stoopid python question (language, not reptile)

[rmd]

ETA: solved!

Very basic python question for folks what know python.

So, I'm messing about with python a bit (rev 2.7.3), and I'm doing some very basic just-above-hello-world stuff. I want to take two lines of input and merge them, but if the second line starts with a + (like some text clients do when inserting a line break), I want to delete the +. But this:

import sys
line1 = sys.stdin.readline()[:-1]
line2 = sys.stdin.readline()[:-1]
if line2[0] == '\+'
 line1 = line1 + line2[1:]
print line1

fails with this error message:

  File "foo.py", line 4
    if line2[0] == '\+'
                      ^
SyntaxError: invalid syntax

I tried using " quotes and with and without the \ escape character. What's the right syntax, here? (google was unhelpful, or at least I had too much trouble distilling the results down to things that were relevent.)
also, man, python is weird!

Comments

[annathepiper]

Python is DEFINITELY weird. I've been writing a crapton of automation in at work for the last couple of years, and it's very much one of the weirder languages I've ever done anything in!

I STILL haven't really managed to bend my brain around lambda statements. :D

[charleshaynes.livejournal.com]

print reduce(lambda x, y: x + y.lstrip('+'), sys.stdin.read().split('\n'))

[annathepiper]

I rest my case. :D

[charleshaynes.livejournal.com]

you can think of a lambda as a shorthand for an anonymous function (I'm sure you know this) so the lambda in my example (lambda x, y: x+y.lstrip('+')) is really just

def fn(x,y): return x + y.lstrip('+')

[annathepiper]

Yep! I've played with them just a little bit in code at work. They're still just damned weird to me. ;)

[charleshaynes.livejournal.com]

Ok, partially prompted by this thread, I decided to write a Sieve of Erathosthenes in python using lambdas and generators. If you like puzzles, here it is:

__author__ = 'chaynes'

from itertools import chain, count, cycle, ifilter, repeat

def primes():
    sieve = []
    for p in ifilter(lambda _: reduce(lambda x, isntMultipleOf: isntMultipleOf.next() and x, sieve, True), count(2)):
        sieve.append(cycle(chain(repeat(True, p-1), (False,))))
        yield p


def main():
    for p in primes():
        print p


if __name__ == "__main__":
    main()

the sieve list holds a list of generators, one for each prime, that generates "False" if a number is not a multiple of that prime, and "True" if it is. It really just generates the sequence (True, True, ... True, False) with a period the length of the prime, over and over.

It has to use reduce(lambda x, y: f(x) and y) instead of "all()" because any/all do lazy/short-circuit evaluation and the generators depend on being called for each integer. That's also why the "isMultipleOf" comes before the "and" and/or also short circuit.

Happy to explain further if anyone is interested.

[rmd.livejournal.com]

... interesting! i'm going to have to stare at that for a while and maybe read up more on python before I come back to it again.

Thanks!

[charleshaynes.livejournal.com]

You're welcome. It's a horrible algorithm though. N^2. This one is based on priority queues and is a better sieve:

__author__ = 'chaynes'

from heapq import heappushpop, heappush
from itertools import count

def primes():
    """
    generate a list of primes

    sieve contains a heap of tuples, (nextLargestMultipleOfPrime, prime)
    top of the heap is the smallest composite bigger or equal to our last prime
    we repeatedly update top of heap until either:
        we find our candidate, in which case it isn't a prime so we increment the candidate
        or the smallest composite is bigger than our candidate
            in which case we push the new prime and its next multiple
    """
    sieve = []
    yield 2
    p = 3
    while True:
        heappush(sieve, (p * p, p))
        yield p
        p += 2
        while sieve[0][0] <= p:
            if (sieve[0][0] == p): p += 2
            heappushpop(sieve, (sieve[0][0] + sieve[0][1], sieve[0][1]))


def main():
    """
    print a generated list of primes
    """
    for (p, _) in zip(primes(), xrange(10000)):
        print p


if __name__ == "__main__":
    main()

Enjoy! and as always happy to answer questions. BTW I just installed Python and Emacs on my phone! I love living in the future.