I"ve got some example Python code that I need to mimic in C++. I do not require any specific solution (such as co-routine based yield solutions, although they would be acceptable answers as well), I simply need to reproduce the semantics in some manner.
This is a basic sequence generator, clearly too large to store a materialized version.
def pair_sequence(): for i in range(2**32): for j in range(2**32): yield (i, j)
The goal is to maintain two instances of the sequence above, and iterate over them in semi-lockstep, but in chunks. In the example below the
first_pass uses the sequence of pairs to initialize the buffer, and the
second_pass regenerates the same exact sequence and processes the buffer again.
def run(): seq1 = pair_sequence() seq2 = pair_sequence() buffer =  * 1000 first_pass(seq1, buffer) second_pass(seq2, buffer) ... repeat ...
The only thing I can find for a solution in C++ is to mimic
yield with C++ coroutines, but I haven"t found any good reference on how to do this. I"m also interested in alternative (non general) solutions for this problem. I do not have enough memory budget to keep a copy of the sequence between passes.
Equivalent C++ to Python generator pattern find: Questions
Finding the index of an item in a list
Given a list
["foo", "bar", "baz"] and an item in the list
"bar", how do I get its index (
1) in Python?
>>> ["foo", "bar", "baz"].index("bar") 1
Reference: Data Structures > More on Lists
Note that while this is perhaps the cleanest way to answer the question as asked,
index is a rather weak component of the
list API, and I can"t remember the last time I used it in anger. It"s been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about
list.index follow. It is probably worth initially taking a look at the documentation for it:
list.index(x[, start[, end]])
Return zero-based index in the list of the first item whose value is equal to x. Raises a
ValueErrorif there is no such item.
The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.
Linear time-complexity in list length
index call checks every element of the list in order, until it finds a match. If your list is long, and you don"t know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give
index a hint. For instance, in this snippet,
l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight
l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:
>>> import timeit >>> timeit.timeit("l.index(999_999)", setup="l = list(range(0, 1_000_000))", number=1000) 9.356267921015387 >>> timeit.timeit("l.index(999_999, 999_990, 1_000_000)", setup="l = list(range(0, 1_000_000))", number=1000) 0.0004404920036904514
Only returns the index of the first match to its argument
A call to
index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.
>>> [1, 1].index(1) 0 >>> [i for i, e in enumerate([1, 2, 1]) if e == 1] [0, 2] >>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1) >>> next(g) 0 >>> next(g) 2
Most places where I once would have used
index, I now use a list comprehension or generator expression because they"re more generalizable. So if you"re considering reaching for
index, take a look at these excellent Python features.
Throws if element not present in list
A call to
index results in a
ValueError if the item"s not present.
>>> [1, 1].index(2) Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: 2 is not in list
If the item might not be present in the list, you should either
- Check for it first with
item in my_list(clean, readable approach), or
- Wrap the
indexcall in a
try/exceptblock which catches
ValueError(probably faster, at least when the list to search is long, and the item is usually present.)
One thing that is really helpful in learning Python is to use the interactive help function:
>>> help(["foo", "bar", "baz"]) Help on list object: class list(object) ... | | index(...) | L.index(value, [start, [stop]]) -> integer -- return first index of value |
which will often lead you to the method you are looking for.
The majority of answers explain how to find a single index, but their methods do not return multiple indexes if the item is in the list multiple times. Use
for i, j in enumerate(["foo", "bar", "baz"]): if j == "bar": print(i)
index() function only returns the first occurrence, while
enumerate() returns all occurrences.
As a list comprehension:
[i for i, j in enumerate(["foo", "bar", "baz"]) if j == "bar"]
Here"s also another small solution with
itertools.count() (which is pretty much the same approach as enumerate):
from itertools import izip as zip, count # izip for maximum efficiency [i for i, j in zip(count(), ["foo", "bar", "baz"]) if j == "bar"]
This is more efficient for larger lists than using
$ python -m timeit -s "from itertools import izip as zip, count" "[i for i, j in zip(count(), ["foo", "bar", "baz"]*500) if j == "bar"]" 10000 loops, best of 3: 174 usec per loop $ python -m timeit "[i for i, j in enumerate(["foo", "bar", "baz"]*500) if j == "bar"]" 10000 loops, best of 3: 196 usec per loop
Equivalent C++ to Python generator pattern repeat: Questions
Create list of single item repeated N times
I want to create a series of lists, all of varying lengths. Each list will contain the same element
n times (where
n = length of the list).
How do I create the lists, without using a list comprehension
[e for number in xrange(n)] for each list?
You can also write:
[e] * n
You should note that if e is for example an empty list you get a list with n references to the same list, not n independent empty lists.
At first glance it seems that repeat is the fastest way to create a list with n identical elements:
>>> timeit.timeit("itertools.repeat(0, 10)", "import itertools", number = 1000000) 0.37095273281943264 >>> timeit.timeit(" * 10", "import itertools", number = 1000000) 0.5577236771712819
But wait - it"s not a fair test...
>>> itertools.repeat(0, 10) repeat(0, 10) # Not a list!!!
itertools.repeat doesn"t actually create the list, it just creates an object that can be used to create a list if you wish! Let"s try that again, but converting to a list:
>>> timeit.timeit("list(itertools.repeat(0, 10))", "import itertools", number = 1000000) 1.7508119747063233
So if you want a list, use
[e] * n. If you want to generate the elements lazily, use
What is the best way to repeatedly execute a function every x seconds?
I want to repeatedly execute a function in Python every 60 seconds forever (just like an NSTimer in Objective C). This code will run as a daemon and is effectively like calling the python script every minute using a cron, but without requiring that to be set up by the user.
while True: # Code executed here time.sleep(60)
Are there any foreseeable problems with this code?
If your program doesn"t have a event loop already, use the sched module, which implements a general purpose event scheduler.
import sched, time s = sched.scheduler(time.time, time.sleep) def do_something(sc): print("Doing stuff...") # do your stuff s.enter(60, 1, do_something, (sc,)) s.enter(60, 1, do_something, (s,)) s.run()
If you"re already using an event loop library like
kivy, and many others - just schedule the task using your existing event loop library"s methods, instead.
Lock your time loop to the system clock like this:
import time starttime = time.time() while True: print "tick" time.sleep(60.0 - ((time.time() - starttime) % 60.0))