r - Incorrect vector length after replacing certain values with NA -

i encountered strange problem:

say make following data frame

test<-as.data.frame(matrix(c(2,4,5,2,4,6),2,3,byrow=t)) #      v1 v2 v3 # 1    2  4  5 # 2    2  4  6

then replace number 5 in column v3 row 1 na:

test$v3[test$v3==5]<-na #      v1 v2 v3 # 1    2  4 na # 2    2  4  6

strangely, length of vector value 6 incorrect:

length(test$v3[test$v3==6]) # 2

how come output 2 instead of 1?

you can take apart expression see what's happening:

test$v3==6 # [1]   na true

as can see, there na value missing element. causes na when subsetting test$v3:

test$v3[test$v3==6] # [1] na  6

since vector of length 2, explains why code returns 2.

it sounds want count number of elements equal 6, ignoring missing values. with:

sum(test$v3 == 6, na.rm=true) # [1] 1

sum(!is.na(test$v3) & test$v3 == 6) # [1] 1

Call