select columns based on multiple strings with dplyr contains()

Question

I want to select multiple columns based on their names with a regex expression. I am trying to do it with the piping syntax of the dplyr package. I checked the other topics, but only found answers about a single string.

With base R:

library(dplyr)    
mtcars[grepl('m|ar', names(mtcars))]
###                      mpg am gear carb
### Mazda RX4           21.0  1    4    4
### Mazda RX4 Wag       21.0  1    4    4

However it doesn't work with the select/contains way:

mtcars %>% select(contains('m|ar'))
### data frame with 0 columns and 32 rows

What's wrong?

Solution 1

You can use matches

 mtcars %>%
        select(matches('m|ar')) %>%
        head(2)
 #              mpg am gear carb
 #Mazda RX4      21  1    4    4
 #Mazda RX4 Wag  21  1    4    4

According to the ?select documentation

‘matches(x, ignore.case = TRUE)’: selects all variables whose name matches the regular expression ‘x’

Though contains work with a single string

mtcars %>% 
       select(contains('m'))

Solution 2

You can use contains from package dplyr, if you give a vector of text options, like this:

mtcars %>% 
       select(contains(c("m", "ar"))

Solution 3

You could still use grepl() from base R.

df <- mtcars[ , grepl('m|ar', names(mtcars))]

...which returns a subset dataframe, df, containing columns with m or ar in the column names

Comments

[agenis]

Thank you @akrun, i feel stupid now :-). But one question, still: given that, why should we even use contains(), if matches() does the same and even better?

[akrun]

@agenis There are several options in ?select for flexibility of use, I guess. contains take a single string, but when you do this regex type matching, it is better to use matches...

[hadley]

@agenis Because you might want to match "." and not have to think about how to escape it in a regular expression

[Michael Bellhouse]

Is there a way to not have to pipe the matches, suppose I have a character vector of 30 different matches I am looking for, how can I read that in?

[akrun]

@MichaelBellhouse In that case you use paste ie. paste(yourvec, collapse="|") and use that in matches

[Michael Bellhouse]

akrun, thank you so much. I;ve been doing a lot of digging and experimenting for this. All the best.

[Michael Bellhouse]

equivalent_for_filter <- df %>% filter(!grepl(paste(exclude_filter, collapse="|"),variable))

[Ömer An]

use matches('m*.ar') for "AND" operator

[akrun]

@titeuf it is a regex code to check either 'm' or 'ar'. If you want both use the code as stated by OmerAn

[Admin]

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