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 +
Your question is unclear. If the calculation of B[3] now depends on the
newly calculated value of B[2], then the answer is no: you must loop. If
B[3] is based on the original B[2] value, then the answer is yes, and the
solution is just a matter of simple indexing, which I leave as an exercise.
--
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
Hi Marcus,
I am guessing you are thinking in terms of Excel, where in column B,
you could enter the formula =(A2 + 0.5*B1) and just drag it down
however many cells you wanted. In R, it would be expressed a bit
differently. If k indexes your rows, I believe you want:
B[k] = A[k] + 0.5 * B[k -
For this particular case there is a nice shortcut (there is still looping , but
it is internal and quick):
a - c(2,2,4, sample(1:10, 97, TRUE) )
b - cumsum( (1/2)^(99:0)*a )/( (1/2)^(99:0) )
## compare
bb - numeric(100)
bb[1] - a[1]
for( i in 2:100 ) {
bb[i] - 1/2 * bb[i-1] + a[i]
}
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