Computer Science 121

Scientific ComputingWinter 2014

Chapter 5Files and Scripts

Files and Scripts

● File (non-technical): (Word) document, image,

recording, video, etc.

● File (technical): a named collection of bytes on

disk.

● 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

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, ...)

Aside: File Deletion

foo.m 011010

110101

000100

111011

OMFG.jpg

hamlet.doc

sort.m

● Q.: What happens when you “delete” a file?

● (Drag OMFG.jpg to trash and empty trash…)

Aside: File Deletion

● A.: What appears to happen...

foo.m 011010

110101

111011hamlet.doc

sort.m

Aside: File Deletion

● A.: What actually happens ...

● Then use WinUnDelete (e.g.) to get back

OMFG.jpg

foo.m 011010

110101

111011hamlet.doc000100

sort.m

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.

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 ...

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!!

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...

Set Path

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, ...

2. Looks in the current directory for a file named foo.m. If not found, ... 3. Searches the directories on the MATLAB search path, in order, for foo.bi (built-in function) or foo.m. If not found, ... 4. Reports ERROR

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:

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'

)

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

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)

Spreadsheet data should have all cells filled (“flat format”), or Matlab will get confused:

YES NO

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

>> d = csvread('sunspots.csv', 1)

d = 1749 1 58

1749 2 62.6

1749 3 70

etc.

5.3 Importing and Exporting Data

5.3 Importing and Exporting Data

● 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', ...

Non-numerical ASCII Files

s = 32 67 97 ... % 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

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 asstrings

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

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 = myprogERROR: 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.