[R] matrix looping accessing previous column

[R_novice]

Thank you for the quick response :-). I've applied your suggestion to my
code, but I still receive an error:

> CEM1_PARTIAL$V3[which(CEM1_PARTIAL$V4 == "DOWN") - 1] <-
> CEM1_PARTIAL$V3[which(CEM1_PARTIAL$V4 == "DOWN") - 1] * -1
Warning message:
In Ops.factor(CEM1_PARTIAL$V3[which(CEM1_PARTIAL$V4 == "DOWN") -  :
  * not meaningful for factors

I would like to perform this evaluation in a loop b/c my data matrix
actually contains 109 columns, and 32,000 rows.

More suggestions are appreciated!

Thanks!
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