how to make a generator use the last yielded value when it regains control

[Ben Cartwright]

John Salerno wrote:
> It
> is meant to take a number and generate the next number that follows
> according to the Morris sequence. It works for a single number, but what
> I'd like it to do is either:
>
> 1. repeat indefinitely and have the number of times controlled elsewhere
> in the program (e.g., use the morris() generator in a for loop and use
> that context to tell it when to stop)
>
> 2. just make it a function that takes a second argument, that being the
> number of times you want it to repeat itself and create numbers in the
> sequence

Definitely go for (1).  The Morris sequence is a great candidate to
implement as a generator.  As a generator, it will be more flexible and
efficient than (2).

def morris(num):
    """Generate the Morris sequence starting at num."""
    num = str(num)
    yield num
    while True:
        result, cur, run = [], None, 0
        for digit in num+'\n':
            if digit == cur:
                run += 1
            else:
                if cur is not None:
                    result.append(str(run))
                    result.append(cur)
                cur, run = digit, 1
        num = ''.join(result)
        yield num

# Example usage:
from itertools import islice
for n in islice(morris(1), 10):
    print n

# Output:
"""
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
"""

--Ben