[R] How to sum one column in a data frame keyed on other columns

[Simon Blomberg]

You could look at the reshape package, using sum as the aggregate function.

HTH,

Simon.

George Nachman wrote:
> I have a data frame that looks like this:
>
> url         time somethingirrelevant visits
> www.foo.com 1:00 xxx                 100
> www.foo.com 1:00 yyy                 50
> www.foo.com 2:00 xyz                 25
> www.bar.com 1:00 xxx                 200
> www.bar.com 1:00 zzz                 200
> www.foo.com 2:00 xxx                 500
>
> I'd like to write some code that takes this as input and outputs
> something like this:
>
> url         time total_vists
> www.foo.com 1:00 150
> www.foo.com 2:00 525
> www.bar.com 1:00 400
>
> In other words, I need to calculate the sum of visits for each unique
> tuple of (url,time).
>
> I can do it with this code, but it's very slow, and doesn't seem like
> the right approach:
>
> keys = list()
> getkey = function(m,cols,index) { paste(m[index,cols],collapse=",")  }
> for (i in 1:nrow(data)) { keys[[getkey(data,1:2,i)]] = 0 }
> for (i in 1:nrow(data)) { keys[[getkey(data,1:2,i)]] =
> keys[[getkey(data,1:2,i)]] + data[i,4] }
>
> I'm sure there's a more functional-programming approach to this
> problem! Any ideas?
>
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>   


-- 
Simon Blomberg, B.Sc.(Hons.), Ph.D, M.App.Stat. 
Centre for Resource and Environmental Studies
The Australian National University              
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