Subset data to contain only columns whose names match a condition

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 and some like XYZ_1, XYZ_2,XYZ_3 let's say.

How can I subset my df based only on columns containing the above portions of text (lets say, ABC or XYZ)? I can use indices, but the columns are too scattered in data and it becomes too much of hard coding.

Also, I want to only include rows from each of these columns where any of their value is >0 so if either of the 6 columns above has a 1 in the row, it makes a cut into my final data frame.

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最佳答案

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
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Using dplyr you can:

df <- df %>% dplyr:: select(grep("ABC", names(df)), grep("XYZ", names(df)))
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You can also use starts_with and dplyr's select() like so:

df <- df %>% dplyr:: select(starts_with("ABC"))
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This worked for me:

df[,names(df) %in% colnames(df)[grepl(str,colnames(df))]]
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Just in case for data.table users, the following works for me: 

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

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Try this (here, looking for variables whose name contains 'date', including all case combinations):

df %>%  dplyr::select(contains("date", ignore.case = TRUE))
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Building off the above, I think it is the most flexible. Note that you'll need to use dplyr, but that's not a terrible thing.

Advantage: You can search for more than "contains". Here, I use "starts_with" for a relatively common string "ST". Using "grep" here could easily have driven you mad; mad, I say!

library(dplyr)


df %>% dplyr::select(starts_with("ST",ignore.case = TRUE))
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Many of the tidyselect options have been mentioned already. contains and starts_with work very well with this specific problem. For more complicated conditions/matching you can use matches, which will select columns using a regular expression match:

library(dplyr)


df %>%
select(matches("^ABC")) # starts with "ABC"


# case insensitive match
df %>%
select(matches("(?i)^abc")) # starts with "ABC", "Abc", "abc", etc.
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I'll provide alternative solutions to subset one or multiple columns, based on @Simon O'Hanlon's sample data:

> library(tidyverse)
> library(dplyr)
>
> df <- data.frame(D=runif(3),
+   ABC_1 = runif(3),
+   ABC_2 = runif(3),
+   XYZ_1 = runif(3),
+   XYZ_2 = runif(3)
+   )
>
> df %>%
+   dplyr::select(matches('ABC'))
ABC_1      ABC_2
1 0.06445754 0.16957677
2 0.75470562 0.06221405
3 0.62041003 0.10902927
>
> df %>%
+   dplyr::select(matches('ABC|XYZ'))
ABC_1      ABC_2     XYZ_1     XYZ_2
1 0.06445754 0.16957677 0.3817164 0.1922095
2 0.75470562 0.06221405 0.1693109 0.2571700
3 0.62041003 0.10902927 0.2986525 0.1812318

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