CHARACTERS AND STRINGS: Going Beyond Numbers

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CHARACTERS AND STRINGS Going Beyond Numbers

• June 7, 99

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WHAT DO WE WANT TO DO?

• MATLAB is first and foremost a numerical
  computation platform
• In practice, however, there are many occasions
  where one has to work with characters or
  alpha-numeric quantities

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EXAMPLES

• Here are a few examples
• placing titles and labels on graphs
• working with file names such as day1.dat,
  day2.dat, etc.
• comparing two strings, one user supplied and one
  in the library
• decimal to binary string conversion

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CHARACTER ARRAY

• How does MATLAB define a character array?
• Any expression enclosed inside single quotes is
  converted to a character array
• uvillanova
• u is defined as a 1x9 character array taking up
  18 bytes

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OTHER FORMS

• All the usual MATLAB conventions apply. Type
  these out and see
• uvillanova,university is a 1x19 array
• ucharlesrichard. Check size of u
• uvillanovauniversity. Does this work? why

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STRING COMPARISON

• One of the key operations performed on character
  arrays is string comparison
• Take
• s1ece 2290
• s2ece 4790
• cstrcmp(s1,s2)
• c0 means s1 and s2 are not equal. c1 means they
  are

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PARTIAL COMPARISON

• It is possible to do partial matching, say, the
  first 3 characters
• cstrncomp(s1,s2,4)
• The result of this comparison will be c1,
  indicating that the first 4 positions are the same

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RELATIONAL OPERATORS

• Relational operators (,lt, gt) can be applied
  to character arrays
• s1james
• s2jenny
• s1s2 returns a binary array with 1s pointing
  to positions of equality
• In one statement find out in how many positions
  do s1 and s2 differ?

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SEARCHING FOR A STRING

• We can find the occurrence of a string by using
  findstr
• labelvillanova
• positionfindstr(nova,label)
• findstr points to the beginning position of the
  string. Here, position6

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REPLACING A STRING

• We can replace the occurrence of a string using
  strrep
• strrep(label,vill,Vill)
• Try changing Bill Clinton to William Clinton
  using strrep

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NUMBER/STRING CONVERSION

• Sometimes we need control over individual digits
• Take
• n2290
• mint2str (n)
• As a result, m becomes a 1x4 array with m(1)2.
  You can set, for example, m(2)3

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Explore!

• Lets set n2290. If you type n, you will get
  2290 but this is not the same as two thousand two
  hundred 90.
• Define m2409 and do nm. What do you get and
  why?

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LABELING PLOTS AT RUNTIME

• Sometimes plot labels are not known before
  execution
• For example, plot title should say... voltage
  ranges from 5 to 9 volts. The problem is these
  numbers are determined at runtime
• How do we pass 5 and 9 to the title command at
  runtime?

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Try it!
Takes input from keyboard
Predefined function in MATLAB
Convert xmax to string
Form a long string to go in the title command
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MIXING STRINGS AND NUMBERS

• Lets say we want to create a text string that
  says
• temperature plot4pm to 6pm
• Call the string s
• stemperature plot,int2str(4),pm to
  ,int2str(6),pm

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TITLING A PLOT TRY IT

• The following code inserts a parameter,
  determined at runtime, in a graph title
• for i14
• fi pick a frequency
• xcos(2pift) signal to plot
• figure(i)number figures
• plot(t,x)grid
• s'a sinusoid of frequency ',int2str(f),'
  Hz'construct title
• title(s)
• end

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SAVING LARGE NUMBER OF FILES AT ONCE

• In many situations your code generates a large
  number of files which you want them saved, each
  under a different name, like
• datafile1.mat
• datafile2.mat
• etc

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ILLUSTRATING WHAT WE WANT

• Say we want to generate 10 sinusoids each at a
  different frequency then save them to individual
  files
• t00.011
• for f110
• xcos(2pitf)
• save datafile x
• end

• What we really want save statement to do is
• save datafile1 x
• save datafile2 x
• save datafile3 x
• ...

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SOLUTIONeval

• eval(s) evaluates string s as if it was an
  explicit MATLAB statement typed in the command
  window
• For example,
• eval(plot(x))
• is equivalent to plot(x)

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CUSTOMIZING STRING AT RUNTIME

• First, put together the string piece by piece.
  Here we want the filename appended by the loop
  index, i.e. datafile1, datafile2 etc
• Then insert the string inside eval

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ANATOMY OF A STRING

• Here is what you need to do
• ssave datafile,int2str(i), x
• then do eval(s)

blank
digit to be appended to file name
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LOADING MULTIPLE DATA FILES

• The inverse of saving many files at once is
  loading them
• If you are given hundreds of files, how would you
  read them?

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EXAMPLE LOADING DIGITAL VIDEO

• Video is a collection of thousands of still
  frames that are saved in files named
  frame1,frame2,frame3 etc.
• Question is how do you import them into MATLAB
  frame by frame, but not manually?

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COMPOSING THE LOAD STRING

• To load file hundreds of image frames, here is
  the code we can use

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HOMEWORK

• Generate 5 sinusoidal functions. The i th signal
  is at frequency 10i Hz. Plot each in a separate
  figure window with its title stating the
  frequency of the sinusoid
• Save each sinusoid in a separate datafile calling
  them sine1.dat, sine2.dat etc. Check to see if
  the files were actually created