[R] How to calc ratios base on current and previous row?

[(Ted Harding)]

On 27-Aug-10 17:17:50, Marcus Drescher wrote:
> Hi all,
> I want to calculate in each row a ratio based on number in the current
> row and the previous row.
> Is there a way to do this without for-loops because that is extremely
> slow.
> 
>       A       B       
> [1]   2       2       
> [2]   2       3       
> [3]   4       5,5
> ...
> 
> B2 = A2 + 0.5*B1
> 
> Thanks in advance.
> Best
> Marcus

If n is the total number of rows, then :

  B[(2:n)] <- A[(2:n)] + 0.5*B[1:(n-1)]

should work! This assumes that A and B have been extracted as vectors,
or can be referred to separately as in a dataframe (say D), like

  D$B[(2:n)] <- D$A[(2:n)] + 0.5*D$B[1:(n-1)]

Ted.

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Date: 27-Aug-10                                       Time: 18:34:27
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