Span: A Predicate Functional that finds the location of the longest sequential run of elements where the predicate is true

Span <- function(f, x)
{ 
    ## Purpose: A Predicate Functional that finds the location of the longest sequential run
    ##          of elements where the predicate is true. 
    ## Arguments:
    ##   f: a predicate (a function that returns a single TRUE or FALSE) 
    ##   x: a list or data frame
    ## Return: The location of the longest sequential run of elements where the predicate is true. 
    ## Author: Feiming Chen, Date: 28 Jun 2017, 09:45
    ## ________________________________________________

    y <- rle(Where(f, x))
    z <- y$lengths
    w <- which.max(z)          # position of the segment with longest sequential run of TRUE
    pos.end <- sum(z[1:w])
    pos.start <- pos.end - z[w] + 1
    c(start=pos.start, end=pos.end)
}
if (F) {                                # Unit Test
    x <- c(3, 1, NA, 4, NA, NA, NA, 5, 9, NA, NA)
    Span(is.na, x)
    ## start   end 
    ##     5     7 
}

Where <- function(f, x)
{ 
    ## Purpose: A Predicate Functional
    ## See other Predicate Functionals: Filter(), Find(), Position()
    ## See other Functionals: Map(), Reduce() 
    ## Arguments:
    ##   f: a predicate (a function that returns a single TRUE or FALSE) (e.g. is.character, all, is.NULL)
    ##   x: a list or data frame
    ## Return: a logical vector 
    ## Author: Feiming Chen, Date: 28 Jun 2017, 09:45
    ## ________________________________________________

    vapply(x, f, logical(1))
}
if (F) {                                # Unit Test
    df <- data.frame(x = 1:3, y = c("a", "b", "c"), stringsAsFactors = T)
    Where(is.factor, df)
    ## Compare with: 
    Filter(is.factor, df)
    Find(is.factor, df)
    Position(is.factor, df)
}