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

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

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’

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

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' ]

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

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)

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

#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 ]

#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!’)

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

#3: Comparison Remember: strings are vectors of chars s1 = 'abcd'; s2 = 'xyzw'; s3 = 'cat'; Avoid normal comparison operators! ◦s1 == s2, 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

#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( i ) % ans = ‘ i’ disp([‘You took ’ num2str(num_guesses) ‘ tries’]); str2num( ‘ i’ ) % ans = i

#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]

Exercise Write a function that given a string str, returns a string replaced_str which is the same as str except all occurrences of ‘great’ in str have been replaced by ‘little’ Write a function that given strings str,a,b replaces all occurrences of a in str by b. ◦ Modify the function so that it also returns the number of replacements.

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

Review Strings doc strings From MATLAB Help window, Contents tab, choose MATLAB → Programming Fundamentals → Characters and Strings.