Is there an R idiom for obtainig the index of the minimum element in a vector after filtering by a boolean index vector?

After you do x[b], the resulting vector has no memory of the original indexes of the values. That information is lost. An alternative would be to alter the values for FALSE to be something very large. For example

which.min(ifelse(b, x, Inf))
# [1] 5

Another alternative is

which(b)[which.min(x[b])] 

Because which(b) is basicially the same as (1:6)[b]

If you have unique values in x :

which(x == min(x[b]))
#[1] 5

If there could be duplicates in x :

which(x == min(x[b]) & b)
#[1] 5

I suggest replace(),

which.min(replace(x, !b, NA))

which is similar to one of GKi's great solutions, but still works if all b are FALSE.

Instead of subsetting x with b the FALSE positions could be set to NA using [<-.

which.min("[<-"(x, !b, NA))
#[1] 5

Alternatively it could also be set to e.g. Inf, as given in the answer from @mrflick, but this will limit the general applicability, to use for which.max it need to be set to -Inf and in case of no TRUE it will return an index.

The logical vector could be converted to indices by using which and those indices could be used for subsetting and be subseted, similar to the solution of @mrflick, but avoiding using which twice.

i <- which(b)
i[which.min(x[i])]
#[1] 5

In case the values in x are all positive you can divide by b what gives for the cases of b == FALSE Inf (and in case of negative x -Inf) - This way is not recommended.

which.min(x / b)
#[1] 5

Comparing with bench::mark:

n <- 1e6
set.seed(42)
x <- sample(0:99, n, TRUE)
b <- sample(c(TRUE,FALSE), n, TRUE)

bench::mark(which.min(ifelse(b, x, Inf))
, which(b)[which.min(x[b])]
#, which(x == min(x[b]))             #Result not equal to others
#, which(x == min(x[b]) & b)         #Result not equal to others
, which.min("[<-"(x, !b, NA))
, which.min("[<-"(x, !b, Inf))
, which.min(x / b)
)
#  expression                        min   median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time
#  <bch:expr>                   <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm>
#1 which.min(ifelse(b, x, Inf))  13.75ms  13.95ms      70.6   30.52MB     61.2    15    13      212ms
#2 which(b)[which.min(x[b])]      5.13ms    5.2ms     192.    19.07MB     92.5    58    28      303ms
#3 which.min(`[<-`(x, !b, NA))    3.58ms   3.67ms     271.    11.44MB     51.2   106    20      391ms
#4 which.min(`[<-`(x, !b, Inf))   4.85ms   4.96ms     200.    19.07MB    100.     50    25      250ms
#5 which.min(x/b)                 3.99ms   4.05ms     246.     7.63MB     22.6   109    10      442ms

b <- logical(n) #No True
bench::mark(#which.min(ifelse(b, x, Inf)) #Wrong result
  which(b)[which.min(x[b])]
, which.min("[<-"(x, !b, NA))
#, which.min("[<-"(x, !b, Inf))           #Wrong result
#, which.min(x / b)                       #Wrong result
)
#  expression                     min median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result
#  <bch:expr>                  <bch:> <bch:>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>
#1 which(b)[which.min(x[b])]   1.18ms 1.21ms      826.    7.63MB     72.9   340    30      412ms <int …
#2 which.min(`[<-`(x, !b, NA)) 5.36ms 5.49ms      181.   15.26MB     38.3    71    15      392ms <int …

b <- !logical(n) #All True
bench::mark(which(b)[which.min(x[b])]
, which.min("[<-"(x, !b, NA))
)
#  expression                     min median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result
#  <bch:expr>                  <bch:> <bch:>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>
#1 which(b)[which.min(x[b])]   5.06ms 5.25ms      184.    19.1MB     92.0    54    27      293ms <int …
#2 which.min(`[<-`(x, !b, NA)) 3.59ms 3.81ms      261.    11.4MB     48.5   102    19      391ms <int …