Referring to a value by row position with dplyr

Question

I have `df` that have 12 columns: ``` df<-read.table(header=T,text="V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12 A01 10378809 10379882 Contig1401|m.3412 101 - 10378809 10379882 255,0,0 1 1073 0 A01 10469105 10469293 Contig1755|m.4465 48 + 10469105 10469293 255,0,0 2 188 0 A01 10469429 10469630 Contig1755|m.4465 5 + 10469429 10469630 255,0,0 NA 201 0") ``` First I want to group them by contig and then generate the following values for 12th column. I figured out how to do this with `dplyr` but I have some errors. ``` as.data.frame(df %.% group_by(V4) %.% summarise(V12=apply(df[2], 2, function(x)x-x[1]))) ``` The error: `Error in summarise_impl(.data, named_dots(...), environment()) : attempt to use zero-length variable name`. For each group I want to subtract the 2nd value from the 1st value from 2nd column. I can do this easily if there are only 2 rows (max-min) but if there are more than 2 then I will miss the middle rows. So I thought I will write a function and insert into `dplyr` but it…

Accepted Answer

I suppose you're looking for this:

```
library(dplyr)
df %.% 
  group_by(V4) %.% 
  mutate(V12 = V2 - V2[1])
```

Referring to a value by row position with dplyr

Solution

Problem