Removal of multiples of a number on a list: - python

I have a number two, I have this list, how would I go about removing multiples of the number 2 from that list and update it?
10 [2, 3, 4, 5, 6, 7, 8, 9, 10] 2

Assuming you have a list l and a number n you can remove all multiples of n from l with a list comprehension:
l = [i for i in l if i%n]
writing if i%n here is the same as writing if i%n != 0, and n divides i iff i%n==0

Methods:
Using a generator
One of the first option that comes to mind is to make use of a generator. The generator would iterate through a sequence, and test if the current element is divisible by n. This allows you to have a more generic solution as well:
def filter_by_multiple(seq, n):
for i in seq:
if i % n:
yield i
Usage:
>>> filter_by_multiple([2, 3, 4, 5, 6, 7, 8, 9, 10], 2)
<generator object filter_by_multiple at 0x000000374ED30258>
>>> list(filter_by_multiple([2, 3, 4, 5, 6, 7, 8, 9, 10], 2))
[3, 5, 7, 9]
>>>
Using a generator expression
While the above solution is fine, is can be shortened even more by using generator expressions. generator expression are like list comprehensions, but unlike them, they return a generator iterator instead of a list.
Usage:
>>> l = [2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> list(el for el in l if el % 2)
[3, 5, 7, 9]
>>>
Using filter():
Among many of the builtin functions in Python, there is one for filtering list called filter(). The usually way to use filter() is to pass in the function you want to use to filter your list, and then the actual list you want filtered. In your case, you want to filter out every element the is not a multiple of two:
Usage:
>>> l = [2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> list(filter(lambda x: x % 2, l))
[3, 5, 7, 9]
>>>
Using a list comprehension
While all of the above are fine ways for filtering a list, probably the most obvious and canonical, is to use a list comprehension. In your case, your list comprehension, is dead simple.
Usage:
>>> l = [2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> [el for el in l if el % 2]
[3, 5, 7, 9]
>>>

Related

Python: Take Every First, Second, Third Element in Sublist

I'm using Python 2.7 and have the following:
my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
I'd like create a 1-d list where the elements are ordered by position in sublist and then order of sublist. So the correct output for the above list is:
[1, 4, 7, 2, 5, 8, 3, 6, 9]
Here's my (incorrect) attempt:
def reorder_and_flatten(my_list):
my_list = [item for sublist in my_list for item in sublist]
result_nums = []
for i in range(len(my_list)):
result_nums.extend(my_list[i::3])
return result_nums
result = reorder_and_flatten(my_list)
This flattens my 2-d list and gives me:
[1, 4, 7, 2, 5, 8, 3, 6, 9, 4, 7, 5, 8, 6, 9, 7, 8, 9]
The first half of this list is correct but the second isn't.
I'd also like my function to be able to handle only 2 sublists. For instance, if given:
[[1, 2, 3], [], [7, 8, 9]
the correct output is:
[1, 7, 2, 8, 3, 9]
Any thoughts?
Thanks!
You're attempting to flatten, and then reorder, which makes things a lot harder than reordering and then flattening.
First, for your initial problem, that's just "unzip", as explained in the docs for zip:
>>> my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> list(zip(*my_list))
... [(1, 4, 7), (2, 5, 8), (3, 6, 9)]
(In Python 2.7, you could just write zip(…) here instead of list(zip(…)), but this way, the same demonstration works identically in both 2.x and 3.x.)
And then, you already know how to flatten that:
>>> [item for sublist in zip(*my_list) for item in sublist]
[1, 4, 7, 2, 5, 8, 3, 6, 9]
But things get a bit more complicated for your second case, where some of the lists may be empty (or maybe just shorter?).
There's no function that's like zip but skips over missing values. You can write one pretty easily. But instead… there is a function that's like zip but fills in missing values with None (or anything else you prefer), izip_longest. So, we can just use that, then filter out the None values as we flatten:
>>> my_list = [[1, 2, 3], [], [7, 8, 9]]
>>> from itertools import izip_longest
>>> list(izip_longest(*my_list))
[(1, None, 7), (2, None, 8), (3, None, 9)]
>>> [item for sublist in izip_longest(*my_list) for item in sublist if item is not None]
[1, 7, 2, 8, 3, 9]
(In Python 3, the function izip_longest is renamed zip_longest.)
It's worth noting that the roundrobin recipe, as covered by ShadowRanger's answer, is an even nicer solution to this problem, and even easier to use (just copy and paste it from the docs, or pip install more_itertools and use it from there). It is a bit harder to understand—but it's worth taking the time to understand it (and asking for help if you get stuck).
result = [l[i] for i in range(max(len(v) for v in my_list)) for l in my_list if l]
i.e.
my_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
[l[i] for i in range(max(len(v) for v in my_list)) for l in my_list if l]
# => [1, 4, 7, 2, 5, 8, 3, 6, 9]
my_list = [[1, 2, 3], [], [7, 8, 9]]
[l[i] for i in range(max(len(v) for v in my_list)) for l in my_list if l]
# => [1, 7, 2, 8, 3, 9]
The itertools module's recipes section provides a roundrobin recipe that would do exactly what you want. It produces a generator, but your expected behavior would be seen with:
# define roundrobin recipe here
from itertools import cycle, islice
def roundrobin(*iterables):
"roundrobin('ABC', 'D', 'EF') --> A D E B F C"
# Recipe credited to George Sakkis
pending = len(iterables)
nexts = cycle(iter(it).next for it in iterables)
while pending:
try:
for next in nexts:
yield next()
except StopIteration:
pending -= 1
nexts = cycle(islice(nexts, pending))
def reorder_and_flatten(my_list):
return list(roundrobin(*my_list))
Your original code's main issue is that it looped over for i in range(len(my_list)):, extending with my_list[i::3]. Problem is, this ends up duplicating elements from index 3 onwards (index 3 was already selected as the second element of the index 0 slice). There are lots of other small logic errors here, so it's much easier to reuse a recipe.
This will be fairly performant, and generalize better than most hand-rolled solutions (it will round robin correctly even if the sublists are of uneven length, and it doesn't require second pass filtering or special handling of any kind to allow None as a value like zip_longest does).
If you are happy to use a 3rd party library, you can use NumPy and np.ndarray.ravel:
import numpy as np
A = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
res_a = A.ravel('F') # array([1, 4, 7, 2, 5, 8, 3, 6, 9])
For the case where you have one or more empty lists, you can use filter to remove empty lists:
B = np.array(list(filter(None, [[1, 2, 3], [], [7, 8, 9]])))
res_b = B.ravel('F') # array([1, 7, 2, 8, 3, 9])
Both solutions require non-empty sublists to contain the same number of items. If list conversion is necessary you can use, for example, res_a.tolist().
While these "black box" methods won't teach you much, they will be faster for large arrays than list-based operations. See also What are the advantages of NumPy over regular Python lists?

