Making an Irregular Sequence Iterator

Create an Iterator object, where the user defines a sequence and a set of initial values, and then calls yield_next() to generate the next element of the sequence. Iterators are R environments, which means they are modified in place, even when passed as arguments to functions. To make a copy of an Iterator that can be modified separately, see clone().

Iterator(result, initial, yield)

Arguments

result	R expression to run each time 'yield_next' is called
initial	named list or vector; declare and initialize every variable that appears in 'result'
yield	variable to yield when 'yield_next()' is called

Value

An environment object of S3 type Iterator

Note

The expression to be evaluated can include constant values not defined in $initial as long as they are defined in the enclosure of where yield_next() is called, not where the Iterator is created. These values will not vary from iteration to iteration (unless you do something strange in the code, like including <<- in $result.)

See also

yield_next(), yield_while(), current() rlang::qq_show()

Examples

#Create the Collatz sequence starting with 50 and print out the first 30 elements
collatz <- Iterator({
            if (n %% 2 == 0) n <- n / 2 else n <- n*3 + 1
           },
           initial = c(n = 50),
           yield = n)

seq <- yield_more(collatz, 30)

# If you want to define the expression outside the Iterator, use [quote()] and `!!`:
expr <- quote(if (n %% 2 == 0) n <- n / 2 else n <- n*3 + 1)
collatz <- Iterator(!!expr,
                    c(n = 50),
                    n)

# using objects defined outside `$initial`:
# Note that `n` in `$initial` overrides the global `n`
m <- 100
n <- 10
it <- Iterator({out <- n + m},
               initial = c(n = -10),
               yield = out)

yield_next(it)
#> [1] 90

# environments are modified in place, so be aware:
it <- Iterator({m <- m + 1}, c(m = 0), m)
other <- it
yield_next(it)
#> [1] 1
current(other)
#> [1] 1