itertools – Make Me Engineer

N-D version of itertools.combinations in numpy

November 4, 2022 by Tarik

You can use itertools.combinations() to create the index array, and then use NumPy’s fancy indexing: import numpy as np from itertools import combinations, chain from scipy.special import comb def comb_index(n, k): count = comb(n, k, exact=True) index = np.fromiter(chain.from_iterable(combinations(range(n), k)), int, count=count*k) return index.reshape(-1, k) data = np.array([[1,2,3,4,5],[10,11,12,13,14]]) idx = comb_index(5, 3) print(data[:, idx]) output: … Read more

importing izip from itertools module gives NameError in Python 3.x

November 2, 2022 by Tarik

In Python 3 the built-in zip does the same job as itertools.izip in 2.X(returns an iterator instead of a list). The zip implementation is almost completely copy-pasted from the old izip, just with a few names changed and pickle support added. Here is a benchmark between zip in Python 2 and 3 and izip in … Read more

itertools.groupby() not grouping correctly

October 5, 2022 by Tarik

itertools.groupby collects together contiguous items with the same key. If you want all items with the same key, you have to sort self.data first. for mid, group in itertools.groupby( sorted(self.data,key=operator.itemgetter(1)), key=operator.itemgetter(1)):

How to apply itertools.product to elements of a list of lists? [duplicate]

August 10, 2022 by Tarik

>>> list(itertools.product(*arrays)) [(-1, -2, -3), (-1, -2, 3), (-1, 2, -3), (-1, 2, 3), (1, -2, -3), (1, -2, 3), (1, 2, -3), (1, 2, 3)] This will feed all the pairs as separate arguments to product, which will then give you the cartesian product of them. The reason your version isn’t working is that … Read more

Python how to read N number of lines at a time

August 9, 2022 by Tarik

islice() can be used to get the next n items of an iterator. Thus, list(islice(f, n)) will return a list of the next n lines of the file f. Using this inside a loop will give you the file in chunks of n lines. At the end of the file, the list might be shorter, … Read more

itertools product speed up

July 21, 2022 by Tarik

The NumPy equivalent of itertools.product() is numpy.indices(), but it will only get you the product of ranges of the form 0,…,k-1: numpy.rollaxis(numpy.indices((2, 3, 3)), 0, 4) array([[[[0, 0, 0], [0, 0, 1], [0, 0, 2]], [[0, 1, 0], [0, 1, 1], [0, 1, 2]], [[0, 2, 0], [0, 2, 1], [0, 2, 2]]], [[[1, 0, … Read more

permutations with unique values

April 30, 2022 by Tarik

Because sometimes new questions are marked as duplicates and their authors are referred to this question it may be important to mention that sympy has an iterator for this purpose. >>> from sympy.utilities.iterables import multiset_permutations >>> list(multiset_permutations([1,1,1])) [[1, 1, 1]] >>> list(multiset_permutations([1,1,2])) [[1, 1, 2], [1, 2, 1], [2, 1, 1]]

How do I use itertools.groupby()?

April 30, 2022 by Tarik

IMPORTANT NOTE: You have to sort your data first. The part I didn’t get is that in the example construction groups = [] uniquekeys = [] for k, g in groupby(data, keyfunc): groups.append(list(g)) # Store group iterator as a list uniquekeys.append(k) k is the current grouping key, and g is an iterator that you can … Read more