Removing the duplicate entries from a list by editing the list

Have a list arr = [1,3,4,5,2,3,4,2,5,7,3,8,1,9,6,2,1,2,1,3,4,3,4,6,9]
want to remove the duplicate values so that the original list should contains single instances of all elements. Do not want to create a extra list and append the elements from list. Also do not want to use inbuilt "set".
Tried to do that with some code as below:
l = len(arr)
for x in range(l):
for y in range(x+1,l):
if arr[x] == arr[y]:
del arr[y]
Tried the above code and its throwing error
"IndexError: list index out of range"
What I understand is whiling deleting the value the size of the list is changing for which its throwing the error. So I made the below changes. But still its failing with same error:
l = len(arr)
for x in range(l):
for y in range(x+1,l):
if arr[x] == arr[y]:
t = y
del arr[y]
y = t - 1
Can some one help me out on this?
Thanks in Advance.
You are trying to make the code more efficient by caching the length of the list in the local variable l. However, that is not helpful because the list is being trimmed inside the loop, and you are not keeping the cached length variable in sync.
for index in range(len(arr)-1,0,-1):
if arr[index] in arr[:index]:
del arr[index]
By going backwards through the array and looking for earlier occurrences of each element, you can avoid having to worry about the length of the list changing all the time.
This method also preserves the order in which elements occur in the original array. Note the instruction is to only remove duplicates (a.k.a. subsequent occurrences).
For example the list [9,3,4,3,5] should reduce to [9,3,4, 5] as the second occurrence of 3 is considered a duplicate and should be removed.
How about this approach:
>>> set(arr)
set([1, 2, 3, 4, 5, 6, 7, 8, 9]) #Just to compare it with the results below.
>>> arr = [1,3,4,5,2,3,4,2,5,7,3,8,1,9,6,2,1,2,1,3,4,3,4,6,9]
>>> arr.sort()
>>> arr
[1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 6, 6, 7, 8, 9, 9]
>>> for i in arr:
while arr.count(i) > 1:
del arr[i]
>>> arr
[1, 2, 3, 4, 5, 6, 7, 8, 9]
Another approach is to find, after sorting your list, the length of the sublist to delete for each number:
>>> arr = [1,3,4,5,2,3,4,2,5,7,3,8,1,9,6,2,1,2,1,3,4,3,4,6,9]
>>> arr.sort()
>>> arr
[1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 6, 6, 7, 8, 9, 9]
>>> for i,j in enumerate(arr):
del arr[i+1:i+arr.count(j)]
>>> arr
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Smallest number in a particular part of my list

