I have an python 3 code as follow:
a = []
b = [[0] * len(a[0]) for _ in range(len(a))]
The above code works fine, but the follow code does not work:
a = []
m, n = len(a), len(a[0])
len(a[0]) apppears in both codes, why the list comprehension does not through IndexError: list index out of range.
Thanks,
range(len(a)) in this case is essentially range(0), which is an empty range:
>>> list(range(0))
[]
Because the collection being iterated over is empty, the comprehension never runs, so a[0] is never evaluated.
It's similar to how this loop prints nothing:
for _ in []:
print("SOME TEXT!")
[] is empty, so the for loop never iterates.
With m, n = len(a), len(a[0]) however, a[0] is run regardless of the length of a, so a[0] is evaluated, and you get an error.
Related
I am trying to do an if/else with a list comprehension (I think). I tried to follow a normal if conditional format with the if/else at the end. I tried to add 'num for num in nums'. A few other things like the commented code at the bottom...but the best I can do is make res[] a list of 'None's.
I'm sure this will never be crucial and a normal if/else in a for loop works, it's just that I read Trey Hunner's visual take on List Comps and wondered about an else.
nums = [2,53,4,7,11,1]
print(nums)
res = []
for num in nums:
if num <= 7:
res.append("less")
else:
res.append("more")
print(res)
nums = [2,53,4,7,11,1]
print(nums)
res = []
res = [res.append("less") if num <= 7
else res.append("more")
for num in nums
]
print(res)
# [a if tC else b for i in items if fC]
# [item
for sublist in myList
for item in (sublist if type(sublist) is list else sublist['val'])
]
You can do it this way:
["less" if num <= 7 else "more" for num in nums]
Your second chunk of code is broken: you call res.append inside the list comprehension. Don't do that, you just need to produce individual values as I've done above.
I am running Python 3.6.2 on Windows 10 and was learning about the zip() function.
I wanted to print part of the object returned by the zip() function.
Here is my code, without the troublesome print statement:
a = ("John", "Charles", "Mike")
b = ("Jenny", "Christy", "Monica", "Vicky")
x = zip(a, b)
tup = tuple(x)
print(tup)
print(type(tup))
print(len(tup))
print(tup[1])
Here is my code with the troublesome print statement:
a = ("John", "Charles", "Mike")
b = ("Jenny", "Christy", "Monica", "Vicky")
x = zip(a, b)
print(tuple(x)[1])
tup = tuple(x)
print(tup)
print(type(tup))
print(len(tup))
print(tup[1])
The print(tuple(x)[1]) statement appears to change the tuple 'tup' into a zero-length one and causes the print(tup[1]) to fail later in the code!
In this line, you create an iterator:
x = zip(a, b)
Within the print statement, you convert the iterator to a tuple. This tuple has 3 elements. This exhausts the iterator and anytime you call it afterwards, it will return no further elements.
Therefore, upon your creation of tup, your iterator does not return an element. Hence, you have a tuple with length 0. And of course, this will raise an exception when you try to access the element with index 1.
For testing, consider this:
a = ("John", "Charles", "Mike")
b = ("Jenny", "Christy", "Monica", "Vicky")
x = zip(a, b)
tup1 = tuple(x)
tup2 = tuple(x)
print(tup1)
print(tup2)
It will give you the following result:
(('John', 'Jenny'), ('Charles', 'Christy'), ('Mike', 'Monica'))
()
This is basically what you do when creating a tuple out of an iterator twice.
so i have a bunch of numbers i've tupled but am having difficulty remove an item from the zipped list.
so far i've tried .remove on the list but that gave me an error.
is there an easy way of doing this?
this is my current code:
Example data:
QueenRowColumn: 3,3
TheComparisonQueen: 7,3
def CheckQueenPathDown(self, QueenRowColumn, TheComparisonQueen):
row = []
column = []
CurrentLocation = QueenRowColumn
#MoveLocation = TheComparisonQueen
a = QueenRowColumn[0]
b = QueenRowColumn[1]
for i in range (-7,0):
row.append(CurrentLocation[1] - i)
column.append(a)
Down = zip(row,column)
#Down.remove(TheComparisonQueen)
return Down
if i, for example were to remove "TheComparisonQueen" from the list of tuples, how would i do it?
