kaos1511 - 4 months ago 21

R Question

Is there a way for me to subset data based on column names starting with a particular string? I have some columns which are like

`ABC_1 ABC_2 ABC_3`

`XYZ_1, XYZ_2,XYZ_3`

How can I subset my

`df`

`ABC`

`XYZ`

Also, I want to only include rows from each of these columns where any of their value is

`>0`

`6`

`1`

Answer

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.

```
# 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
```