More efficient strategy for which() or match()

Question

I have a vector of positive and negative numbers

vec<-c(seq(-100,-1), rep(0,20), seq(1,100))

the vector is larger than the example, and

Answer 1

Use sum() and logical comparison:

sum( vec < 0 )
[1] 100

This will be pretty quick, and when you sum a logical, TRUE is 1 and FALSE is 0 so the total will be the number of negative values.

Uh oh, I feel the need for a benchmarking comparison... :-) Vector length is 2e5

library(microbenchmark)
vec<-c(seq(-100,-1,length.out=1e5), rep(0,20), seq(1,100,length.out=1e5))
microbenchmark( (which.min(vec < 0) - 1L) , (sum( vec < 0 )) )

Unit: milliseconds
                      expr      min       lq   median       uq       max neval
 (which.min(vec < 0) - 1L) 1.883847 2.130746 2.554725 3.141787 75.943911   100
            (sum(vec < 0)) 1.398100 1.500639 1.508688 1.745088  2.662164   100