# [R] Odp: How to change row value based on previous row value

Petr PIKAL petr.pikal at precheza.cz
Mon Oct 8 15:05:16 CEST 2007

```Hi

r-help-bounces at r-project.org napsal dne 08.10.2007 14:24:13:

> Hi,
>
> I would like to fix the data in the following data.frame, and I am
> having trouble coding this in R - help please!
>
> > x
>   A    B   x    y     z
> 1 1 10.0 100 1000 10000
> 2 2 19.8 200 2000 20000
> 3 3 20.1 300 3000 30000
> 4 4 20.3 400 4000 40000
> 5 5 30.0 500 5000 50000
>
> Column B is the problem.
>
> The logic I need to apply (this is in pseudo code ... I don't know how
> to do this in R)
>
> for all rows, r
> if x\$B[r] <= x\$B[r-1] + a
>    then x\$B[r] = x\$B[r-1]
>
> i.e. If a=1, then I would end up with the data.frame
>
> > x
>   A    B   x    y     z
> 1 1 10.0 100 1000 10000
> 2 2 19.8 200 2000 20000
> 3 3 19.8 300 3000 30000
> 4 4 19.8 400 4000 40000
> 5 5 30.0 500 5000 50000

Maybe not the simplest one:

> test
A    B   x    y     z
1 1 10.0 100 1000 10000
2 2 19.8 200 2000 20000
3 3 20.1 300 3000 30000
4 4 20.3 400 4000 40000
5 5 30.0 500 5000 50000

> b.na <- test\$B
> b.na[c(F,diff(test\$B)<1)] <- NA
> b.na
[1] 10.0 19.8   NA   NA 30.0
> library(zoo)
> b.na <- na.locf(b.na)
[1] 10.0 19.8 19.8 19.8 30.0

and you can put the vector back do data frame by

test\$B<-b.na

But it works only if there are chunks of values separated by larger steps.

Regards
Petr

>
> Many thanks in advance for any help,
>
> Regards,
> David
>
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