If you just looking to drop TheComparisonQueen from iterator of tuples you can return values that are not equal to TheComparisonQueen using a list comprehension or a generator expression.
# List Comprehension
Down = [(i,j) for i,j in zip(row,column) if (i,j) != TheComparisonQueen]
# Generator Expression
Down = ((i,j) for i,j in zip(row,column) if (i,j) != TheComparisonQueen)
Am trying to check if elements of a list match elements of another. But there is a slight twist to the problem.
alist = ['949', '714']
blist = ['(714)824-1234', '(419)312-8732', '(949)555-1234', '(661)949-2867']
Am trying to match the elements of alist to the blist, but only the area code part(in blist). Here is my current code:
def match_area_codes(alist, blist):
clist =[]
for i in alist:
for j in blist:
if i in j:
clist.append(j)
return clist
The code works for the most part, except when there is a string matching the area code anywhere else in the list. It should only print:
['(714)824-1234', '(949)555-1234']
but it ends up printing
['(714)824-1234', '(949)555-1234', '(661)949-2867']
as there is a '949' in the last phone number. Is there a way to fix this?
You can use a regular expression to get the part within (...) and compare that part to alist.
import re
def match_area_codes(alist, blist):
p = re.compile(r"\((\d+)\)")
return [b for b in blist if p.search(b).group(1) in alist]
Example:
>>> alist = set(['949', '714'])
>>> blist = ['(714)824-1234', '(419)312-8732', '(949)555-1234', '(661)949-2867']
>>> match_area_codes(alist, blist)
['(714)824-1234', '(949)555-1234']
If you really really want to do it without regular expressions, you could, e.g., find the position of the ( and ) and thus get the slice from the string corresponding to the region code.
def match_area_codes(alist, blist):
find_code = lambda s: s[s.index("(") + 1 : s.index(")")]
return [b for b in blist if find_code(b) in alist]
However, I would strongly suggest to just take this as an opportunity for getting started with regular expressions. It's not all that hard, and definitely worth it!
from math import ceil
def merge(all_lst):
sorted_lst = []
while all_lst:
min_value,index = all_lst[0][0],0
for lst in all_lst:
if lst[0]<min_value:
min_value = lst[0]
index = all_lst.index(lst)
sorted_lst.append(min_value)
all_lst[index].pop(0)
if not all_lst[index]:
all_lst.remove(all_lst[index])
return sorted_lst
def merge_sort(lst, k):
def split(lst):
split_lst = []
j = ceil(len(lst)/k) if len(lst)>=k else 1
for i in range(0,len(lst),j):
split_lst.append(lst[i:i+j])
return split_lst
lst=split(lst)
if len(lst[0])==1:
return lst
else:
for i in range(len(lst)):
lst[i]=merge(merge_sort(lst[i],k))
return merge(lst)
Above is my code for k-way merge sort. Basically what it does is split the list into k smaller list by calling the split function until each sublist in the list is a single element. Then the list containing sublists will be merged into one single list.
My code works fine when splitting is done twice. (eg.[3,6,8,5,2,1,4,7] -> [3,6,8],[5,2,1],[4,7] -> [3],[6],[8],[5],[2],[1],[4],[7]). But when the splitting is done more than twice, (eg,[3,6,8,5,2,1,4,7] -> [3,6,8,5],[2,1,4,7] -> [3,6],[8,5],[2,1],[4,7] -> [3],[6],[8],[5],[2],[1],[4],[7]), the code will fail. Can anyone help find me find out what goes wrong in my code? Thanks in advance.
I believe the problem you're having is that merge_sort sometimes returns a flattened list and other times returns a list of lists. You should probably return a flat list in all cases. There's some other cruft: You don't need split to be its own function, since you only call it the one time.
Here's a greatly simplified version of your code:
def merge_sort(lst, k):
if len(lst) == 1: # simpler base case
return lst
j = ceil(len(lst)/k) # no need to check for k < len(lst) (ceil handles it)
#split and recursively sort in one step
lst = [merge_sort(lst[i:i+j], k) for i in range(0, len(lst), j)]
return merge(lst) # always return a merged list (never a list of lists)