[R] column subtraction by row

[Pete Brecknock]

Vining, Kelly wrote:
> 
> Dear UseRs,
> 
> I have a data frame that looks like this:
> 
> head(test2)
>     attributes start   end StemExplant Callus RegenPlant
> 1  LTR_Unknown   120   535       3.198  1.931      1.927
> 3  LTR_Unknown  2955  3218       0.541  0.103      0.613
> 6  LTR_Unknown  6210  6423       6.080  4.650      9.081
> 9  LTR_Unknown  9658 10124       0.238  0.117      0.347
> 14 LTR_Unknown 14699 14894       3.545  3.625      2.116
> 25 LTR_Unknown 33201 33474       1.275  1.194      0.591
> 
> 
> I need to subtract each value in the "end" column from its corresponding
> value in the "start" column, then append that difference as a new column
> in this data frame. 
> 
> It seems like apply could be the way to approach this, but I can't see any
> easy way to designate "difference" as a function, like, say, sum or mean.
> Plus, all the apply/lapply examples I'm looking at seem to depend on a
> data frame being just the two columns on which to operate, without any way
> to designate which columns to use in the function(x,y) part of an lapply
> statement.  Another alternative would be a for loop, but when I try this:
> 
> for(i in 1:nrow(test2)) {
> 	testout[i] <- (test2$end[i] - test2$start[i])
> 	}
> 
> I get an error. So I'm stuck at the first step here. I think that once I
> can figure out how to get the differences, I can use cbind to append the
> data frame. But if there is a better way to do it, I'd like to know that
> as well.
> 
> Any help is appreciated.
> 
> --Kelly V.
> ______________________________________________
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
> 


IS this what you are looking for?

# Data
lines <- "attributes start end StemExplant Callus RegenPlant
LTR_Unknown   120   535 3.198  1.931 1.927
LTR_Unknown  2955  3218 0.541  0.103 0.613
LTR_Unknown  6210  6423 6.080  4.650 9.081
LTR_Unknown  9658 10124 0.238  0.117 0.347
LTR_Unknown 14699 14894 3.545  3.625 2.116
LTR_Unknown 33201 33474 1.275  1.194 0.591"

d = read.table(textConnection(lines), header=TRUE) 

# Create new variable
d$new=d$end - d$start

print(d)

   attributes start   end StemExplant Callus RegenPlant new
1 LTR_Unknown   120   535       3.198  1.931      1.927 415
2 LTR_Unknown  2955  3218       0.541  0.103      0.613 263
3 LTR_Unknown  6210  6423       6.080  4.650      9.081 213
4 LTR_Unknown  9658 10124       0.238  0.117      0.347 466
5 LTR_Unknown 14699 14894       3.545  3.625      2.116 195
6 LTR_Unknown 33201 33474       1.275  1.194      0.591 273

HTH

Pete

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