I am looking for a fast way to preserve large numpy arrays. I want to save them to the disk in a binary format, then read them back into memory relatively fastly. cPickle is not fast enough, unfortunately. I found numpy.savez and numpy.load. But the weird thing is, numpy.load loads a npy file into “memo…
Tag: numpy
Concatenating two one-dimensional NumPy arrays
How do I concatenate two one-dimensional arrays in NumPy? I tried numpy.concatenate: But I get an error: TypeError: only length-1 arrays can be converted to Python scalars Answer Use: The arrays you want to concatenate need to be passed in as a sequence, not as separate arguments. From the NumPy documentation…
How do you get the magnitude of a vector in Numpy?
In keeping with the “There’s only one obvious way to do it”, how do you get the magnitude of a vector (1D array) in Numpy? The above works, but I cannot believe that I must specify such a trivial and core function myself. Answer The function you’re after is numpy.linalg.norm. (I reckon…
How does one find the largest consecutive set of numbers in a list that are not necessarily adjacent?
For instance, if I have a list This algorithm should return [1,2,3,4,5,6,7,8,9,10,11]. To clarify, the longest list should run forwards. I was wondering what is an algorithmically efficient way to do this (preferably not O(n^2))? Also, I’m open to a solution not in python since the algorithm is what mat…
Extracting specific columns in numpy array
This is an easy question but say I have an MxN matrix. All I want to do is extract specific columns and store them in another numpy array but I get invalid syntax errors. Here is the code: It seems like the above line should suffice but I guess not. I looked around but couldn’t find anything syntax wise…
Add single element to array in numpy
I have a numpy array containing: I want to create an array containing: That is, I want to add the first element on to the end of the array. I have tried the obvious: But I get an error saying ValueError: arrays must have same number of dimensions I don’t understand this – the arrays are both just …
Vectorize over the rows of an array
I have an array X and I want to apply a function f to all the rows of X: Now, y should be array([15,30], ‘i’). Which method or slicing magic will implement rows in the most efficient way? Answer NumPy implements the concept of “action over a particular axis”. The general function is nu…
How do I remove all zero elements from a NumPy array?
I have a rank-1 numpy.array of which I want to make a boxplot. However, I want to exclude all values equal to zero in the array. Currently, I solved this by looping the array and copy the value to a new array if not equal to zero. However, as the array consists of 86 000 000 values and I have
How does python numpy.where() work?
I am playing with numpy and digging through documentation and I have come across some magic. Namely I am talking about numpy.where(): How do they achieve internally that you are able to pass something like x > 5 into a method? I guess it has something to do with __gt__ but I am looking for a detailed expla…
Immutable numpy array?
Is there a simple way to create an immutable NumPy array? If one has to derive a class from ndarray to do this, what’s the minimum set of methods that one has to override to achieve immutability? Answer You can make a numpy array unwriteable: Also see the discussion in this thread: http://mail.scipy.org…