Functions and text files

Transcription

1 Functions and text files Francesco Vespignani DiSCoF Università degli Studi di Trento. December 3, 2009

2 Today Functions and Scripts Latex Strings Text Files Practice

3 Function and Scripts

4 Finalizing a script Once you have solved a problem you can save it in a.m file and generalize the results. Where to save it? In the current directory or in the matlab path. The matlab path is simply a list or directories where to look for user-defined scripts and functions.

5 Types of scripts Matlab has three ways to create new commands: scripts functions mex-files

6 Scripts and functions Scripts are just a sequence of commands that run in the same workspace: all variables defined within the caller workspace may be changed, every variables defined by the script are visible in the caller workspace. Functions strictly define which variables of the caller are visible within the script (input arguments) and which one are returned back to the caller workspace output arguments. Moreover functions are compiled the first time that are called, while scripts are line-by-line interpreted as if they are typed on a terminal. Advantages of functions: it guarantees information encapsulation. Disadvantages of functions: when a large amount of data is passed as an argument a copy is created in memory and this can reduce performances.

7 Function s syntax The difference between scripts and functions depends on the first line of the.m file Syntax of a function: function [out1, out2,...] = MyFunctionName ( in1, in2,...) %MYFUNCTIONNAME makes something interesting % MYFUNCTIONNAME(a, b) returns some values... % function, argument, output description... % % example... %Written by Francesco Vespignani, version 0.98, date January 20, Function body... out1 = ciao... end A function must be stored in a file with the same name as the function name (in case of disagreement the files name dominates).

8 Global variables The function workspace is distinct from the caller s workspace. If you need you can create global variables, that are visible from within every function that has the declaration. Globals are useful to avoid a copy of a large data set at every function call or to share values between functions. Other programming languages (C) have a argument passing by address possibility. Example from Mastering matlab: function mytic global mytictoc mytictoc = clock; function mytoc global mytictoc disp(etime, clock,mytictoc);

9 Arguments A function can have a variable number of input and output arguments. Function behavior can be programmatically changed as a function of the number and types or arguments. For these purposes there are special variables: nargin: number or input argument in the actual call to the function nargout: number or output argument in the actual call to the function varargin: cell array containing the input argument values varargout: cell array containing the output argument values

10 Arguments Cell arrays are similar to arrays but can contain different types ordata. You can access to the elements by using Braces instead of Square brackets: varargout{12} In order to use varargin, varargout this must be explicitly declared in the function statement. Otherwise a function called with too much input or output arguments rises an error. Good practice is to do argument checking to prevent unexpected results. With if... then... else... end construct you can check if the arguments are of the right datatype or if numbers are they are within a specific range of values (e.g. positive, even...). A function typically exits at the end of the script. You can return to the caller execution stream with the return or with the error command.

11 Practice We can try to generate a function that loads a file with a bw snodgrass bmp image, transforms it in a rgb or indexed image and changes foreground and background colors. Let s make some input and output optional (returning the image or writing on a new file, changing only fg or both...) Provide an help.

12 Latex

13 A brief history of L A TEX L A TEXis one of the evolution of TEX, written by Donald Ervin Knuth, Stanford University starting from 1978 (see the wikipedia entry: TEX). TEXwas primary intended for typesetting mathematical formulas, given that new press technology were very poor compared to classical typographical techniques. i A B = P r ˆn da = P r dv. (1) Further examples of basics mathematical typesetting by Harvey Gould at

14 L A TEXrepositories and documentation Resources: The Tex users community web page : the CTAN archive: A Windows distribution: A text editor with syntax highlights and buttons for running the program may be useful (Emacs, WinEdt, TeXnic,...) Manuals and tutorial: There are many on the web... The not so short introduction to LaTeX learning by example (many authors public their source.tex files...)

15 How does L A TEXwork? It is a markup language, similar to html and xml. It is somewhat a script mixing data with tags (preceded by a backslash) that are commands that tell to the program how to render a particular type of text or how to use some data to create graphic objects in the output file. The output file is a vector graphic data file: dvi: (device independent) a typical latex generated format postscript: a language originally developed for interfacing in the same way to different printers and plotters (see Ghostscript, Ghostview and GSview). pdf: the file format created for Acrobat Reader by Adobe svg: scalable vector graphics an xml format

16 Halloworld in L A TEX Write in a file prova.tex the following: \documentclass{article} \begin{document} halloworld! \end{document} Then run from a command line >pdflatex prova.tex. A note about Miktex installation on windows (installing the full distribution with most of the packages takes time and disk space). On-the-fly package installation is possible (if allowed by the network firewall settings).

17 L A TEXDocument basic structure The structure of a L A TEXdocument: \documentclass{article} Preamble containing style-packages, definitions, macros \begin{document} % % Body of the document with sections, tables, figures etc. % \end{document}

18 learn L A TEX See simple examples from the web: download small2e.tex sample2e.tex Read the html help (cs.wlu.edu/~necaise/refs/latex2e/ or a local one) to understand how to make paragraphs, sectioning, environments (equation, table, figure, tabular), cross-references within the document. Figure numbering, section pages and bibliographic references are automatically managed. Complete refresh (e.g. of table of contents) may require to run pdflatex multiple times, since the output creation is strictly sequential. BibTeX is a different TEXvariant to print references on different formats (apastyle...).

