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Computer Science 121 Scientific Computing Winter 2012 Chapter 5 Files and Scripts.

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1 Computer Science 121 Scientific Computing Winter 2012 Chapter 5 Files and Scripts

2 ● File (non-technical): (Word) document, image, recording, video, etc. ● File (technical): a named collection of bytes on disk.

3 ● ASCII vs. Binary ● “ASCII file” means “file that can be viewed as text by a program (Notepad) that interprets each byte as an ASCII code”. ● Binary file is anything that cannot be viewed that way ● “JPEG file” means “file that can be viewed as an image by using a program (Photoshop) that interprets the bytes as JPEG-encoded image. ● “MP3 File” means “file that can be watched/heard as a video/audio recording by using a program that interprets the bytes as an MP3- encoded video / audio stream”. ● “Foo File” means “file whose contents can be experienced by using a program that interprets the bytes as a Foo encoding”. ● XML (eXtensible Markup Language) is an attempt to compromise between binary and ASCII: make all data human-readable

4 5.1 Filenames ● General format: name. extension ● For historical reasons, extension is usually three characters. ● Extension tells OS what program to use to open file (MS Word, Excel, Matlab,...)

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6 Aside: File Deletion foo.m OMFG.jpg hamlet.doc sort.m ● Q.: What happens when you “delete” a file? ● (Drag OMFG.jpg to trash and empty trash…)

7 Aside: File Deletion ● A.: What appears to happen... foo.m hamlet.doc sort.m

8 Aside: File Deletion ● A.: What actually happens... ● Then use WinUnDelete (e.g.) to get back OMFG.jpg foo.m hamlet.doc sort.m

9 Directory Structure ● Directories (folders) are organized hierarchically (one inside another) ● So we are forced to choose a single organization method (like library with card catalog indexed only by author) ● But we can use links (shortcuts) to add additional organization, without copying files.

10 Pathnames ● Pathname is “full name” of directory in a linear form –e.g., C:\MyDocuments\cs121\myproj\new\ ● Complete filename includes path – e.g., C:\MyDocuments\cs121\myproj\new\myprog.m ● This becomes important because of the...

11 Working Directory >> pwd % print working directory ans = C:\MATLAB\work ● Without extra effort, we can only access files in our working directory >> myprog % run myprog.m script ERROR: myprog? LOL!!

12 Working Directory ● Solutions ● Make shortcuts from working directory (annoying) ● >> cd('C:\MyDocuments\cs121\myproj\new\') >> myprog ERROR: Can't find someOther.m… loser! ● Use Matlab File menu to add paths: File / Set Path...

13 Set Path

14 How Matlab Uses Paths ● When we type a name foo into the interpreter, Matlab follows this sequence: 1. Looks for foo as a variable. If not found, Looks in the current directory for a file named foo.m. If not found, Searches the directories on the MATLAB search path, in order, for foo.bi (built-in function) or foo.m. If not found, Reports ERROR

15 5.2 File operators ● File write/read operators allow us to save/restore values from previous Matlab sessions. ● File / Save Workspace As... is simplest way to do this – saves everything to a.mat file ● If we want to save/restore specific variables, we can use the save and load commands:

16 5.2 File operators >> a = 'foo'; b = 2; c = pi; >> save myvariables a b >> clear >> load myvariables >> who Your variables are: a b – I never use the other syntax ( >> save('myvariables', 'a', 'b' )

17 5.3 Importing and Exporting Data Often want to get data from other programs (Excel, LabView, text editor) into Matlab, and save data in a format that other programs can read. Excel saves data in binary, proprietary (of course!). xls format

18 5.3 Importing and Exporting Data Generally, other formats will all be text- based (ASCII) –.csv : comma-delimited values (no commas in vals) –.dlm : other delimiter (allows commas in vals) –.xml : eXtensible Markup Language (newer)

19 Spreadsheet data should have all cells filled (“flat format”), or Matlab will get confused: YES NO

20 csvread operator allows us to read numerical data, but we need to cut off the header in the file: Remove it by hand from the file: >> d = csvread('sunspots-no- header.csv'); Specify # of lines to cut ignore in cvsread: >> d = csvread('sunspots.csv', 1); % ignore first line 5.3 Importing and Exporting Data

21 >> d = csvread('sunspots.csv', 1) d = etc. 5.3 Importing and Exporting Data

22 ● importdata command is useful for heterogeneous data. ● Returns a data structure: >> d = importdata('sunspots.csv') d = data: [2820x3 double] textdata : {'Year', 'Month',... colheaders : {'Year', 'Month',...

23 Non-numerical ASCII Files s = % need to munge this txt files : anything we want to treat as text (ASCII characters) >> fid = fopen('mobydick.txt'); >> s = fread(fid); >> fclose(fid) >> s

24 Non-numerical ASCII Files >> s = char(s') % transpose, textify ans = Call me Ishmael. Some years ago- never mind how long precisely -having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world.... textread does this for us, and tokenizes words into cell array: >> s = textread('mobydick.txt‘, ‘%s’) s = {‘Call’, ‘me’, ‘Ishmael.’, … Treat as strings

25 5.4 Scripts ● You know most of this stuff already ☺ ● You can run a script (e.g., myprog.m ) from the interpreter: >> myprog ● Tips − Don't name any variables myprog − Don't use any blank spaces in script names − Re-read search path stuff from a few pages back

26 5.5 Scripts as Computations ● Scripts are (mostly) like typing directly into the interpreter – so variables can get overwritten ● This also means that there is no ans value: >> x = myprog ERROR: loser trying to execute SCRIPT myprog as a program. ● Nor can we pass arguments: >> myprog(7) ERROR: My name is Donnie, and you suck at Matlab.


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