On Saturday, September 22, 2018 at 12:20:08 PM UTC-7, Thomas Jollans wrote: > On 22/09/2018 20:18, Victor via Python-list wrote: > > On Saturday, September 22, 2018 at 6:22:32 AM UTC-7, Peter Otten wrote: > >> Victor via Python-list wrote: > >> > >>> Let me use a different input args and display them below. Basically, I am > >>> hoping to add up all elements of each nested list. So at first it should > >>> start with [1,11,111] ==> 1+11+111 = 123. But instead, it appears to take > >>> the 1st element from each nested list to add up [1,2,3] = 6. How should > >>> it be corrected? Thx. > >> > >> I see three options. You can > >> > >> (1) use a list comprehension > >> > >> [add_all_elements(*sub) for sub in alist] > >> > >> (2) replace map() with itertools.starmap() > >> > >> list(itertools.starmap(add_all_elements, alist)) > >> > >> (3) change your function's signature from add_all_elements(*args) to > >> add_all_elements(args), either by modifying it directly or by wrapping it > >> into another function > >> > >> list(map(lambda args: add_all_elements(*args), alist)) > >> > >> (3a) My personal preference would be to change the signature and then use > >> the list comprehension > >> > >> def add_all_elements(args): ... > >> [add_all_elements(sub) for sub in alist] > > > > Hi Peter, > > Thank you for your suggested solutions. They all work. But I just want to > > know what is wrong with my doing: > > > > list(map(add_all_elements,*alist)) > > > > Theoretically, each list element is passed to add_all_elements. And if my > > alist is [[1, 11, 111], [2, 22, 222], [3, 33, 333]], then the 1st list > > element must be this [1,11,111] passed as args into add_all_elements. > > Now, > > alist = [[1,11,111], [2,22,222], [3,33,333]] > > so `map(add_all_alements, *alist)` is equivalent to > > map(add_all_elements, > [1,11,111], > [2,22,222], > [3,33,333]) > > According to the docs [1], map(function, iterable, ...) > > "applies function to every item of iterable, yielding the results. If > additional iterable arguments are passed, function must take that many > arguments and is applied to the items from all iterables in parallel." > > So map takes the first item(s) of the argument(s), and applies the > function to them, followed by the second item(s), and so on. > > In other words: > > def map(function, *iterables): > for args in zip(iterables): > yield function(*args) > > > [1] https://docs.python.org/3/library/functions.html#map > > Come to think of it, this suggests a rather silly alternative "solution": > > map(add_all_elements, *zip(*alist)) > > > > > In other words, the following should have happened: > > > >>>> add_all_elements (*[1,11,111]) > > My args = (1, 11, 111) > > > > i = 1 > > BEFORE total = 0 > > AFTER total = 1 > > > > i = 11 > > BEFORE total = 1 > > AFTER total = 12 > > > > i = 111 > > BEFORE total = 12 > > AFTER total = 123 > > > > FINAL total = 123 > > > > 123 > > > > Again, thanks! > >
Thanks all, I got it now! -- https://mail.python.org/mailman/listinfo/python-list
