7, June `99
CHARACTERS AND STRINGS: Going Beyond Numbers
1999 BG Mobasseri
1
06/11/09
WHAT DO WE WANT TO DO?
x
MATLAB is first and foremost a "numerical" computation platform x In practice, however, there are many occasions where one has to work with characters or alpha-numeric quantities
1999 BG Mobasseri
2
06/11/09
EXAMPLES
x
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
1999 BG Mobasseri
3
06/11/09
CHARACTER ARRAY
x
How does MATLAB define a character array? x Any expression enclosed inside single quotes is converted to a character array
u=`villanova' u is defined as a 1x9 character array taking up 18 bytes
1999 BG Mobasseri 4 06/11/09
OTHER FORMS
x
All the usual MATLAB conventions apply. Type these out and see
u=[`villanova','university'] is a 1x19 array u=[`charles';'richard']. Check size of u u=[`villanova';'university']. Does this work? why
1999 BG Mobasseri
5
06/11/09
STRING COMPARISON
x
One of the key operations performed on character arrays is string comparison x Take
s1=`ece 2290' s2=`ece 4790' c=strcmp(s1,s2)
x
c=0 means s1 and s2 are not equal. c=1 means they are
1999 BG Mobasseri 6 06/11/09
PARTIAL COMPARISON
x
It is possible to do partial matching, say, the first 3 characters
c=strncomp(s1,s2,4)
x
The result of this comparison will be c=1, indicating that the first 4 positions are the same
1999 BG Mobasseri
7
06/11/09
RELATIONAL OPERATORS
x
Relational operators (==,<=, >=) can be applied to character arrays
s1=`james' s2=`jenny' s1==s2 returns a binary array with 1's pointing to positions of equality
x
In one statement find out in how many positions do s1 and s2 differ?
1999 BG Mobasseri 8 06/11/09
SEARCHING FOR A STRING
x
We can find the occurrence of a string by using findstr
label=`villanova' position=findstr(`nova',label)
x
findstr points to the beginning position of the string. Here, position=6
1999 BG Mobasseri
9
06/11/09
REPLACING A STRING
x
We can replace the occurrence of a string using strrep
strrep(label,'vill','Vill')
x
Try changing Bill Clinton to William Clinton using strrep
1999 BG Mobasseri
10
06/11/09
NUMBER/STRING CONVERSION
x
Sometimes we need control over individual digits x Take
n=2290 m=int2str (n)
x
As a result, m becomes a 1x4 array with m(1)=2. You can set, for example, m(2)=3
1999 BG Mobasseri 11 06/11/09
Explore!
x
Let's set n=`2290'. If you type n, you will get 2290 but this is not the same as two thousand two hundred 90. x Define m=`2409' and do n+m. What do you get and why?
1999 BG Mobasseri
12
06/11/09
LABELING PLOTS AT RUNTIME
x
Sometimes plot labels are not known before execution x For example, plot title should say... voltage ranges from 5 to 9 volts. The problem is these numbers determined are at runtime x How do we pass 5 and 9 to the title command at runtime?
1999 BG Mobasseri 13 06/11/09
Takes input from keyboard
Try it!
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
x
Let's say we want to create a text string that says
temperature plot:4pm to 6pm
x
Call the string s
s=[`temperature plot:',int2str(4),'pm to `,int2str(6),'pm']
1999 BG Mobasseri
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06/11/09
TITLING A PLOT: TRY IT
x
The following code inserts a parameter, determined at runtime, in a graph title
for i=1:4 f=i;% pick a frequency x=cos(2*pi*f*t); %signal to plot figure(i);%number figures plot(t,x);grid; s=['a sinusoid of frequency ',int2str(f),' Hz'];%construct title title(s) end
1999 BG Mobasseri 16 06/11/09
SAVING LARGE NUMBER OF FILES AT ONCE
x
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
1999 BG Mobasseri 17 06/11/09
ILLUSTRATING WHAT WE WANT
x
Say we want to generate 10 sinusoids each at a different frequency then save them to individual files t=0:0.01:1; for f=1:10 x=cos(2*pi*t*f); save datafile x end
1999 BG Mobasseri 18
x
What we really want save statement to do is
save datafile1 x save datafile2 x save datafile3 x ...
06/11/09
SOLUTION:eval
x
eval(s) evaluates string s as if it was an explicit MATLAB statement typed in the command window x For example,
eval(`plot(x)')
is equivalent to plot(x)
1999 BG Mobasseri
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06/11/09
CUSTOMIZING STRING AT RUNTIME
x
First, put together the string piece by piece. Here we want the filename appended by the loop index, i.e. datafile1, datafile...