19 Packages Basic latex allow to do relatively few thinks. It evolves by packages provided by the developers community. Some very useful commonly used packages: geometry: to personalize page borders, spaces etc. fancyhdr: full left-right alternating customization of page headers savetrees: basic latex has a elegant large borders page layout, however trees are important babel: support for many languages inputenc: defines the encoding of special characters (accents, ümlaut...) graphicx: to include vector and raster images

20 modern L A TEXexamples Making slides or posters: Beamer! beamer Beamer User Guide a beamer quickstart beamerposter (or sciposter) Drawings, complete control of arrows, overlays etc.: TiKz TikZ and PGF manual Examples web page L A TEXfor linguistics: Latex4Ling If you want to write a real book in L A TEXsee the class memoir: memoir

21 L A TEX I ll provide some examples from my own works on the website. In particular there are some basic document classes: letter article report book L A TEXis not easy to learn (steep beginners learning curve) but after some experience you can adapt many examples you find in the web to your purposes. Unlike microsoft word or other similar application it is easy to integrate into a script and execute it automatically, in order to extend graphic capability of a program (as matlab).

22 Install and configure MikTeX and TeXnic Two problems: access to the network installing software (user privileges) We are not directly connected to the web, we need to manually configure the proxy in the applications that need a web connection: address: proxy.unitn.it port: 3128

23 Why we need that Opensource languages and applications, but also Matlab sometimes need to download and install additional components (packages). Packages are bundles of scripts that provide additional features, typically contributed by users and not only by the application core development team. Package differs from simple scripts in that they follows specific principles that minimize interference with the core functions and with the other packages and scripts. Packages may depend on other (user-contributed) packages, this give rise to a somewhat complex hierarchy. Packages needs to be aligned in that some package needs a specific version of the environment or of other packages. Why not installing all the packages? They are a lot (more popular an application is, more packages, you will probably never use, exists).

24 How to install packages, manually Typically packages can be installed either manually or automatically. Manual installation requires the download of the package from some web site, in the form of a tarball or a zipped bunch of files. The tarball has to expanded into a specific place within the directory tree of the application. After that, for most applications, some command has to be run in order to refresh an internal list of the available packages.

25 How to install packages, automatically Usually applications such as MikTex have an automatic package download and installation system. This is available only for popular packages that are added in the software distribution. (Some new or experimental packages may not be available in the distribution and this is the only situation where manual installation is mandatory). Distributions are usually stored on a large number of computers that are kept aligned with the main distribution source (Mirror sites). Automatic installation guarantees that the right package version for the installed distribution/version is installed.

26 Knowing the structure of the application On windows usually installing and configuring an application involves many system directories, on unix-style applications (latex, R, matlab) usually everything is installed in the application directory that may have a rather complex structure. Typically what we need to know of this complex structure is where the executable files are and where the add-on packages and documentation are placed.

27 Let s see the MikTex hierarchy executables: MikTex X.Y/miktex/bin latex packages: MikTex X.Y/tex/latex packages documentation: MikTex X.Y/doc/latex

28 Practice Configuring MikTex for network packages installation. Telling TeXnic (or other editors) where the latex executables are. Practice with on-the fly package download and installation. Hoping that now the questionario and other latex examples works...

29 Strings

30 Text processing in matlab Strings in matlab are just arrays of characters. In other scripting languages there are more sophisticated text processing tools. However there are specialized functions for strings: let s see doc strings. For today s practice see in particular strtok for segmenting a string. Advanced string functions are regular expressions (see regexp).

31 Text Files

32 Files There are specific functions in matlab for reading and writing to files (similar to C). Basic concepts about files: special commands allow to open and close files. This guarantees that during file processing other applications does not change the file content. file access is typically sequential (read or write a piece after the other) or random (using special function that places reading or writing operation at specific points of the file). files can contain text or binary data. The open instruction can specify the type of file. Text files automatically recognize end-of-line and other special characters. When opening a text file you may specify the type of encoding (accent letters, chinese characters...).

33 Files For reading and writing on files matlab has a full set of functions (see halp iofun) at different level: dlmread, dlmwrite read and write entire delimited files (as.csv tables), does not need open/close functions. fprintf, fscanf read and write text files using c-style formatting fgets fgets gets line-by-line from a text file fread, fwrite low level io functions for binary data The same instructions can be used for virual files that can be pipeline communication with peripherals, processes or serial and parallel ports.

34 Practice

35 Today and homework practice Take a text (atext.txt) from a text file and format it using L A TEX. After this try to generate from matlab a number of pdf copies of the same text replacing at random words with space for completion. Completion texts were used in the fifties as a psycholinguistic tool to evaluate text readability (application to propaganda during the cold war). The idea is to replace words and see it the people completes with the right words. Our scripts should be able to produce many versions of the same text with different random words substituted with spaces.

36 A word about randomization Matlab has a function rand that produces a pseudo-casual sequence of number uniformly distributed between zero and 1. Pay attention that every time you launch rand after opening matlab the exact same sequence is returned. This is done so the same sequence can be produced when testing an algorithm. To change the sequence you have to change the seed that is an initial value of the random algorithm that determines causally the sequence. To set a random seed as a function of the current date and time you can run: rand( twister,sum(100*clock)). See the help page for further information.