Strings

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• Question 1
• Question 2
• Question 3

GROW STEM

Speaker Thursday (April 27), 5:15, Newton 214

Reginald Byron, Southwestern University and Geneseo Alum

“Implicit Bias, Microaggressions, and the March Toward More Inclusive Pedagogies”

SOFIs

You get paid (with $1 added to your printing balance) for completing SOFIs! (And thanks to those of you who have already completed them for this course.)

Questions?

Strings

Sections 7.1 and 7.2

String Input

What happens if you try to reply to an input command with a string? Alternatively, how do you reply to an input command with a string?

• input turns out to be more complicated than it might look. It normally tries to interpret anything you enter as a Matlab expression to evaluate, so strings have to be enclosed in single quotation marks. But adding a second argument of 's' to input tells it to always treat all input as a string, without quotation marks.

>> input( 'Enter a string ' )
Enter a string 'good morning'
ans =
good morning
>> input( 'What is your name? ' )
What is your name? Joe
Error using input
Undefined function or variable 'Joe'. 
What is your name? 3*sin(pi) - 17^3
ans =
       -4913
>> input( 'What is your name? ', 's' )
What is your name? Joe
ans =
Joe
>> input( 'What is your name? ', 's' )
What is your name? 3* sin(pi) - 17^3
ans =
3* sin(pi) - 17^3

Write a script that prompts for and inputs a description of today’s weather, and then creates a string of the form “I’m glad it’s <description of weather> today.”

Relevant ideas or questions from the reading:

• What was the reading saying about uint8, codes, etc?
  • In a computer, characters are represented by numeric codes. Historically the mapping of codes to letters was the ASCII system, with possible codes from 0 to 127. Today it’s Unicode, with around 2 million possible codes to handle the full range of symbols, alphabets, etc. used around the world. The Matlab function uint16 (uint16 is actually preferable to the uint8 described by the book) translates characters to numeric codes, while char translates the other way.

>> s = 'ABCabc 012 &*. Σ'
s =
ABCabc 012 &*. Σ
>> uint16( s )
ans =
  Columns 1 through 7
     65     66     67     97     98     99     32
  Columns 8 through 14
     48     49     50     32     38     42     46
  Columns 15 through 16
     32   8721
>> char( 6 )    % 6 doesn't correspond to a printable character
ans =

>> char( 97 )
ans =
a
>> char( [ 89, 3412, 87, 999 ] )
ans =
YൔWϧ

How do you get a single quotation mark into a string?

• Represent it by a pair of single quotation marks:

>> 'I''m glad'
ans =
I'm glad

Solution (in script file “weather.m”):

word = 'sunny';
phrase1 = 'I''m glad it''s';
phrase2 = 'today';
sentence = [ phrase1, blanks(1), word, blanks(1), phrase2 ]

Running this produces:

>> weather
sentence =
I'm glad it's sunny today

Comments:

• Strings get made in several ways, including as constants between single quotes, from input commands with the 's' option, or by concatenating shorter strings together.
• Use the blanks function to insert spaces

Comparing Strings

Extend the script from above so that it generates different strings for different kinds of weather, e.g., “I’m sorry....” if the weather is “rainy,” “I’m glad...” if the weather is “sunny,” maybe a couple of others.

Relevant ideas or questions from the reading:

• The strcmp family of functions (strcmp, strcmpi, strncmp, strncmpi) lets you test whether strings are equal to each other.

>> strcmp( word, 'rainy' )
ans =
     0
>> word
word =
warm and sunny
>> strcmp( word, 'warm and sunny' )
ans =
     1

• Use these functions as you would use any other logical expression, e.g., as the test in an if statement.

Solutions (still in script file “weather.m”):

word = input( 'Weather description: ', 's' );
if strcmp( word, 'rainy' )
    fprintf( 'I''m sorry it''s rainy today\n' );
elseif strcmp( word, 'sunny' )
    fprintf( 'That''s nice!\n' );
elseif strcmpi( word, 'sunny' )
    fprintf( 'I''m glad\n' );
else
    fprintf( 'I''ve never seen %s weather\n', word )
end

Running this now produces interactions such as...

>> weather
Weather description: sunny
That's nice!
>> weather
Weather description: SUNNY
I'm glad
>> weather
Weather description: foggy
I've never seen foggy weather
>> weather
Weather description: sunny
That's nice!

Comments:

• The strcmp family of functions is a good way to compare strings, with options for making the comparison case-sensitive or -insensitive and using all the characters in the strings or only some.
• The weather script also demonstrates that you can embed strings in output from fprintf by using the %s directive.

Overall Comments

The string construction, comparison, and output operations discussed here allow you to do a great deal of text processing in Matlab.

You can do even more by remembering that strings are really vectors of integers, and using standard vector operations on them.

Next

Cell arrays

Read textbook sections 14.1 through 14.3

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