[R] Iteration within data blocks

[Michaell Taylor]

I know there must be an easy way to do this, but I am too new to R and it 
must be late because I can't seem to get myself around this.  Assume I have,

scenario 	year	growth	value
1		1	.05	1
1		2	.03	0
1		3	.01	0
1		4	0	0
2		1	.01	1
2		2	.01	0
2		3	.01	0
2		4	0	0

What I want to do is within each scenario "grow" the value by the growth rate 
so that I get :  
scenario 	year	growth	value
1		1	.05	1
1		2	.03	1.03
1		3	.01	1.0403
1		4	0	1.0403
2		1	.01	1
2		2	.01	1.01
2		3	.01	1.0201
2		4	0	1.0201

There are 5,000 scenarios and 10 years. The only thing dancing in my head are 
giant for loops, which I know isn't right.

Any suggestions?

=========================================
Michaell Taylor, PhD
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