>> I'm trying to generate a list of values > > Better to think of a sequence of values, whether materialized as a 'list' or > not.
The final value will actually be a string but it seems it is usually faster to join a list of strings than to concat them one by one. >> where each value is dependent >> on the previous value in the list and this bit of code needs to be >> repeatedly so I'd like it to be fast. It doesn't seem that >> comprehensions will work as each pass needs to take the result of the >> previous pass as it's argument. map() doesn't seem likely. filter() or > > Comprehensions combine map and filter, both of which conceptually work on > each item of a pre-existing list independently. (I am aware that the > function passed can stash away values to create dependence. The problem is I don't really have a pre-existing list. I just want a tight loop to generate the list from the initial value. Probably I could do something similar to my filter() method below but it seems really convoluted. I thought maybe there was a better way I just didn't know about. Python is full of neat tricks but it can be hard to remember them all. >> reduce() seem workable but not very clean. Is there a good way to do >> this? About the best I can get is this: >> >> l = [ func ( start ) ] >> f = lambda a: func ( l[-1] ) or a >> filter ( f, range ( big_number, -1, -1 ) ) >> >> >> I guess I'm looking for something more like: >> >> l = do ( lambda a: func ( a ), big_number, start ) > > Something like > > def do(func, N, value): > yield value > for i in range(1,N): > value = func(value) > yield value > > ? > For more generality, make func a function of both value and i. > If you need a list, "l = list(do(f,N,x))", but if you do not, you can do > "for item in do(f,N,x):" and skip making the list. Generators seem considerably slower than using comprehension or map/filter/reduce. I have tons of memory to work in (~100GB actual RAM) and the resulting data isn't that big so I'm more concerned about CPU. Thanks. -- http://mail.python.org/mailman/listinfo/python-list
