All Questions
2,375
questions
568
votes
7
answers
249k
views
What are the advantages of NumPy over regular Python lists?
What are the advantages of NumPy over regular Python lists?
I have approximately 100 financial markets series, and I am going to create a cube array of 100x100x100 = 1 million cells. I will be ...
- 24.4k
434
votes
10
answers
1.2m
views
Creating a Pandas DataFrame from a Numpy array: How do I specify the index column and column headers?
I have a Numpy array consisting of a list of lists, representing a two-dimensional array with row labels and column names as shown below:
data = np.array([['','Col1','Col2'],['Row1',1,2],['Row2',3,4]])...
- 5,405
423
votes
6
answers
631k
views
265
votes
9
answers
452k
views
List of lists into numpy array
How do I convert a simple list of lists into a numpy array? The rows are individual sublists and each row contains the elements in the sublist.
- 2,997
243
votes
1
answer
96k
views
What is the inverse function of zip in python? [duplicate]
I've used the zip function from the Numpy library to sort tuples and now I have a list containing all the tuples. I had since modified that list and now I would like to restore the tuples so I can use ...
- 2,687
211
votes
7
answers
181k
views
Filtering a list based on a list of booleans
I have a list of values which I need to filter given the values in a list of booleans:
list_a = [1, 2, 4, 6]
filter = [True, False, True, False]
I generate a new filtered list with the following ...
- 41.7k
204
votes
5
answers
325k
views
How to convert list of numpy arrays into single numpy array?
Suppose I have ;
LIST = [[array([1, 2, 3, 4, 5]), array([1, 2, 3, 4, 5],[1,2,3,4,5])] # inner lists are numpy arrays
I try to convert;
array([[1, 2, 3, 4, 5],
[1, 2, 3, 4, 5],
[1, 2, ...
- 13.4k
200
votes
12
answers
337k
views
Index all *except* one item in python
Is there a simple way to index all elements of a list (or array, or whatever) except for a particular index? E.g.,
mylist[3] will return the item in position 3
milist[~3] will return the whole list ...
- 3,619
135
votes
9
answers
297k
views
How to save a list as numpy array in python?
Is possible to construct a NumPy array from a python list?
- 40.9k
120
votes
1
answer
133k
views
Convert 2d numpy array into list of lists [duplicate]
I use an external module (libsvm), which does not support numpy arrays, only tuples, lists and dicts. But my data is in a 2d numpy array. How can I convert it the pythonic way, aka without loops.
&...
- 34.3k
109
votes
6
answers
644k
views
List to array conversion to use ravel() function
I have a list in python and I want to convert it to an array to be able to use ravel() function.
- 1,559
86
votes
4
answers
196k
views
How to use python numpy.savetxt to write strings and float number to an ASCII file?
I have a set of lists that contain both strings and float numbers, such as:
import numpy as num
NAMES = num.array(['NAME_1', 'NAME_2', 'NAME_3'])
FLOATS = num.array([ 0.5 , 0.2 , 0.3 ])
...
- 1,034
81
votes
5
answers
168k
views
python pandas flatten a dataframe to a list
I have a df like so:
import pandas
a=[['1/2/2014', 'a', '6', 'z1'],
['1/2/2014', 'a', '3', 'z1'],
['1/3/2014', 'c', '1', 'x3'],
]
df = pandas.DataFrame.from_records(a[1:],columns=a[0])
I ...
- 4,093
81
votes
3
answers
63k
views
Python Array Slice With Comma?
I was wondering what the use of the comma was when slicing Python arrays - I have an example that appears to work, but the line that looks weird to me is
p = 20*numpy.log10(numpy.abs(numpy.fft.rfft(...
- 1,229
77
votes
7
answers
476k
views
'list' object has no attribute 'shape'
how to create an array to numpy array?
def test(X, N):
[n,T] = X.shape
print "n : ", n
print "T : ", T
if __name__=="__main__":
X = [[[-9.035250067710876], [7.453250169754028], [33....
- 19k
71
votes
10
answers
136k
views
How to create a numpy array of lists?
I want to create a numpy array in which each element must be a list, so later I can append new elements to each.
I have looked on google and here on stack overflow already, yet it seems nowhere to be ...
- 1,203
64
votes
6
answers
83k
views
How to check if a variable is either a python list, numpy array or pandas series
I have a function that takes in a variable that would work if it is any of the following three types
1. pandas Series
2. numpy array (ndarray)
3. python list
Any other type should be rejected. ...
- 4,890
63
votes
10
answers
133k
views
numpy-equivalent of list.pop?
Is there a numpy method which is equivalent to the builtin pop for python lists?
