HOWTO Find the Number of Pixels in a MM with Javascript

This is a simple trick to find the number of pixels in a millimeter using jQuery.

Code

function pixelsPerMM()
{
    $("body").append("<div id='onebyone' style='width:1mm;height:1mm;display:hidden;'></div>");
    var pixels = $("#onebyone").width();
    $("#onebyone").remove();
    return pixels;
}

How It Works

1. Adds a new element that is 1mm x 1mm to the page.
2. Requests the width (in pixels) of that element.
3. Deletes the element.

It is probably a good idea not to call this repeatedly in your code, as changing the web document in this manner is slow.

Why Use It

If you have an interface that you want to be exactly the same on every device, but you can't lay out the entire thing using CSS i.e. some elements are dynamically generated using Javascript, this would be a good method.

When You Are Justified in Complaining About FOSS

LibreOffice 4.0 has recently been released, and has received a lot of unfounded criticism over its new ability to be themed using Firefox Personas. Now would be a good time to go over when it is acceptable or not to complain about FOSS, as there seems to be quite a lot of confusion about that.

Most of the time, you should not complain about FOSS like this because:

• You got it for free.
• If you don't like it, you don't have to use it.
• You are encouraged to fix the things that annoy you.
• You did not do any work.

However, there are a few cases where you are allowed to complain about FOSS:

• When the product destroys your data.
• When there is no way to contribute to the project other than forking it. (e.g. Android)
• When you have contributed code/patches that are widely desired, but the maintainers will not accept them.
• When you are paying the developers to make changes, and they are not.

Back to Personas!

All of the complaints the LO team have received over this issue are unfounded because the software is free, does not force you to update to the newest version, has a remarkably open ecosystem (1500 merges from 3.0 to 4.0), and the product still works without ever using that feature.

Not only does LO 4.0 work better than the 3.X branches, it is vastly superior in speed, supported formats, and code maintainability.

This whole issue will seem silly in two months when Ubuntu releases a branded LO using a theme that adds subtleties to the document editor that perfectly integrate it with the platform, but until then hopefully these guidelines will suffice for indignant posters.

HOWTO Make a Quick and Dirty (Pseudo)Random Number Generator

If you ever need to build your own quick and dirty (pseudo-)random number generator it actually isn't that hard. This generator is used in various real-world systems:

• Visual Basic (to version 6)
• A few ANSI C implementations, including glibc
• Java's Random library

The implementation will iterate over all numbers [0,m) when using proper a, m ,c's. ¹

Implementation

#include <stdio.h>
#include <unistd.h>

const long m = 4294967296; // 2 ^ 32
const long a = 1103515245;
const long c = 12345;

long lastX = 0;

long nrandom() {
    lastX = (a * lastX + c) % m;
    return lastX;
}

void seed(int num)
{
    lastX = num;
}

int main()
{
    int i;

    seed(getpid());

    for(i = 0; i < 1000; i++)
        printf("%li\n", nrandom());
}

Note that this implementation only generates m pseudorandom numbers, in this case 4,294,967,296; so if you need more, you'll want to use a different algorithm.

---

1. These conditions require some discrete mathematics that are too long to go in to here; it involves coprime numbers and recurrence relations. (Please point out if I've missed some simple explaination in the comments, I'm fairly new to this!)↩

HOWTO Fix Flash in Chrome + Ubuntu 12.10

After a recent Chrome update, I was left without flash for this browser, which by the way handles Pandora way better than Firefox does on the same platform.
A quick fix is all that is needed:

1. Type chrome://plugins/ in to your URL bar.
2. At the upper right, press "Details"
3. Scroll down to the Adobe Flash Player entry, there should be two sub-entries, these are versions of Flash Chrome can use.
4. Disable the entry that looks like this (should be the first one):
   

   Name:   Shockwave Flash
   Location:   /home/user/.config/google-chrome/PepperFlash/11.5.31.138/libpepflashplayer.so
   Type:   PPAPI (out-of-process)
   
   MIME types:
   MIME type                       Description         File extensions
   application/x-shockwave-flash   Shockwave Flash     .swf
   application/futuresplash        Shockwave Flash     .spl




5. Restart the browser.

For some reason the flashplayer using the pepper API appears not to be working, so you'll just have to stick with the one that was installed to your platform through the package manager (a safer bet anyway).

HOWTO Use Python "for in" and "for else" Loops

This is a short guide to the subtleties of Python for loops, especially for those programmers from Java/C++ that may miss some of the best features because those languages don't have any equivalent, it covers:

• The basic for loop
• Looping over dictionaries
• Using the for else loop

The Basic for loop

If you come from a Java/C++/PHP background, you're used to two different for loops foreach and for. Python only has one of these, the more powerful foreach.
In Java, your for loop would look something like this:

for(int i = 0; i < 10; i += 2)
{
    System.out.println(i);
}

In Python, the same loop would look like;

for i in range(0, 10, 2):
    print(i)

The range function returns a list of numbers 0 to 10 incremented by 2.
If you wanted 0-9 inclusive, the loop would be even more simple.

for foo in range(10):
    print(foo)

In this same manner, you can replace the range() with any list:

for j in [1,1,2,3,5]:
    print(j)

Looping over Dictionaries (maps)

Maps are a very handy Python builtin feature, allowing you to create arbitrary key->value pairs.

