it should help me speed up. However, I've been totally unable to walk through the example and understand what is going on.

One example is the following:

1> aggregate(state.x77, list(Region = state.region), mean)
         Region Population Income Illiteracy Life Exp Murder HS Grad  Frost   Area
1     Northeast       5495   4570      1.000    71.26  4.722   53.97 132.78  18141
2         South       4208   4012      1.738    69.71 10.581   44.34  64.62  54605
3 North Central       4803   4611      0.700    71.77  5.275   54.52 138.83  62652
4          West       2915   4703      1.023    71.23  7.215   62.00 102.15 134463

The output here is exactly what I would expect. So I try to understand what is going on. So I look at state.x77

1> head(state.x77)
           Population Income Illiteracy Life Exp Murder HS Grad Frost   Area
Alabama          3615   3624        2.1    69.05   15.1    41.3    20  50708
Alaska            365   6315        1.5    69.31   11.3    66.7   152 566432
Arizona          2212   4530        1.8    70.55    7.8    58.1    15 113417
Arkansas         2110   3378        1.9    70.66   10.1    39.9    65  51945
California      21198   5114        1.1    71.71   10.3    62.6    20 156361
Colorado         2541   4884        0.7    72.06    6.8    63.9   166 103766

OK, that's odd to me. I would expect to see a column in state.x77 named state.region or something. So state.region must be its own object. So I do a str() on it:

1> str(state.region)
 Factor w/ 4 levels "Northeast","South",..: 2 4 4 2 4 4 1 2 2 2 ...

It looks like state.region is just a factor. Somehow there HAS to be a connection between state.region and state.x77 in order for aggregate() to group state.x77 b

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