Question

I’ve got a matrix with “A”, “B” and NA values, and I would like to count the number of “A” or “B” or NA values in every column.

sum(mydata[ , i] == "A")

and

sum(mydata[ , i] == "B")

worked fine for columns without NA. For columns that contain NA I can count the number of NAs with sum(is.na(mydata[ , i]). In these columns sum(mydata[ , i] == "A") returns NA as a result instead of a number.

How can i count the number of “A” values in columns which contain NA values?

Thanks for your help!

Example:

> mydata
    V1  V2  V3  V4 
V2 "A" "A" "A" "A"
V3 "A" "A" "A" "A"
V4 "B" "B" NA  NA 
V5 "A" "A" "A" "A"
V6 "B" "A" "A" "A"
V7 "B" "A" "A" "A"
V8 "A" "A" "A" "A"

sum(mydata[ , 2] == "A")
# [1] 6

sum(mydata[ , 3] == "A")
# [1] NA

sum(is.na(mydata[ , 3]))
# [1] 1

Answer 1

The function sum (like many other math functions in R) takes an argument na.rm. If you set na.rm=TRUE, R removes all NA values before doing the calculation.

Try:

sum(mydata[,3]=="A", na.rm=TRUE)