Question

I’ve got a data frame that contains several interleaved values that occurred in a timeline. I’d like to create a new data frame that contains line numbers (row IDs, basically), a file descriptor, operation and a “size” value.

Example:

  line fd syscall       size
1    1  1   lseek 1289020416
2    2  1   lseek 1289021440
3    3  2   lseek 1289024512
4    4  1   lseek 1289025536
5    5  2   lseek 1289026560
6    6  1   lseek 1289027584

I’d like to compute a diff of the size values per fd and show the starting point of the diff. The diff function itself throws away a lot of data. Is there something similar that will help me have context (e.g. where the beginning of each line was)?

I’d like results that look like the following where I know how far each fd has moved since the previous line, and what the previous line was.

  line fd diff
1    1  1 1024
2    2  1 4096
3    3  2 2048
4    4  1 2048

Is there something I can do that’s easier than tearing it all apart and looping? I have to believe someone has a slightly better diff out there.

Example input:

structure(list(line = 1:6, fd = c(1, 1, 2, 1, 2, 1), syscall = structure(c(1L, 
1L, 1L, 1L, 1L, 1L), class = "factor", .Label = "lseek"), size = c(1289020416, 
1289021440, 1289024512, 1289025536, 1289026560, 1289027584)), .Names = c("line", 
"fd", "syscall", "size"), row.names = c(NA, -6L), class = "data.frame")

Answer 1

Use plyr to cut the data.frame in pieces and transform to attach the new vector.

library(plyr)
ddply(dtf, .(fd), function(x) transform(x, diff = c(x$size[1], diff(x$size))))