Popping obviously doesn't work on numpy arrays, and I want to avoid a list conversion.
- 743
59
votes
3
answers
117k
views
Python List of np arrays to array
I'm trying to turn a list of 2d numpy arrays into a 2d numpy array. For example,
dat_list = []
for i in range(10):
dat_list.append(np.zeros([5, 10]))
What I would like to get out of this list ...
- 23.3k
55
votes
2
answers
121k
views
How to access a column in a list of lists in python
I have a 2D array in python modeled by a list of lists and I want to extract the column. I made a quick research and I found a way that uses numpy arrays. The problem is that I do not want to use ...
- 821
51
votes
5
answers
222k
views
Convert list or numpy array of single element to float in python
I have a function which can accept either a list or a numpy array.
In either case, the list/array has a single element (always). I just need to return a float.
So, e.g., I could receive:
list_ = [4]...
- 7,276
48
votes
6
answers
6k
views
Is there a difference between `board[x, y]` and `board[x][y]` in Python?
I'm working through a tutorial on GeekforGeeks website and noticed that they are checking a point in an array using board[x,y], which I've never seen before. I don't think this would work, but when I ...
- 769
44
votes
2
answers
82k
views
concatenate numpy arrays which are elements of a list
I have a list containing numpy arrays something like L=[a,b,c] where a, b and c are numpy arrays with sizes N_a in T, N_b in T and N_c in T.
I want to row-wise concatenate a, b and c and get a numpy ...
- 1,411
43
votes
9
answers
79k
views
Failed to find data adapter that can handle input: <class 'numpy.ndarray'>, (<class 'list'> containing values of types {"<class 'int'>"})
history = model.fit(X, y, batch_size=32, epochs=40, validation_split=0.1)
the line problem was this
Showing error:
ValueError: Failed to find data adapter that can handle input: <class 'numpy....
- 431
42
votes
10
answers
67k
views
How to make List from Numpy Matrix in Python [duplicate]
I using the dot() function from numpy to multiply a matrix of 3x3 with a numpy.array of 1x3. The output is for example this:
[[ 0.16666667 0.66666667 0.16666667]]
which is of type:
<class '...
- 3,808
38
votes
2
answers
50k
views
Merge two arrays vertically to array of tuples using numpy
I have two numpy arrays:
x = np.array([-1, 0, 1, 2])
y = np.array([-2, -1, 0, 1])
Is there a way to merge these arrays together like tuples:
array = [(-1, -2), (0, -1), (1, 0), (2, 1)]
- 1,143
38
votes
1
answer
59k
views
AttributeError: 'numpy.ndarray' object has no attribute 'toList'
I'm trying to append certain columns of Pandas Data Frames from CSV files into a numpy array. I have no idea how to instantiate an empty numpy array, so I'm testing it first with a list.
def windows(...
user2398046
37
votes
6
answers
67k
views
How to delete an object from a numpy array without knowing the index
Is it possible to delete an object from a numpy array without knowing the index of the object but instead knowing the object itself?
I have seen that it is possible using the index of the object using ...
- 483
34
votes
2
answers
12k
views
Numpy individual element access slower than for lists
I just started using Numpy and noticed that iterating through each element in a Numpy array is ~4x slower than doing the same but with a list of lists. I know now that this defeats the purpose of ...
- 1,659
30
votes
1
answer
65k
views
TypeError: only integer arrays with one element can be converted to an index 3
I'm having this error in the title, and don't know what's wrong. It's working when I use np.hstack instead of np.append, but I would like to make this faster, so use append.
time_list a list of ...
- 526
25
votes
1
answer
41k
views
How to create a dynamic array
As I understand, the list type in Python is a dynamic pointer array, which will increase it's capacity when items are appended to it. And an array in NumPy uses a continuous memory area to hold all ...
- 96k
22
votes
4
answers
10k
views
Difference between list(numpy_array) and numpy_array.tolist()
What is the difference between applying list() on a numpy array vs. calling tolist()?
I was checking the types of both outputs and they both show that what I'm getting as a result is a list, however,...
- 425
21
votes
4
answers
43k
views
Grab unique tuples in python list, irrespective of order
I have a python list:
[ (2,2),(2,3),(1,4),(2,2), etc...]
What I need is some kind of function that reduces it to its unique components... which would be, in the above list:
[ (2,2),(2,3),(1,4) ]
...
- 30k
20
votes
3
answers
26k
views
How to return back a list instead of tuple in psycopg2
I have the following query
cursor.execute(
"""
SELECT transform(row_to_json(t)) FROM
(select * from table
where a = %s
and b = %s limit 1000) t;
"""
, ...
