Posts with the category Computations

Pandas / PyTables Error Trying to store a string with len [42] in [values_block_0] column

If you get an error message like

Traceback (most recent call last):
  File "./append_to_hdf5.py", line 81, in <module>
    main()
  File "./append_to_hdf5.py", line 63, in main
    store.append('df', df, format='t', complib=None)
  File "/home/philipp/.pyvenv/playground-3.4/lib/python3.4/site-packages/pandas/io/pytables.py", line 904, in append
    **kwargs)
 ...

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Counting bits set to 1 in bytes with Python / Popcount or Hamming Weight

I want to run popcount on a large set of 144-byte-long bytearrays. Doing so in Python can be a pita! First, I was using the naive approach with bin(b).count('1') but it was very slow. I did some research on the internet for faster algorithms and I found some. But running...

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False Color Plots

False color (or more exactly pseudo color) can be useful when a device / detector records a grayscale image and a human has to interpret it. Fine differences are hard to see in grayscale images und a false color mapping allows us to map the 255 shades of gray...

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Maximum Value in 2D Array in Matlab

Finding the value and the position of the maximum value in a 2D array (or the first one if there are more than one maximum values) can be done like this:

[v,ind]=max(X);
[v1,ind1]=max(max<span...

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Creating a Gaussian Window in 3D Using Matlab

If you want to create a gaussian window with Matlab in three dimensions (as you can do in two dimensions with the built-in gausswin()) you can use the function gauss3D() found on MATLAB Central:

S = gauss3D(20,20,7);
slice(<span...

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German Tax ID – Calculate the Check Sum using Python/ Steuer-Identifikationsnummer – Prüfsummenberechnung mit Python

Calculate the Check Sum for a German Tax ID number using Python:

#!/usr/local/env python
# -*- coding: utf-8 -*-

class CalcCheckCipher(object):
    @staticmethod
    def getCheckCipher( idnrString):
...

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ezyfit - An Alternative to Matlab's Curve Fitting Toolbox

ezyfit can prevent you from buying the Curve Fitting Tool that Matlab does not come with by default. This is how I use it to fit data according to a function :

x=15:45
y = 30 +...

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Handle 16bit TIFF Images in Python

There seem to be quite a lot of problems with 16-bit grey scale TIFF images (especially with the Python Imaging Library - PIL). If you can, you may want to use FITS instead of TIFF. There are good and up-to-date libraries for Python: PyFITS.

For me, however, the FreeImage...

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hub teaches git about GitHub

Hub is a useful extension to make git GitHub aware.

catch lines:

hub introduces git to GitHub
and
hub teaches git about GitHub

Installation

brew install hub

Usage

See hub's webpage for introductory examples and its man page for further reference.

...

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Light Wavelength from Energy

You can use the following Python function to convert a photon energy (in electron volts) to the corresponding wavelength (in nano meters):

def wavelength_from_energy ( electron_volts):
    """Returns a photon wavelength in nm from a photon energy given in eV."""
 ...

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Physical Constants in Python

SciPy (updated to CODATA 2010)

Installation (easiest using pip):

  1. Install Fortran as described for your OS on http://www.scipy.org/Installing_SciPy (or via brew install gfortran on Max OS X)
  2. Install SciPy itself: pip install scipy

Usage:

from scipy.constants import *
print "The Planck constant h:", <span...

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Install matplotlib on Mac OS X

Installation of the pre-built binaries for OS X

Read Installing OSX binaries in the matplotlib Installation FAQ.

Installation via pip

This is a good way if you have an up-to-date Python installation on your computer and pip installed. It takes, however, some time (~ 20 mins for me) to install as it...

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Image Functions in Matlab

Matlab has several functions to handle images:

info = imfinfo(filename)
A = imread(filename)
% http://www.mathworks.com/help/techdoc/ref/image.html
% http://octave.sourceforge.net/image/function/imread.html
image(A)

Create an animated GIF image

http://www.mathworks.com/support/solutions/en/data/1-48KECO/?solution=1-48KECO

x = 0:0.01:1;
<span...

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Use XML files in Matlab

Here are the solutions I've found on the web:

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A PID Controller in Python

...

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Estimate the transform limited beam duration of a laser pulse from its spectral width in nm in Python

A transform limited pulse has a time-bandwidth product of for Gaussian beams.

Here are a couple of Python functions to find the minimum duration of a gaussian pulse with a certain spectral FWHM (measured in nm) at a given central wavelength plus the...

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Newport Power Meter 2935T-C via libusb on Linux

This power meter from Newport has a serial RS232 and a USB connection. This page tries to find out how it's possible to talk to the device via USB. For RS232 connection, look at the blog post Newport Power Meter 2935T-C – Serial Connection via Python.

USB Connection Info

Output of...

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Octave on Mac OS X

You can either download a *.dmg from http://octave.sourceforge.net/.

Or you can also install it via homebrew using:

brew install octave

I also wanted to install gnuplot including its AquaTerm terminal via homebrew (see Soeren's comment on http://slashusr.wordpress.com/2010/01/17/gnuplot-with-aquaterm-on-osx-snow-leopard/). But the build for AquaTerm failed :( :

cd /usr/local/Library/Formula/
wget...

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Convert between Power and dBm

Many devices that send & receive data via antennas give you information about their power in units of dBm. If you wan to know how to convert this unit to a power value given in mW (and vice versa) you may read on.

The formulae found in the Wikipedia article...

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Convert between plus-cylinder and minus-cylinder notation in eyeglass prescription writings using Python

Doctors in Germany usually give out prescriptions with negative values for the cylinder. Some eyeglass manufacturers list their products with plus-cylinder notation so the optometrists have to convert the notations.

Online Converter

If you want to convert your prescription writings, I created this online tool to convert your values:

Convert Cylindrical Notation

It...

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Chinese OCR

OCR for Windows

OCR for Mac OS

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