Closure

In JavaScript, we can create closure like this,

var foo = (() => {
  let x = 0;
  return () => {
    return x++;
  }
})();

for (var i = 0; i < 10; i++) {
  var x = foo();
  console.log(x);
}

But translating above code into C++, it cannot work as expected for the variable, unless the variable is outside the lambda function.

// In a function
int x; // variable here
auto foo = [&x]() {
  x = 0;
  return [&x]() {
    return x++;
  };
}();
for (int i = 0; i < 10; i++) {
  int x = foo();
  cout << x << endl;
}

This is because C++ variable can be only accessed in the function scope. After the function return the value, the variable is not accessible anymore.

Similarly, Python can also return the function, but it cannot work as closure like JavaScript. However, Python can create non-primitive variable such as object or list so that the variable will be accessed by reference.

def _foo():
    x = [0]
    def increment():
        x[0] += 1
        return x[0]
    return increment

foo = _foo()
for i in range(10):
    x = foo()
    print(x)

Using Python as the most powerful calculator

Once I was looking for “expression calculator”. The expression calculator is different from normal calculator like calc.exe, which cannot use variable freely as algebra. But the expression calculator, we can predefine a variable with a value, then evaluate the variables in algebraic expression. It is very useful. My favourite expression calculator is SpeedCrunch.

Then, recently, I want to test a math function in SpeedCrunch, but this is impossible, since expression calculator can only define the variable, but not define a function. I remembered the title of Python Tutorial, “Using Python as a Calculator“. Yes, expression calculator, with ability to define function, then Python will be the one.

So, I tried Python, writing the function in the interactive console, easy as it is. Not only that, one can do the fractions and also calculating the date time using Python. So, Python is now my #1 expression calculator. The only drawback I found is that Python does not have the mathematic constants as SpeedCrunch. That means, one needs to find the constants, and also the the unit conversion by our own.

Besides that, to use math functions such as logarithm and trigonometry functions, one must import the math module.

from math import *
from fractions import Fraction #to use the fraction
from datetime import *

Comic Downloader alpha stage

I wrote several Greasemonkey userscripts. However, the Javascript has its own limitation for security reason, especially when a website uses scripts from cross domain. As a result, Javascript cannot get the information, and my userscript fails to work. Therefore, I start another project, in order to solve the cross domain problem using Python, and targeted on Linux (Ubuntu).

The current development stage is alpha, and targeted only on one website, since the other websites I frequently visit still work with Greasemonkey userscript. The alpha stage of Comic Downloader is currently work in command-line. GUI feature will be added in future with PyGTK. Currently, the command-line is able to solve my problem for offline comic reading.

Feel free to visit the project page.