When a subset of R needs to include `which` or can I just put a logic test?

Question

Let's say I have a data frame dfand want a subset of it based on the value of column a.

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

Do I need to include a function in brackets whichor can I just include a logic test?

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

Everything seems to work the same way ... I was getting some strange results in a large data frame (i.e. getting blank lines as well as correct ones), but as soon as I cleaned up the environment and restarted my script it worked fine.

Answer 1

df$a == "2" , which(df$a=="2") . , , which .

:

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