- 1,067
19
votes
6
answers
6k
views
Finding the index of a numpy array in a list
import numpy as np
foo = [1, "hello", np.array([[1,2,3]]) ]
I would expect
foo.index( np.array([[1,2,3]]) )
to return
2
but instead I get
ValueError: The truth value of an array with more ...
- 1,255
18
votes
1
answer
10k
views
Numpy: Difference between a[i][j] and a[i,j]
Coming from a Lists background in Python and that of programming languages like C++/Java, one is used to the notation of extracting elements using a[i][j] approach. But in NumPy, one usually does a[i,...
- 11.2k
17
votes
10
answers
4k
views
Python function that identifies if the numbers in a list or array are closer to 0 or 1
I have a numpy array of numbers. Below is an example:
[[-2.10044520e-04 1.72314372e-04 1.77235336e-04 -1.06613465e-04
6.76617611e-07 2.71623057e-03 -3.32789944e-05 1.44899758e-05
5.79249863e-05 ...
- 2,244
17
votes
11
answers
23k
views
return index of last non-zero element in list
I get data in the following format:
[-2, -2, 0, 0, 0, 0, 0]
[-2, 20, -1, 0, 3, 0, 0]
with each line being a different input. The lists could be longer than 7 elements. I need to return the index ...
- 2,739
17
votes
6
answers
2k
views
Get second minimum values per column in 2D array
How can I get the second minimum value from each column? I have this array:
A = [[72 76 44 62 81 31]
[54 36 82 71 40 45]
[63 59 84 36 34 51]
[58 53 59 22 77 64]
[35 77 60 76 57 44]...
- 221
17
votes
2
answers
9k
views
Python Slice Notation with Comma/List
I have come across some python code with slice notation that I am having trouble figuring out.
It looks like slice notation but uses a comma and a list:
list[:, [1, 2, 3]]
Is this syntax valid? If ...
- 17.3k
17
votes
3
answers
995
views
Why can itertools.groupby group the NaNs in lists but not in numpy arrays
I'm having a difficult time to debug a problem in which the float nan in a list and nan in a numpy.array are handled differently when these are used in itertools.groupby:
Given the following list and ...
- 149k
17
votes
4
answers
10k
views
Comparing two large lists in python
I have one list which contains about 400 words. And another list of lists, in which each list contains about 150,000 words. This list has 20 such lists.
Now I want to see how many of these 400 words ...
- 9,506
16
votes
4
answers
15k
views
bitwise operations between elements in a list
I have a of list of bitwise elements, e.g. [1,1,1], and I want to do a bitwise OR operation between every element in the list. So, e.g.
for [1,1,1] do
1 | 1 | 1 = 1
or for [1,17,1] do
1 | 17 ...
- 557
16
votes
2
answers
1k
views
numpy.unique gives wrong output for list of sets
I have a list of sets given by,
sets1 = [{1},{2},{1}]
When I find the unique elements in this list using numpy's unique, I get
np.unique(sets1)
Out[18]: array([{1}, {2}, {1}], dtype=object)
As can ...
- 664
15
votes
2
answers
3k
views
Find nearest indices for one array against all values in another array - Python / NumPy
I have a list of complex numbers for which I want to find the closest value in another list of complex numbers.
My current approach with numpy:
import numpy as np
refArray = np.random.random(16);
...
- 333
15
votes
3
answers
18k
views
Fastest way to check if duplicates exist in a python list / numpy ndarray
I want to determine whether or not my list (actually a numpy.ndarray) contains duplicates in the fastest possible execution time. Note that I don't care about removing the duplicates, I simply want to ...
- 1,572
14
votes
2
answers
15k
views
Deleting masked elements in numpy array
I have some arrays that contain masked elements (from Numpy.MaskedArray), e.g.
data = [0,1,masked,3,masked,5,...]
Where the mask doesn't follow a regular pattern.
I want to iterate through the ...
- 920
14
votes
2
answers
12k
views
Finding the indices of the top three values via argmin() or min() in python/numpy without mutation of list?
So I have this list called sumErrors that's 16000 rows and 1 column, and this list is already presorted into 5 different clusters. And what I'm doing is slicing the list for each cluster and finding ...
- 639
14
votes
1
answer
18k
views
What is the difference between a NumPy array and a python list? [duplicate]
Why do we use numpy arrays in place of lists in python? What is the main difference between them?
- 615
13
votes
7
answers
24k
views
Python numpy array vs list
I need to perform some calculations a large list of numbers.
Do array.array or numpy.array offer significant performance boost over typical arrays?
I don't have to do complicated manipulations on ...
- 8,742