32
votes
Accepted
Looking for a way to create a particular matrix in python in a less amount of time
The NumPy Reference should be the first place you look when you have a problem like this. The operations you need are nearly always in there somewhere. And functions that you find while browsing the ...
- 49.2k
31
votes
Accepted
Scale Numpy array to certain range
NumPy provides numpy.interp for 1-dimensional linear interpolation. In this case, where you want to map the minimum element of the array to −1 and the maximum to +1,...
- 49.2k
28
votes
Accepted
Crop black border of image using NumPy
Vectorization with NumPy
When read with cv2.imread or skimage.io.imread or scipy.misc.imread,...
- 886
25
votes
Accepted
Remove points in a straight line
A look at the second plot in the post shows that something has gone wrong. There are four points in a line here:
This shouldn't be possible, since "points that create a straight line when plotted" ...
- 49.2k
22
votes
Crop black border of image using NumPy
As you've discovered, looping over individual pixels in Python is very slow. You need to organize your computation so that it uses a series of NumPy (or SciPy, or Scikit-Image, or OpenCV) operations ...
- 49.2k
21
votes
19
votes
Efficient numpy cosine distance calculation
Your code does not run: there are missing import statements:
import numpy as np
import scipy.spatial.distance
Your algorithms ...
- 49.2k
18
votes
Generating a 3D Point Cloud
1. Review
No docstring. What does this function do? What parameters does it take? What shape must the depth_image parameter be? What does it return?
In Python, ...
- 49.2k
18
votes
Accepted
Remove pixel patch in image which is stored as array
On my computer it takes 1.745 seconds to run the code in the post.
There's no need for the array of random indexes to be two-dimensional:
...
- 49.2k
18
votes
Accepted
Iterate through two arrays calculating a statistic from indexes of zeros
First, some basics of your implementation:
Type-hint your arguments; I had to read and investigate a bunch to deduce that tau is a positive integer
...
- 47.9k
16
votes
Accepted
Sending a C++ array to Python/NumPy and back
1. Review
It's not recommended to use using namespace std; — the problem is that this imports all of the identifiers from std, ...
- 49.2k
15
votes
Accepted
Approximate (250 over 100) permutation best fitting certain criteria
1. Analysis
In the general case, the problem of finding the sample of words with the "most even" distribution of letters is NP-hard. Here I'm considering a general instance of this problem to be:
...
- 49.2k
15
votes
Accepted
Comparing pixels against RGB value in NumPy
Your original code can be rewritten as:
mask = np.all(img == (255, 127, 63), axis=-1)
It is a little cleaner, but not more efficient, as it still has to allocate ...
- 6,512
14
votes
Accepted
Possible optimizations for calculating squared euclidean distance
You can decrease Python's overhead by parsing the args tuple and creating the return value manually.
Try changing these portions of your code:
...
- 351
14
votes
Accepted
K-Mean with Numpy
1. Code review
There's no documentation. What do your functions do and how am I supposed to call them?
There are no test cases. What gives you confidence that the algorithm is correct?
The algorithm ...
- 49.2k
13
votes
Accepted
Optimizing very simple piece of “Game of Life” code by taking advantage of NumPy's functionality
I'll just be cheeky and post a slightly modified version of my SO answer here.
So first things first, you want to get rid of the loops. They are slow to execute.
The first loop:
...
- 586
13
votes
Accepted
Balanced Random Assignment in Python
Just create a list of 18 ones and 18 twos, shuffle it, then reshape to 6x6:
...
- 401
13
votes
Accepted
Python program that draws the Mandelbrot set fractal
The key thing to remember when working with numerical code is that the CPython interpreter is pretty slow (it trades speed for flexibility) and so you must avoid running in the interpreter whenever ...
- 49.2k
13
votes
12
votes
Accepted
Groupby in NumPy
The function scipy.ndimage.find_objects returns exactly the slices that you are looking for:
...
- 49.2k
12
votes
Accepted
Processing an image to extract green-screen mask
There's a simpler way to create the empty image using numpy.zeros_like:
empty_img = numpy.zeros_like(img)
As Austin Hastings ...
- 293
12
votes
Accepted
Optimize Performance of Region Checking in List Comprehension
Loops over large arrays are not really a good idea in Python. This is why your original list comprehension is not terribly fast.
Your numpy version is loop free, but as far as I know, ...
- 7,184
12
votes
Accepted
How to clean the indexes, and ideally not create an additional array
Here is a review of the solution.
^ is xor in Python. It is not for computation of exponentials.
When running code outside a method / class, it is a good practice ...
- 2,281
11
votes
Accepted
Order a list of tuples or a NumPy array in a specific format
This is really a graph problem! Draw a graph with a vertex for every number in your list, and an edge between vertices \$a\$ and \$b\$ if there is a pair \$(a, b)\$ or \$(b, a)\$ in your list:
What ...
- 49.2k
11
votes
Accepted
Combining two clock tick vectors with NumPy
Everything apart from the for loop in your code is already vectorized which leaves only the for loop to be optimized.
You ...
- 226
11
votes
Accepted
Recursively Save Python Dictionaries to HDF5 Files Using h5py
Here's what I tested.
I took out the classmethod stuff to make it easier to read, and simplified names a bit. I'll defer judgement on whether that stuff is needed as part of a larger package or not.
...
- 1,441
11
votes
Accepted
Plot timings for a range of inputs
matplotlib adding stuff to the current figure is because you are not using the OO-interface. It is slightly clunkier, but allows way more freedom
In my view, you ...
- 8,950
11
votes
Accepted
Table of Tribonacci sequence using NumPy and PANDAS
You’re using the wrong tool for the job. Basically, you do all the computation in Python, use numpy for intermediate storage and ...
- 28.1k
11
votes
Accepted
Solve the phase state between two haplotype blocks using markov transition probabilities
Before we get to the interesting stuff, we should handle some stylistic niggles. Note that PEP 8 is a de-facto style for Python code. First, imports should be sorted
...
- 9,523
11
votes
Remove points in a straight line
You can use Ramer Douglas Peuker algorithm. RDP takes a curve and eliminates points that are close to straight lines. It is distance based. Starting with the two endpoints, it forms a line and picks ...
- 543
Only top scored, non community-wiki answers of a minimum length are eligible