Filtering observations by using grep the reverse way in R

问题

Shown as below:

    df <- data.frame(X1 = rep(letters[1:3],3),
                     X2 = 1:9,
                     X3 = sample(1:50,9))
    df
    ind<- grep("a|c", df$X1)
    library(data.table)
    df_ac <- df[ind,]
    df_b <- df[!ind,]

df_ac is created using the regular grep command. If I want to use the grep the reverse way: to select all observations with X1 == 'b'. I know I can do this by:

ind2<- grep("a|c", df$X1, invert = T)
df_b <-df[ind2,]

But, in my original script, why does the command df_b <-df[!ind,] return a data frame with zero observation? Anyone can explain to me why my logic here is wrong? Is there any other way to select observations in a data.frame by using the grep reversely without specifying invert = T? Thank you!

回答1:

You may be more interested in grepl instead of grep:

ind<- grepl("a|c", df$X1)
df[ind,]
#   X1 X2 X3
# 1  a  1 16
# 3  c  3 38
# 4  a  4 10
# 6  c  6 18
# 7  a  7 33
# 9  c  9 49

df[!ind,]
#   X1 X2 X3
# 2  b  2  5
# 5  b  5 14
# 8  b  8 50

---

Alternatively, go ahead an make use of "data.table" and try out %in% to see what else might work for you. Notice the difference in the syntax.

ind2 <- c("a", "c")
library(data.table)
setDT(df)
df[X1 %in% ind2]
#    X1 X2 X3
# 1:  a  1 16
# 2:  c  3 38
# 3:  a  4 10
# 4:  c  6 18
# 5:  a  7 33
# 6:  c  9 49

df[!X1 %in% ind2]
#    X1 X2 X3
# 1:  b  2  5
# 2:  b  5 14
# 3:  b  8 50

来源：https://stackoverflow.com/questions/36091366/filtering-observations-by-using-grep-the-reverse-way-in-r