{
    "key":"value",
    "bob":"smith",
    "age":13,
    "dob":(2012,01,01)
}

You can quickly loop over key->value pairs in a dictionary by using the following:

for key, value in a.items():
    print("{} -> {}".format(key, value))

Note that this is much faster than looping over the keys, then looking up the value for each key inside the for loop. The items function returns a list of tuples where the first value is the key, and the second a value.

In fact, you can loop over any list of tuples in a similar manner:

b = [("Perro", 1, ["Canis", "lupis"]),
 ("Gato", 2, ["Felis", "catus"])]

for name, id, scientific in b:
    print("Name: {} ID: {} Breed: {}".format(name, id, " ".join(scientific)))

The Magic of the for else Loop

If you've ever wanted to check if something happened within a for loop you can use an else block after your for loop:

for i in ["foo","bar"]:
    if i == "baz":
        break
else:
    # happens if break is not called, meaning "baz" was not found
    return -1

# i is set to the location of "baz"
return i

The example above tries to find the location of a specified string in a list of strings, if found it returns the index, otherwise -1. Of course there are far easier ways to do this, but you get the idea.

HOWTO Debug Crashes in C/C++ Applications on Ubuntu

In this howto we'll cover:

• Compiling C/C++ code for debugging
• Allowing debugging
• Viewing errors
• Fixing common gdb issues

Your computer is happily humming along and your program is progressing fine, suddenly disaster strikes! Segmentation fault (core dumped) your computer yells before it plunges back to darkness and your friendly shell prompt reappears.

The way of debugging in Ubuntu when coding by hand is not nearly as nice as popping up an IDE, but when done right can be much faster.

Our Problematic Code

For demonstration we'll be using this bit of code that is written to cause crashes.

int main()
{
    int* p = 0x0000007b; // The cause of many a Windows XP BSOD
    int j;

    for(j = 0; j < 10000000; j++)
    {
        p++;
        *p = j;
    }

    return 1;
}

In order to get the best debugging results you'll have to compile your code with the -ggdb flag, in this instance: gcc -ggdb killer.c.
If you run this program, it will die nearly right away with the error: Segmentation fault (core dumped).

The -ggdb flag to the compiler instructs it to include lots of debugging information. However, you won't want this on production executables because it takes up a lot of space. This example program was 9.6K with symbols, and 6.2K without!

Enabling the Dump

By default, Ubuntu 12.10 won't output crash information for programs you make yourself, to fix this you'll need to run the command:

ulimit -c unlimited

This will need to be run every time you log back on to your system.

Debugging

Once you have logging enabled and your program crashes a file called core should appear in the directory from which you ran the program. Use the gdb command to see what information it contains about the crash.

gdb a.out core

Where a.out is the name of your program that crashed and created the core file.

Analyzing the Debug Output

GNU gdb (GDB) 7.5-ubuntu
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/joseph/Desktop/a.out...done.

[New LWP 13663]

warning: Can't read pathname for load map: Input/output error.
Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
#0  0x00000000004004ed in main () at breakme.c:9
9           *p = j;

This is the output you'll get from the gdb command, all you want is the last three lines, from the top they tell you:

1. What happened.
2. Where the error happened (in main in file breakme.c on line 9)
3. What the line was.

Common gdb Errors

• warning: exec file is newer than core file. means the core file you're using wasn't made by your executable, you'll need to delete it and run your program again.
• warning: core file may not match specified executable file. means the core file you're running against probably wasn't made by the program you specified, gdb will probably report wrong results.

Extended Euclidean Algorithm in Python

The Extended Euclidean Algorithm is a simple extension to the Euclidean Algorithm that in addition to returning the greatest common denominator also gives numbers s and t such that gcd = s * a + t * b.
Specifically, you may want to find 1 = sa + tb which is very useful in cryptography.

The Code

def extended_euclidean(a,b):
    origa = a
    origb = b

    x = 0
    lastx = 1
    y = 1
    lasty = 0

    while b != 0:
        q = a // b
        a, b = b, a % b

        full = "%d = (%d * %d + %d * %d) - %d * (%d * %d + %d * %d) " % ( b, origa, lastx, origb, lasty, q, origa, x, origb, y )
        short = "%d = %d * %d + %d * %d" % ( b, origa, lastx - x * q, origb, lasty - y * q)
        print("%s\t%s\t%s\t%s" % (a, b, full, short))

        x, lastx = (lastx - q * x, x)
        y, lasty = (lasty - q * y, y)

    return (lastx, lasty)

The Output

Take the numbers 120 and 23:

>>> extended_euclidean(120,23)
23  5   5 = (120 * 1 + 23 * 0) - 5 * (120 * 0 + 23 * 1)     5 = 120 * 1 + 23 * -5
5   3   3 = (120 * 0 + 23 * 1) - 4 * (120 * 1 + 23 * -5)    3 = 120 * -4 + 23 * 21
3   2   2 = (120 * 1 + 23 * -5) - 1 * (120 * -4 + 23 * 21)  2 = 120 * 5 + 23 * -26
2   1   1 = (120 * -4 + 23 * 21) - 1 * (120 * 5 + 23 * -26)     1 = 120 * -9 + 23 * 47
1   0   0 = (120 * 5 + 23 * -26) - 2 * (120 * -9 + 23 * 47)     0 = 120 * 23 + 23 * -120
(-9, 47)