Filtering a dataframe by specified column and specified value

Question

I do not know how to program in R at all, I only know python pandas. How to do the following in R:

my code has two variables, username and asignment
I want to filter my dataframe so that I only get the row that has name == username...
and then I only want the the column whose name matches the variable asignment.

Pictured is how I did it in Python pandas just to make it clear what I want. How do I do this in R?

score 3 · Answer 1 · answered Jul 24 '19 at 23:49

3

If you only want the value you can use:

df$asgn1[df$name == username]

Or, using dplyr to get the column you can use filter and select

library(dplyr)
df %>%
  filter(name == username) %>%
  select(asgn1)

answered Jul 24 '19 at 23:49

Bjørn Kallerud

979
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23

so the problem here is that i dont actually know that the value entered is asgn1. All i know is the variable assignment. How would I change the code to work if all i know is a variable? – ai.jennetta Jul 24 '19 at 23:52
@datasciencephl. If that is the ccase, it would be `df[[assignment]][df$name == username]` – akrun Jul 25 '19 at 01:43

score 0 · Answer 2 · answered Jul 25 '19 at 01:23

You could do

df[df$name == username, assignment]
#[1] 80

Or you can also subset based on column position

df[df[, 1] == username, assignment]

In dplyr you could also use pull to get column name as vector

library(dplyr)
df %>%
  filter(name == username) %>%
  pull(assignment)

data

df <- data.frame(name = c("Jenna", "Alex", "Nick"), asgn1 = c(80, 90, 92), 
                 asgn2 = c(82, 92, 94), stringsAsFactors = FALSE)
username = "Jenna"
assignment = "asgn1"

Filtering a dataframe by specified column and specified value

2 Answers2