Reason for unexpected output in subsetting data frame - R

Question

I have the data frame "a" and it has a variable called "VAL". I want to count the elements where the value of VAL is 23 or 24.

I used two codes which worked Ok:

nrow(subset(a,VAL==23|VAL==24) 
nrow(subset(a,VAL %in% c(23,24)))

But, I tried other code which gives an unexpected output and I don't know why.

nrow(subset(a,VAL ==c(23,24)))

Even if I change the order of 23 and 24, it gives a different unexpected output.

nrow(subset(a,VAL ==c(24,23)))

Why are those codes incorrect ? What are they actually doing?

Answer 1

Working through an example shows where it is going wrong:

a <- data.frame(VAL=c(1,1,1,23,24))
a
#  VAL
#1   1
#2   1
#3   1
#4  23
#5  24

These work:

a$VAL %in% c(23,24)
#[1] FALSE FALSE FALSE  TRUE  TRUE
a$VAL==23 | a$VAL==24
#[1] FALSE FALSE FALSE  TRUE  TRUE

The following doesn't work due to vector recycling when comparing - take note of the warning message below E.g.:

a$VAL ==c(23,24)
#[1] FALSE FALSE FALSE FALSE FALSE
#Warning message:
#In a$VAL == c(23, 24) :
#  longer object length is not a multiple of shorter object length

This last bit of code recycles what you are testing against and is basically comparing:

c( 1,  1,  1, 23, 24) #to
c(23, 24, 23, 24, 23)

...so you don't get any rows returned. Changing the order will give you

c( 1,  1,  1, 23, 24) #to
c(24, 23, 24, 23, 24)

...and you will get two rows returned (which gives the intended result by pure luck, but it is not appropriate to use).