Subset data to contain only columns whose names match a condition

Question

Try grepl on the names of your data.frame. grepl matches a regular expression to a target and returns TRUE if a match is found and FALSE otherwise. The function is vectorised so you can pass a vector of strings to match and you will get a vector of boolean values returned.

Example

#  Data
df <- data.frame( ABC_1 = runif(3),
            ABC_2 = runif(3),
            XYZ_1 = runif(3),
            XYZ_2 = runif(3) )

#      ABC_1     ABC_2     XYZ_1     XYZ_2
#1 0.3792645 0.3614199 0.9793573 0.7139381
#2 0.1313246 0.9746691 0.7276705 0.0126057
#3 0.7282680 0.6518444 0.9531389 0.9673290

#  Use grepl
df[ , grepl( "ABC" , names( df ) ) ]
#      ABC_1     ABC_2
#1 0.3792645 0.3614199
#2 0.1313246 0.9746691
#3 0.7282680 0.6518444

#  grepl returns logical vector like this which is what we use to subset columns
grepl( "ABC" , names( df ) )
#[1]  TRUE  TRUE FALSE FALSE

To answer the second part, I'd make the subset data.frame and then make a vector that indexes the rows to keep (a logical vector) like this...

set.seed(1)
df <- data.frame( ABC_1 = sample(0:1,3,repl = TRUE),
            ABC_2 = sample(0:1,3,repl = TRUE),
            XYZ_1 = sample(0:1,3,repl = TRUE),
            XYZ_2 = sample(0:1,3,repl = TRUE) )

# We will want to discard the second row because 'all' ABC values are 0:
#  ABC_1 ABC_2 XYZ_1 XYZ_2
#1     0     1     1     0
#2     0     0     1     0
#3     1     1     1     0


df1 <- df[ , grepl( "ABC" , names( df ) ) ]

ind <- apply( df1 , 1 , function(x) any( x > 0 ) )

df1[ ind , ]
#  ABC_1 ABC_2
#1     0     1
#3     1     1

You can also use starts_with and dplyr's select() like so:

df <- df %>% dplyr:: select(starts_with("ABC"))

Just in case for data.table users, the following works for me:

df[, grep("ABC", names(df)), with = FALSE]

Using dplyr you can:

df <- df %>% dplyr:: select(grep("ABC", names(df)), grep("XYZ", names(df)))

This worked for me:

df[,names(df) %in% colnames(df)[grepl(str,colnames(df))]]

Simplest solution, given to me by my statistics professor:

df[,grep("pattern", colnames(df))]

That's it. It doesn't give you booleans or anything, it just gives you your dataset that follows that pattern.