define a function using an iterative function

Question

I have some function FUN(a, b): on two characters. I want to define another function foo(s), such that

foo(s):
    FUN(a[0],a[1])
    FUN(a[2],a[3])
    FUN(a[4],a[5])
    ...

for all characters in s (assume s is even length). My thoughts are that we basically need to run FUN(a,b) (len(s)%2) times, but I'm not sure how to iterate a function in that way, while also making sure FUN has the right inputs. Any ideas?

for i in range(0, len(s), 2): FUN(s[i], s[i+1]) or for i in range(0, len(s), 2): FUN(*s[i:i+2]) — falsetru
– falsetru, Commented Oct 31, 2013 at 11:01
@GamesBrainiac well, your answer is less readable and probably worse performance-wise than falsetru's comment. I definitely wouldn't prefer it... — l4mpi
– l4mpi, Commented Oct 31, 2013 at 11:38

Nafiul Islam · Accepted Answer · 2013-10-31 11:10:49Z

2

Well this would be easy to do with zip:

def fun(a, b):
    print a, b


def foo(s):
    for x, y in zip(s[::2], s[1::2]):
        fun(x, y)


foo("12345678")

Output:

A more efficient way, using generators, would be using izip (same output):

from itertools import izip


def fun(a, b):
    print a, b


def foo(s):
    for x, y in izip(s[::2], s[1::2]):
        fun(x, y)


foo("12345678")

edited Oct 31, 2013 at 11:10

answered Oct 31, 2013 at 11:04

Nafiul Islam

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1 Comment

GBN89 Over a year ago

thanks this is similiar to what i wanted. what would the easiest way be to concatenate all the outputs into a single string?

Matthias · Accepted Answer · 2013-10-31 11:56:24Z

1

When a question ist tagged with iteration there must be a way to do it with iter. :-)

values = range(10)

def do_work(x, y):
    print('{}_{}'.format(x, y))

it = iter(values)
try:
    while it:
        do_work(next(it), next(it))
except StopIteration:
    pass

Nice suggestion from l4mpi:

it = iter(values)
for value in it:
    do_work(value, next(it))

edited Oct 31, 2013 at 11:56

answered Oct 31, 2013 at 11:13

Matthias

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3 Comments

Nafiul Islam Over a year ago

Erm, using exceptions for program flow is a bad idea. They're supposed to catch errors. Atleast, thats what I'm used to anyways.

l4mpi Over a year ago

This can be rewritten as a for loop: for x in it: do_work(x, next(it)) - avoids catching the StopIteration, but only works for input with an even number of elements. @GamesBrainiac you don't have any idea how a for loop is implemented in python, do you? It uses the same StopIteration exception to find out when the sequence it iterates over runs out of values - and that's required, as iterators in python by design only have a next method, but no has_more or is_empty method.

Matthias Over a year ago

I knew that there had to be a simple solution. :-( A lot of years with Python and still learning. :-)

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