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prepping for interviews and this is something i couldnt find. https://blog.finxter.com/python-list-reverse/#:~:text=The%20time%20complexity%20of%20the,the%20number%20of%20list%20elements. says .reverse() is the O(n) which I assume is because it replaces every index. is [::-1] just printing the list backwards or is something similar going on
list.reverse operates in place. That means that it almost certainly does something like this under the hood:
for i in range(len(self) // 2):
j = len(self) - 1 - i
self[i], self[j] = self[j], self[i]
The actual method is implemented in C, so of course it will be much more verbose, especially in the swapping operation.
list[::-1] creates a copy of the original. The idea is similar, but since the reversal of not in place, you have the luxury of just writing to a destination index rather than swapping via a temporary object:
new = [None] * len(self)
for i in range(len(self)):
j = len(self) - 1 - i
new[j] = self[i]
The actual slice processing code is much more complex since it has to be able to accept integers and all sorts of unorthodox slice expressions that set custom start and stop bounds, and step sizes.
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for pl in pokemon_list:
if pl["pokemon"] == showtype:
result = f"stamina: {'{:.2f}'.format((pl['stamina']/statmax)*100)}% ("
for i in range(pl["stamina"]):
result += "*"
How do I convert the code above to a more pythonic way?
First, using str.format() inside of an f-string is not sensible. Pick one
technique or the other, depending on the situation. For readability and
maintainability, do the computations outside of the f-string. An f-string is an
awkward context for computation; but it's perfect for simple variable
interpolation. In your case, you'd probably be better off sticking to
format().
Second, list comprehension is just a tool. It's not the holy grail of Python.
Select the tool only when it makes sense. One key test is whether you want
to create a list. You do not, as far as I can tell.
Your comment implies that you want to return the first matching item during the
iteration. If so, just do it. While you're at it, help your reader by using
convenience variables to reduce the visual weight of the code.
fmt = "stamina: '{:.2f}'% ({}"
for pl in pokemon_list:
if pl["pokemon"] == showtype:
s = pl['stamina']
return fmt.format(100 * s / statmax, '*' * s)
Could this be rewritten in the style of a comprehension? Yes, but it would be
less readable, even after breaking it apart into three lines of code. Avoid the
temptation to write overly fancy code like this, which will just leave your
readers scratching their heads.
fmt = "stamina: '{:.2f}'% ({}"
gen = (
fmt.format(100 * pl['stamina'] / statmax, '*' * pl["stamina"])
for pl in pokemon_list:
if pl["pokemon"] == showtype
)
return next(gen, None)
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I've coded a check for integers which returns a boolean value:
def NoZerosFives(n: str):
if '0' in n or '5' in n: #It checks if any of the digits is 0 or 5
return False
return True
which I then put in a script:
for n in range(10**20000, 10**100000000):
d = str(n)
if NoZerosFives(d):
pass
else:
print('Failed the check.')
n is obviously extremely large, and the else:print('Failed the check.') is here just to tell me if the script's going or not, but it's not printing anything.
My first thought is that the problem is not with Python but with my hardware (I have a pretty powerful laptop, but still a laptop). Is my computer not powerful enough? Should I have done something to my PC that I didn't do?
Or is the code not fit for the objective and I just need to do something different?
As #jordanm points out in a comment, NoZerosFives will not return True (which is the only way your code produces any output at all) for a very long time. Think about how long it's going to take, starting from 10**20000 (shown below), to reach a value with no zeros in it by just adding 1 at each step.
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0
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Could you please tell me why it is considered as "not pythonic" when I need the index and the value when looping over a list and use:
a = [1,2,3]
for i in range(len(a)):
# i is the idx
# a[i] is the value
but rather it is recommended to use
for idx, val in enumerate(a):
print idx, val
who defines "pythonic" and why is the latter one better? I mean it's not that much better concerning readability, is it!?
Thanks in advance
First of all, the first way is ugly: You either need a separate variable assignment to get the element or use a[i] all the time which could theoretically be an expensive operation. Imagine a being a database cursor: When you iterate it (a.__iter__ being called) the object can safely assume that you are going to iterate over all its items. So all or at least multiple rows could be retrieved at once. When getting the length such an optimization would be stupid though since you surely don't want to retrieve data just because you want the number of items. Also, when retrieving a specific item you cannot assume that other items will be retrieved, too.
Additionally, using enumerate() works with any iterable while range(len()) only works with countable, indexable objects.
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I am making an attempt to learn recursion better so I have been practicing some problems.
A common problem is flatten an list of lists to a single list. For example
[3,4,[1,2]] so the desired result will be [3,4,1,2].
The way I am approaching the problem is:
is 0 element a list - no
is 1 element a list - no
is 2 element a
list - yes
call the function again and do tests again
My base case will be to return if element is not a list.
Is there a better approach to understand recursion better?
Your approach is in the right direction, but you should not check more than the first element. In general - a "pure" recursive solution will only have to look at the "head" of the list (first element), and the "tail" (list[1:]).
something like (python like pseudo code):
def flatten(l):
if len(l) == 0:
return []
if isinstance(l[0], list):
left = flatten(l[0])
else:
left = [l[0]]
return left + flatten(l[1:])
You example is certainly one that works however it may be slightly too simple for you to get a full understanding. I found that maze problems were the most helpful in me understanding recursive problems. The main preface is there is a 2x2 array consisting of two different values. One value denotes a wall within the maze and the other value denotes empty space. Your job would be to traverse through the maze using recursion. Here is an example problem and solution:
https://www.cs.bu.edu/teaching/alg/maze/
Hope this helps!!
You should.
Create a new list (passed) before calling function. Call function.
Loop through the old one and check if the type of list if so, then call function recursive.
else add element to passed list.
aList = [3,4,[1,2]]
def flatten(inList,outList):
for elem in inList:
if isinstance(elem,list):
flatten(elem,outList)
else:
outList.append(elem)
bList = list()
flatten(aList,bList)
print bList
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Could you please tell me why it is considered as "not pythonic" when I need the index and the value when looping over a list and use:
a = [1,2,3]
for i in range(len(a)):
# i is the idx
# a[i] is the value
but rather it is recommended to use
for idx, val in enumerate(a):
print idx, val
who defines "pythonic" and why is the latter one better? I mean it's not that much better concerning readability, is it!?
Thanks in advance
First of all, the first way is ugly: You either need a separate variable assignment to get the element or use a[i] all the time which could theoretically be an expensive operation. Imagine a being a database cursor: When you iterate it (a.__iter__ being called) the object can safely assume that you are going to iterate over all its items. So all or at least multiple rows could be retrieved at once. When getting the length such an optimization would be stupid though since you surely don't want to retrieve data just because you want the number of items. Also, when retrieving a specific item you cannot assume that other items will be retrieved, too.
Additionally, using enumerate() works with any iterable while range(len()) only works with countable, indexable objects.