I have a question about python. I have to sort a list of random numbers in a particular way (it's not allowed to use sort()). I'll try to explain:
I have to search for the smallest number, and swap this number with the number at the first position in the list.
Then, I search again for the smallest number, but this time ignore the first number in my list because this one is already sorted. So, I should start searching for the smallest number from the second number (index 1) till the end of the list. The smallest number then found, should be swapped with the second number in the list(so the index 1).
I hope you understand my problem. This is the code I wrote so far, but I get errors and/or the sorting isn't correct.
array = random_integers(10,size=10)
my_list = list(array)
for i in range(len(my_list)):
print my_list
a = min(my_list[i:len(my_list)])
b = my_list.index(a)
my_list[i],my_list[b]=my_list[b],my_list[i]
print my_list
I think there's a problem in my range, and a problem with the
a = min(my_list[i:len(my_list)])
I want to search for the smallest number, but not in the ENTIRE list how can I do this?
The problem occurs on this line:
b = my_list.index(a)
since this searches for the first occurrence of a in all of my_list. If the same number occurs twice, then b will always correspond to the smallest such index, which might be less than i. So you might end up moving a number which has already been sorted.
The obvious thing to try is to slice my_list before calling index:
my_list[i:].index(a)
but note that index will return values between 0 and N-i. We want numbers between i and N. So be sure to add i to the result:
b = my_list[i:].index(a)+i
Thus, the easiest way to fix your code as it presently exists is:
for i in range(len(my_list)):
a = min(my_list[i:])
b = my_list[i:].index(a)+i
my_list[i], my_list[b] = my_list[b], my_list[i]
but notice that min is searching through all the items in my_list[i:] and then the call to index is traversing the same list a second time. You could find b in one traversal like this:
b = min(range(i, N), key=my_list.__getitem__)
Demo:
import numpy as np
array = np.random.random_integers(10,size=10)
my_list = list(array)
N = len(my_list)
for i in range(N):
b = min(range(i, N), key=my_list.__getitem__)
my_list[i], my_list[b] = my_list[b], my_list[i]
print my_list
yields
[3, 10, 9, 6, 5, 3, 6, 8, 8, 4]
[3, 3, 9, 6, 5, 10, 6, 8, 8, 4]
[3, 3, 4, 6, 5, 10, 6, 8, 8, 9]
[3, 3, 4, 5, 6, 10, 6, 8, 8, 9]
[3, 3, 4, 5, 6, 10, 6, 8, 8, 9]
[3, 3, 4, 5, 6, 6, 10, 8, 8, 9]
[3, 3, 4, 5, 6, 6, 8, 10, 8, 9]
[3, 3, 4, 5, 6, 6, 8, 8, 10, 9]
[3, 3, 4, 5, 6, 6, 8, 8, 9, 10]
[3, 3, 4, 5, 6, 6, 8, 8, 9, 10]
If you want the smallest number from a list you can use min(). If you want a part of a list you can use list slicing: my_list[1:]. Put the two together and you get the smallest number from a part of your list. However, you don't need to do this, as you can .pop() from the list instead.
sorted_list = []
while my_list:
n = min(my_list)
sorted_list.append(my_list.pop(my_list.index(n)))
If you're using numpy arrays then instead of my_list.index(min(my_list)) you can use the .argmin() method.
While this type of sorting is good for an introduction, it is not very efficient. You may want to consider looking at the merge sort, and also the Python's built-in timsort.

Compare each element in a list to all others

Is there a way to compare all elements of a list (ie one such as [4, 3, 2, 1, 4, 3, 2, 1, 4]) to all others and return, for each element, the number of other elements it is different from (ie, for the list above [6, 7, 7, 7, 6, 7, 7, 7, 6])? I then will need to add the numbers from this list.
li = [4, 3, 2, 1, 4, 3, 2, 1, 4]
from collections import Counter
c = Counter(li)
print c
length = len(li)
print [length - c[el] for el in li]
Creating c before executing [length - c[el] for el in li] is better than doing count(i) for each element i of the list, because that means that count() do the same count several times (each time it encounters a given element, it counts it)
By the way, another way to write it:
map(lambda x: length-c[x] , li)
You can get similar counter with count() method.
And subtract the total number.
Do it in one line with a comprehension list.
>>> l = [4, 3, 2, 1, 4, 3, 2, 1, 4]
>>> [ len(l)-l.count(i) for i in l ]
[6, 7, 7, 7, 6, 7, 7, 7, 6]
For Python 2.7:
test = [4, 3, 2, 1, 4, 3, 2, 1, 4]
length = len(test)
print [length - test.count(x) for x in test]
You could just use the sum function, along with a generator expression.
>>> l = [4, 3, 2, 1, 4, 3, 2, 1, 4]
>>> length = len(l)
>>> print sum(length - l.count(i) for i in l)
60
The good thing about a generator expression is that you don't create an actual list in memory, but functions like sum can still iterate over them and produce the desired result. Note, however, that once you iterate over a generator once, you can't iterate over it again.

Why following code doesn't give "IndexError"?

I have following python code:
a = [0,1,2,3,4,5]
del(a[2:7])
Should not this give "IndexError"? If no then why?
This deletes the list slice from 2 to before 7. List slices do not throw index errors, rather if they extend beyond the end of the list, they return the whole remainder of the list.
>>> a = list(range(10))
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a [5:20]
[5, 6, 7, 8, 9]

Categories

Resources