On Mon, 20 Aug 2018 08:19:12 +1000 Cameron Simpson <c...@cskk.id.au> wrote:
[sorry for late reply] > Someone else has descibed zip tersely: it pairs it the elements of 2 > lists. In fact it joins up matching elements of an arbitrary number > of iterables. Here is a 3 iterable example: > > >>> zip( (1,2,3), (4,5,6), (7,8,9) ) > <zip object at 0x10aa4ce88> > >>> list( zip( (1,2,3), (4,5,6), (7,8,9) ) ) > [(1, 4, 7), (2, 5, 8), (3, 6, 9)] > > See that is has collected the first element of each tuple, then the > second and so forth? Yep, an image or an example says more than 1000 pages of explanation > In my sentence above, "iterable" means any object you can iterate > over - any object you could use in a for-loop. So, obviously, a list > as in your programme. And also a tuple like (1,2,3) as in the example > above (a tuple is an unmodifiable list). ANd any number of other > things that will yield a sequence of values. > > In Python 3 (which you're using) zip returns a lazy object, which > does the zipping as you request the values. That is why the example > shows a base "zip()" returning a "zip object" - we hand it to list() > to get an actual list of all the values. This is fast (instant return > of the zip object without doing much work) and cheap in memory (no > need to make a huge result if you're zipping big lists) and > cheap in time (no need to make the whole result if you're only using > the first part of it). > > So, to the code using the zip: > > for device_nr, (pcf, pcf_value) in enumerate(zip(list_pcf, > list_pcf_value)): > > The inner zip yields (pcf, pcf_value) pairs from zipping list_pcf and > list_pcf_value, an iterable of: > > (pcf[0], pcf-value[0]), (pcf[1], pcf_value[1]), ... > > and you can write a for loop like this: > > for pcf, pcf_value in zip(list_pcf, list_pcf_value): > > to deal with each of those pairs in turn. > > However, since you also need the index (device_nr) in order to update > list_pcf_value: > > list_pcf_value[device_nr] = output > > we then hand the whole thing to enumerate: > > for device_nr, (pcf, pcf_value) in enumerate(zip(list_pcf, > list_pcf_value)): > > The enumerate does just what the previous one did, yields pairs of > (index, value). Each value is a pair from the zip, so it yields: > > (0, (pcf[0], pcf-value[0])), > (1, (pcf[1], pcf_value[1])), > ... > > and Python's tuple unpacking syntax is fairly generous, so you want > write: > > device_nr, (pcf, pcf_value) = (0, (pcf[0], pcf-value[0])) > > and you can stick that in the for loop, getting the syntax you're > using in the programme: > > for device_nr, (pcf, pcf_value) in enumerate(zip(list_pcf, > list_pcf_value)): Well, that's a nice and clear explanation, at the prompt I played a bit with the enumerate and zip functions and after all it's quite simple :) > >The code is now as follows: > > Much nicer. A few tiny nit picks: [..] changes applied > >while True: > > if GPIO.input(23) == 1: # if still 0, another event has occurred > > GPIO.wait_for_edge(23, GPIO.FALLING) > > Can the GPIO ops also raise IOErrors? Just wondering. Probably, sounds logigal, the GPIO pin is read and if something is wrong it will generate an I/O error. R. -- richard lucassen http://contact.xaq.nl/ -- https://mail.python.org/mailman/listinfo/python-list
