I’m writing a program to collect a series of images from a USB microscope and generate an average as a way of denoising. I’m having to convert the image into a numpy array, sum the pixels then divide before converting back to an image. Somewhere along the line the channels get mixed so my output i…
Tag: numpy
Force NumPy ndarray to take ownership of its memory in Cython
Following this answer to “Can I force a numpy ndarray to take ownership of its memory?” I attempted to use the Python C API function PyArray_ENABLEFLAGS through Cython’s NumPy wrapper and found it is not exposed. The following attempt to expose it manually (this is just a minimum example rep…
Convert array 1:n in matlab to python
I have question similar to this, and possibly a much simpler one: We use the a lot in Matlab. But in Python I simply struggle to get this simple thing work. I tried using arange, but never really realized what the error is: I went to the numpy website and tried to give the syntax there, but again: I wouldn…
How do I slice a numpy array to get both the first and last two rows
As far as I can see it, it isn’t covered in the indexing, slicing and iterating scipy tutorial, so let me ask it here: Say I’ve How do I slice the array in order get both the first and last rows: Answer I don’t know if there’s a slick way to do that. You could list the indices explicit…
Pass a 2d numpy array to c using ctypes
What is the correct way to pass a numpy 2d – array to a c function using ctypes ? My current approach so far (leads to a segfault): C code : Python code: Answer This is probably a late answer, but I finally got it working. All credit goes to Sturla Molden at this link. The key is, note that
Multidimensional arrays, using range, while simultaneously having a set start, stop, and step?
Oy mates, I am learning numpy on my own and getting a pretty good handle on it, a few concepts elude me even after reading the documentation though. I am trying to go through this matrix and make every second row have 10s all the way through it. The last line of code is incorrect. I know how to use
Python: datetime64 issues with range
I am trying to have a vector of seconds between two time intervals: For some reason when I print range I get: So the length is 23401 which is what I want but definitely not the correct time interval. Why is that? Also, if I have a DataFrame df with a column of datetime64 format that looks like: Once I
What does axis in pandas mean?
Here is my code to generate a dataframe: then I got the dataframe: When I type the commmand : I got : According to the reference of pandas, axis=1 stands for columns and I expect the result of the command to be So here is my question: what does axis in pandas mean? Answer It specifies the axis along which
Difference between numpy.array shape (R, 1) and (R,)
In numpy, some of the operations return in shape (R, 1) but some return (R,). This will make matrix multiplication more tedious since explicit reshape is required. For example, given a matrix M, if we want to do numpy.dot(M[:,0], numpy.ones((1, R))) where R is the number of rows (of course, the same issue als…
NumPy dtype issues in genfromtxt(), reads string in as bytestring
I want to read in a standard-ascii csv file into numpy, which consists of floats and strings. E.g., Whatever I tried, the resulting array would look like E.g., However, I want to save a step for the byte-string conversion and was wondering how I can read in the string columns as regular string directly. I tri…