how do you extract the columns that contain a certain text/string in R

问题

I need to be able to extract columns that contain exact string that I am looking for. For example, I have this data frame x:

structure(list(Time = structure(1L, .Label = "1/1/2015", class = "factor"), 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.MB. = 3555L, 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.Free.MB. = 55L, 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Swap.Free.MB. = 44L, 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Cache.Free.MB. = 66L, 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Swap.And.Cache.Free.MB. = 44L, 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Percent.Free = 44L, 
    WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Round.Trip.Time = 44L), .Names = c("Time", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.MB.", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.Free.MB.", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Swap.Free.MB.", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Cache.Free.MB.", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Swap.And.Cache.Free.MB.", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Percent.Free", 
"WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Round.Trip.Time"
), class = "data.frame", row.names = c(NA, -1L))

I need to only extract the column that contains this exact match ".Total.Phys.Mem.MB."

When I do this:

x[,grepl(".Total.Phys.Mem.MB.", colnames(x)[2:ncol(x)])]

I dont get the column that contains this string in it ".Total.Phys.Mem.MB.". Is there a better way to extract the columns that contain the string in R?

回答1:

library(dplyr)

select(x, contains(".Total.Phys.Mem.MB."))
  WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.MB.
1                                                                                    3555

回答2:

Unless fixed=TRUE is defined, grepl recognizes the pattern as a regular expression; and in regex the dot is a character of special meaning which must be escaped to match a literal.

> x[grepl("\\.Total\\.Phys\\.Mem\\.MB\\.", colnames(x))]
  WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.MB.
1                                                                                    3555

> x[grepl('.Total.Phys.Mem.MB.', colnames(x), fixed=TRUE)]
  WTAD..Linux..Linux.Percent.of.Physical.Memory.and.Swap.Used.on.web02.Total.Phys.Mem.MB.
1                                                                                    3555

来源：https://stackoverflow.com/questions/28099189/how-do-you-extract-the-columns-that-contain-a-certain-text-string-in-r

标签

regex