Dear R experts,

I would like to substitute the values of a vector, say A, with the average  
values taken over a factor

For example, lets assume


A

[1] 1 2 3 4 5 6 7 8 9 10

B

[1] 1 2 2 4 4 4 7 7 7 7
Levels: 1 2 4 7

I need to have

1.0 2.5 2.5 5.0 5.0 5.0 8.5 8.5 8.5 8.5 8.5

Thank you

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