# What is SVN?

To say it in very simple terms, SVN allows you to store your files on a server with a change history and have a "working copy" on any computer you like. You only work in your working copy and at some intervals tell SVN to copy the changes you make to the server. SVN will then overwrite the files on the server, but at the same time keep a record of what has changed. This means, that you can always go back to some earlier version – no more need for manual backup!

Also, SVN is great for working in groups. Because the files are on the server and everybody can have his own working copy on his own computer, you need not send around files with the changes you make. Every group member just makes her changes whenever she is ready to get the changes to the group, she just tells SVN to copy them to the server. The other group members only have to update their working copy with the newest version on the server and all have the same version of the code.

That’s actually about it, if you only want to use the basic functionalities. Just some terminology: Creating a working copy is called "checkout", copying code from the server to your working copy is called "update" and copying your changes from your working copy to the server is called "commit".

# Installing a LaTeX package

Let’s say you want to create a A0 poster with LaTeX. You find an example on the internet that starts like this:

\documentclass[final]{beamer}
\usepackage[orientation=landscape,size=a0,scale=1]{beamerposter}
\usepackage{lipsum} % lorem ipsum


You download the example ‘example.tex’, run pdflatex on it and it fails like this:

me@mycomputer: pdflatex example.tex
This is pdfTeX, Version 3.1415926-2.3-1.40.12 (TeX Live 2011)
restricted \write18 enabled.
entering extended mode
(./example.tex
LaTeX2e <2011/06/27>
Babel  and hyphenation patterns for english, dumylang, nohyphenation, lo
(/usr/share/texlive/texmf-dist/tex/latex/beamer/beamer.cls

[...]

Type X to quit or  to proceed,
or enter new name. (Default extension: sty)

Enter file name:


This means, that this particular LaTeX package ‘beamerposter’ is not installed on your machine. Bad luck.

## What to do if you have admin permissions

On linux, open your favourite package manager (e.g., Synaptic), type the name of the LaTeX package (in this case ‘beamerposter’). If the result shows a linux package like ‘texlive-latex-extra’ install it and be happy.

## What to do if you do not have admin permissions

Go to CTAN. Search for the missing package name and click on the best result. In the beamerposter case, you will end up here. To get to a page where you can actually download the package, you need to follow the link listed under CTAN path in the box at the bottom of the page. Click on ‘Download’ and save the ‘beamerposter.zip’ somewhere on your computer.

We will also assume that the second package, ‘lipsum’, is also missing, you would find it on CTAN here.

### 2. Extract the package to the correct location

The READMEs of LaTeX package usually contain "Put it in your tex folder" or "Put it somewhere where LaTeX can find it" (if they contain anything on installation at all). What this actually means is, that there are several possibilities. LaTeX searches for sources in a few directories, depending on your system and LaTeX distribution. Some examples for linux and texlive are:

/usr/share/texmf/
/usr/share/texlive/texmf/
/usr/local/share/texmf/tex/latex/
~/texmf/


If you don’t have admin permissions, the easiest is to create a folder ‘texmf’ in your home directory (~). You will need in this folder a subfolder ‘tex’, and then ‘latex’. So in total you should have:

~
|- texmf/
|- tex
|- latex


In this folder, i.e., ~/texmf/tex/latex/, you can put any style files and latex will find them. It is advisable to create separate folders for separate packages, so we will extract the ‘beamerposter.zip’ that we downloaded into the folder ~/texmf/tex/latex/beamerposter/ and ‘lipsum.zip’ into the folder ~/texmf/tex/latex/lipsum/. This is what the folder looks like now:

~
|- texmf/
|- tex
|- latex
|- beamerposter
|- beamerposter.pdf
|- beamerposter.sty
|- beamerposter.tex
|- example.tex
|- lipsum
|- lipsum.dtx
|- lipsum.ins
|- lipsum.pdf


As you can see, we now have a ‘beamerposter.sty’. So if this were the only package we needed, we could skip step 3. Unfortunately we are still missing ‘lipsum.sty’, so this is what step 3 is about.

### 3. Create a style file

As we see, there is no style file ‘lipsum.sty’. There is only a ‘lipsum.ins’ and a ‘lipsum.dtx’ file. The .dtx file is only to create the documentation and we can ignore it here. To create the style file, run latex (latex, not pdflatex!) on ‘lipsum.ins’:

me@mycomputer: latex lipsum.ins


The result should look like this:

~
|- texmf/
|- tex
|- latex
|- beamerposter
|- ...
|- lipsum
|- lipsum.dtx
|- lipsum.ins
|- lipsum.log
|- lipsum.pdf
|- lipsum.sty


### 4. Try pdflatex again

And it should work (unless of course a different package is missing…).

## Updating the Database

If you install fonts and in some other cases you need to update the LaTeX package database. On linux/texlive this is done with ‘texhash’:

me@mycomputer: texhash


## More

This works for regular LaTeX packages. Bibtex packages go to texmf/bibtex. If there are fonts involved, you will need to put them in texmf/fonts and it might get tricky.

# Typesetting text in math mode

In information retrieval and text classification, tf-idf plays a big role. Read the Wikipedia article to learn what it is about, here I want to deal with the problem of typesetting the formula in LaTeX.

The formula is log-weighted term frequency tf times inverse document frequency idf, if we naivly write this down, we arrive at this:

tf-idf_{t,d} = (1 +\log tf_{t,d}) \cdot \log \frac{N}{df_t}


When you look at the LaTeX output, you will see that several things go wrong. In math mode, LaTeX interprets two letters next to each other as a product of two variables. So the name tf becomes the mathematical expression “t times f” and is typeset accordingly. Also, in case of tf-idf, the name contains a hyphen. In math mode a hyphen between two expression is interpreted as a minus sign. So this is definitely not what we want.

How do we solve the problem? What we want is that this part is interpreted as normal text. One possibility to add text to equations is the command \mbox{} (another is the command \text{} which requires the amsmath package). So this is it:

\mbox{tf-idf}_{t,d} = (1 +\log \mbox{tf}_{t,d}) \cdot \log \frac{N}{\mbox{df}_t}

# Change the encoding of a file

My favourite topic is "encoding" (of course that was sarcasm). So my first post is about how to change the encoding of some text file from Latin-1 to UTF-8 on command line:

iconv -f latin1 -t utf8 source_file > target_file
`

Of course we need to know what encoding the file is in… which may be a topic for some future post.