[R] Script for conditional sums of vectors

[Benjamin Gillespie]

Hi guys,

I hope you can help me with this (probably) simple query:

I have a data frame:

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a=c(1,1,1,1,1,1,2,2,2,2,2,2)
b=c(1,1,1,2,3,4,1,1,2,2,3,4)
c=c(400,200,300,100,500,300,200,100,500,400,200,100)


data=data.frame(a=a,b=b,c=c)

--------------------------

And I would like to get the following output:

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	b
a	1	2	3	4
1	900	100	500	300
2	300	900	200	100

--------------------------

The values in the output represent the sum of values "c" in data frame "data", for each "a" and "b" combination.

For example, where "a" = 1 and "b" = 1, the output is 400+200+300 = 900.

Please would anyone be able to provide a script to create my desired output?

Many thanks in advance,
		
Ben Gillespie
Research Postgraduate
 
School of Geography
University of Leeds
Leeds
LS2 9JT