CODEWITHSUNDEEP
rdataframerow
Pyxel
Pyxel

Reputation: 117

Selecting rows from a column which name is in a variable

I have a dataset with a named column. The name of the column is also in a variable. I would like to be able to select all rows that satisfy a condition on the column "col".

Here I would like to select all rows whose value in the "col" column matches the condition "< 2".

name = "col"
dataset = data.frame(col = 1:3)

I tried to use "eval" in subset, or the "select" function of the dplyr package, but it does not do what I want (or I misused it).

Is there a simple way to do this?

Upvotes: 0

Views: 1086

Answers (2)

Eric Burden
Eric Burden

Reputation: 76

If you're new(ish) to R, I'm going to recommend using the tidyverse set of packages, including the ever-useful dplyr for problems like this so you can have more immediately readable and understandable code. You can get this package using install.packages('tidyverse'). With that installed, to answer your question:

library(dplyr)

df <- data.frame(
  col = c(0:10),
  another_col = c(10:20),
  third_col = c(25:35)
)

dynamic_name <- "col"

filter_at(df, dynamic_name, ~ .x < 2)

Note: The tidyverse family of packages typically accept the formula syntax (that ~ expression) as a way to introduce anonymous (lambda) functions, so ~ .x < 2 is a function that returns TRUE if the value passed in is less than 2).

Upvotes: 1

gaut
gaut

Reputation: 5958

please see below.

a <- 1:5
b <- 6:10
namevar <- "a"
df <- data.frame(a,b)
df[df[,namevar] %in% c(1:3),]
  a b
1 1 6
2 2 7
3 3 8

What happens here is that df[,namevar] %in% c(1:3) gives a vector of TRUE, FALSE depending on whether the condition is met.

Then passing this boolean vector as indices to df yields all the rows where the condition is TRUE.

For more details about %in% see help(is.element())

Upvotes: 0

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