A Vectorized if-then-else: The ifelse() Function

In addition to the usual if-then-else construct found in most languages, R also includes a vectorized version, the ifelse() function. The form is as follows:

ifelse(b,u,v)

where b is a Boolean vector, and u and v are vectors.

The return value is itself a vector; element i is u[i] if b[i] is true, or v[i] if b[i] is false. The concept is pretty abstract, so let’s go right to an example:

> x <- 1:10
> y <- ifelse(x %% 2 == 0,5,12)  # %% is the mod operator
> y
 [1] 12  5 12  5 12  5 12  5 12  5

Here, we wish to produce a vector in which there is a 5 wherever x is even or a 12 wherever x is odd. So, the actual argument corresponding to the formal argument b is (F,T,F,T,F,T,F,T,F,T). The second actual argument, ...

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