Conditional expression in Python

It has bothered me for a long time that Python does not provide a conditional expression operator similar to the ?: operator used in C. But by some fortune, I ran across this Python snippet:

("False", "True")[var == True]

which does exactly the same as what you would write in C

var ? "True" : "False"

What a relief!

Erase/Replace

新的一年開始了。

我不要再害怕。

還要活的更從容。

這是這一年的新希望（似乎比往年成熟些？）。唸研究所，不論是知識上或是生活上都真的讓我學到很多很多；假如一直這樣下去，第一個目標也許可以達成一大部分。第二個目標似乎困難許多，但某種程度上也是建立在第一個目標上的吧。

繼續努力。

Regex for matching floating point numbers

As simple as it sounds, there doesn't seem to be a standard regular expression for matching floating point numbers -- for some reason everyone seems to have their own preference: scientific notation or not, decimal points in the exponent or not... etc. Currently I'm settling with

([-+]?[0-9]*\.?[0-9]+([eE][-+]?[0-9]*\.?[0-9]+)?)

Actually for my purposes, I also need an optional unit, so I'm really using this

([-+]?[0-9]*\.?[0-9]+([eE][-+]?[0-9]*\.?[0-9]+)?)([a-zA-Z]*)

Preventing string truncation with snprintf()

One issue with using snprintf() (the buffer-overrun-preventing version of sprintf()) is that since the size of the char[] we're trying to "print" into is limited, truncation would occur if the string is longer than the char[] size. Yet after studying the manual carefully, I noticed that snprintf() actually returns the length of the resultant string, regardless of the size of the buffer. Below is a simple demonstration of how to prevent truncation with this feature and dynamical allocation of the character buffer.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
   #define NC 10
   #define ALONGSTRING "A long string of nonsense that would've been truncated."

   size_t len;
   char *string = calloc(NC, sizeof(*string));

   len = snprintf(string, NC, ALONGSTRING);

   printf("Truncated string: '%s'\n", string);

   if(len >= NC) {
       string = realloc(string, (len + 1) * sizeof(*string));
       snprintf(string, len + 1, ALONGSTRING);
   }

   printf("Non-truncated string: '%s'\n", string);

   free(string);

   return 0;
}

Embedding Python

I've finally written my first embedded Python program! The following is basically a "hello world" program that demonstrates the basics of writing a C program that interacts with Python. It starts the embedded Python environment, parses some input (which is stored in the __main__ namespace) and prints out that input directly from C.

#include <Python.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
  PyObject *globals, *list, *name, *item, *mp;
  Py_ssize_t i, n;

  Py_Initialize();
  PyRun_SimpleString("a = 'some text'; b = 123.456");
  mp = PyImport_AddModule("__main__");
  list = PyObject_Dir(mp);
  n = PyList_Size(list);
  printf("n=%d\n", n);
  for(i = 0; i < n; i++) {
      name = PyList_GetItem(list, i);
      item = PyObject_GetAttr(mp, name);
      if(PyString_Check(item))
          printf("list[%d] (string): '%s' = '%s'\n", i, PyString_AsString(name), PyString_AsString(item));
      else if(PyFloat_Check(item))
          printf("list[%d] (float): '%s' = %g\n", i, PyString_AsString(name), PyFloat_AsDouble(item));
  }
  Py_Finalize();

  return 0;
}

To compile this program, you'll need to do something like

gcc embed_python.c -I/usr/include/python2.5 -lpython2.5

Modern physics course at Stanford

Apparently Stanford University has published a series of modern physics lecture videos (taught by Prof. Leonard Susskind) on Youtube. Although this is a continuing studies course, it is definitely graduate level, but taught in a very friendly manner -- a very good supplement for even graduate students of physics.

The rest of the videos are available via this link.

SI vs. Gaussian units

This is a major source of confusion for many physics students, including me. The following link is an article that explains the differences quite clearly, citing some of the historical reasons for having two popular unit systems in use at the same time.

S.I. versus c.g.s. Units by Jason Harlow