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COMP 116: Introduction to Scientific Programming

In this lecture, we delve into the concept of strings in MATLAB, exploring how they serve as sequences of characters, including letters, digits, and symbols. We cover essential operations such as string concatenation, case conversion, and comparison. Additionally, we demonstrate practical applications like input/output handling and text analysis. With exercises on string manipulation, this lesson aims to equip you with the necessary skills to effectively work with strings in MATLAB programming.

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COMP 116: Introduction to Scientific Programming

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  1. COMP 116: Introduction to Scientific Programming Lecture 24: Strings in MATLAB

  2. So far…. • Vectors, matrices • Of numbers • Linear systems • if-elseif-else-end • Loops: while, for • functions • Today: letters, words, sentences • characters and strings

  3. Characters and strings • Characters: ‘a’, ‘b’, ‘3’, ‘ ’ • Letters, digits, punctuation, symbols, whitespace(space, newline, … ) • Strings are sequences(vectors) of characters • ‘Meep’ is nothing but [‘M’, ‘e’, ‘e’, ‘p’] • [‘Bat’, ‘man’] gives ‘Batman’ • NOTE: Strings are rows (not columns) • ‘’ % empty string, length == 0 • ‘a’ % Single character • ‘COMP 116’

  4. Why do we need strings ? • Text Input / Output • Remember >>input(‘Give me a number:’)? • File Input / Output • Analyzing text

  5. Constructing Strings • Enclose string in single quotes • 'This is a string' • NOTE: the actual quote symbols are apostrophes • Row vector of characters • ['a', 'n', 'o', 't', 'h', 'e', 'r' ]

  6. Strings as a vector of chars • Can be manipulated like any other vector s1 = 'The quick brown fox ' s2 = 'jumped over the lazy dog' s = [s1 s2] % concatenate strings s(5) % ans = q s(17:19) % ans = fox jIdx = find( s == 'j' ) % ans=21 jStr = s(jIdx:jIdx+3) % ans = jump

  7. Exercise • Assume that str=‘this is a string’. What do the following commands do: >>str(1:4) >>str(1)==‘t’ >>str(1)=‘T’ >>str(end:-1:1)

  8. String operations • Combining • Case Conversion (upper/lower) • Conversion • Comparison • Search / Replace • Other useful functions

  9. #1: Combining strings (concatenation) • Given a, b - both string variables • Same as concatenating arrays • [ a b ] a = ‘With great power'; b = ‘comes great responsibility'; space = ' '; s = [ a space b ]

  10. #2: Case conversion • lower(varString) • Converts to lower case • upper(varString) • Converts to upper case • lower( ‘COMP 116' ) • % ans = ‘comp 116' • upper(‘I am not shouting!’)

  11. Exercise • Write a function that given a string str returns a string upper_str that is the same as str except the first character of every word in upper_str is uppercase i.e. given‘this is a string’ the function should return ‘This Is A String’ • HINT: A word starts at the first position (of the string) or after a space character • You will need for loops, if-else,==,upper

  12. #3: Comparison • Remember: strings are vectors of chars s1 = 'abcd'; s2 = 'xyzw'; s3 = 'cat'; • Avoid normal comparison operators! • s1 == s2, s1 < s3, s1 >= s3 • Operators work element by element (on characters) • Thus, strings (i.e., the vector of chars) must be same length • Use string comparison functions instead • strcmp(a,b), true when a and b are the same • strcmpi, string comparison while ignoring case • strncmp, strncmpi: • Similar, but compares first n characters only

  13. #4: Conversion: strings and numbers • num2str() • Converts numbers to strings • Handles real or complex number • str2num() • Converts strings to numbers • Handles real or complex number strings • num2str( 10.1 + 4.25i ) • % ans = ‘10.1 + 4.25i’ • disp([‘You took ’ num2str(num_guesses) ‘ tries’]); • str2num( ‘10.1 + 4.25i’ ) • % ans = 10.1 + 4.25i

  14. #5: Searching • strfind • Search for a string inside another string • returns indices to start of each instance strVal= ['with great power comes great responsibility.']; strfind( strVal, 'great') % ans = [6 24]

  15. Exercise • Write a function that given a string str, returns a string replaced_strwhich is the same as strexcept all occurrences of ‘great’ in strhave been replaced by ‘little’ • Write a function that given strings str,a,b replaces all occurrences of a in strby b. • Modify the function so that it also returns the number of replacements.

  16. Other string functions Look up what these functions do • strcat • strrep • strtok • mat2str • str2angle • strtrim • deblank • strjust • regexp • strread • textread • textscan • …

  17. Review Strings • doc strings • From MATLAB Help window, Contents tab, choose MATLAB→ProgrammingFundamentals→Characters and Strings.

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