R中的ifelse语句

Question

I'm looking at a gene in 10 people. And this gene has two alleles, say a and b . And each allele has 3 forms: type 2, 3 or 4.

a <- c(2, 2, 2, 2, 3, 3, 3, 2, 4, 3)
b <- c(4, 2, 3, 2, 4, 2, 3, 4, 4, 4)

I wish to code a variable that tells me how many type 4 alleles the person has: 0, 1, or 2.

var <- ifelse(a==4 & b==4, 2, 0)

The code above doesn't work since I didn't account for the individuals who have just one copy of the type 4 allele. I feel like I might need 2 ifelse statements that work simultaneously?

Answer 1

EDIT: You don't actually need ifelse or any fancy operations other than plus and equal to.

var <- (a == 4) + (b == 4)

If you're set on ifelse , this can be done with

var <- ifelse(a == 4, 1, 0) + ifelse(b == 4, 1, 0)

However, I prefer the following solution using apply . The following will give you three cases, the result being the number of 4's the person has (assuming each row is a person).

a = c(2, 2, 2, 2, 3, 3, 3, 2, 4, 3)
b = c(4, 2, 3, 2, 4, 2, 3, 4, 4, 4)

d <- cbind(a,b)

apply(d, 1, function(x) {sum(x == 4)})

For this operation, I first combined the two vectors into a matrix since it makes applying the function easier. In R, generally if data are the same type it is easier (and faster for the computer) to combine the data into a matrix/data frame/etc., then create a function to be performed on each row/column/etc.

To understand the output, consider what happens to the first row of d.

> d[1, ]
a b 
2 4

> d[1, ] == 4
a     b 
FALSE  TRUE

Booleans are interpreted as integers under addition, so

> FALSE + TRUE
[1] 1

It doesn't seem to matter whether the 4 came from a or b, so we end up with three cases: 0, 1, and 2, depending on the number of 4's.