Filtering rows by a criteria defined by variables

Question

I've the following data.table

structure(list(val1 = c(1, 2, 1, 3, 4, 5, 3), val2 = c(4, 5, 6, 4, 2, 4, 5)), .Names = c("val1", "val2"), row.names = c(NA, -7L), class = c("data.table", "data.frame"), .internal.selfref = <pointer: 0xedae28>)

What I would like to do is filter the rows in it based on a criteria defined by other variables. For example, I might want all rows that have val1 >= 1. This is easily done as

x[val1 > 1,]

However, I want to be able to specify the column name (i.e. either val1 or val2) as a variable and the filter value as a variable. I tried

cname = 'val1'
cutoff = 1
x[(cname >= cutoff),] # Try 1
x[(cname) >= (cutoff),] # Try 2
x[eval(cname > cutoff),] # Try 3

but it just gave back the original data.table.

List of related questions, though all of the answers are super long and/or don't use `get`, which is simplest: http://stackoverflow.com/q/24833247/1191259 — Frank, Apr 29 '15 at 18:22
@Frank, you actually took the time to read through that linked question? :-) — A5C1D2H2I1M1N2O1R2T1, Apr 29 '15 at 18:24
@AnandaMahto Not quite, but more than I should have :) I had a feeling this was a dupe, but I like this Q&A much better than any of those. Here's my own question on the same thing: http://stackoverflow.com/q/19276194/1191259 I think everyone has asked this at some point. — Frank, Apr 29 '15 at 18:29

score 6 · Accepted Answer · edited Apr 29 '15 at 18:22

6

It sounds like you're looking for get:

x[get(cname) > cutoff,]
#    val1 val2
# 1:    2    5
# 2:    3    4
# 3:    4    2
# 4:    5    4
# 5:    3    5

edited Apr 29 '15 at 18:22

dayne

7,504
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answered Apr 29 '15 at 18:05

A5C1D2H2I1M1N2O1R2T1

190,393
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485

score 3 · Answer 2 · answered Apr 29 '15 at 18:06

3

You could use eval with as.name or as.symbol

x[eval(as.name(cname)) > cutoff]
#    val1 val2
#1:    2    5
#2:    3    4
#3:    4    2
#4:    5    4
#5:    3    5

answered Apr 29 '15 at 18:06

akrun

874,273
37
540
662

Filtering rows by a criteria defined by variables

2 Answers2