CODEWITHSUNDEEP
rsubset
Andrew Jackson
Andrew Jackson

Reputation: 823

When subsetting in R is it necessary to include `which` or can I just put a logical test?

Say I have a data frame df and want to subset it based on the value of column a. 

df <- data.frame(a = 1:4, b = 5:8)
df

Is it necessary to include a which function in the brackets or can I just include the logical test?

df[df$a == "2",]
#  a b
#2 2 6
df[which(df$a == "2"),]
#  a b
#2 2 6

It seems to work the same either way... I was getting some strange results in a large data frame (i.e., getting empty rows returned as well as the correct ones) but once I cleaned the environment and reran my script it worked fine.

Upvotes: 6

Views: 249

Answers (1)

eipi10
eipi10

Reputation: 93851

df$a == "2" returns a logical vector, while which(df$a=="2") returns indices. If there are missing values in the vector, the first approach will include them in the returned value, but which will exclude them.

For example: 

x=c(1,NA,2,10)

x[x==2]

[1] NA  2

x[which(x==2)]

[1] 2

x==2

[1] FALSE    NA  TRUE FALSE

which(x==2)

[1] 3

Upvotes: 1

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