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CS1112 Lecture 17 Oct 28, 2008
Lecture slides 1
Previous Lecture: Working with images
Today’s Lecture: Characters and strings Very brief introduction to recursion—more later
Announcements: Section will be in the classrooms this week Project 4 posted. Due Thurs10/30 at 6pm.
October 28, 2008 Lecture 17 4
Characters & strings We have already used strings: n= input(‘Next number:’) fprintf(‘Answer is %d’, ans) A string is made up of individual characters, so a string is a 1-d array of characters The character array ‘CS1112 rocks!’ is of length 13, has 7 letters, 4 digits, 1 space, and 1 symbol. Use single quotes to enclose characters: ‘100’ is a character array of length 3 100 is a numeric value
October 28, 2008 Lecture 17 7
Strings as vectors Vectors Assignment v= [7 0 5]; Indexing x= v(3); % x is 5 v(1)= 1; % v is [1 0 5] w= v(2:3); % w is [0 5] : notation v= 2:5; % v is [2 3 4 5] Appending v= [7 0 5]; v(4)= 2; % v is [7 0 5 2] Concatenation v= [v [4 6]]; % v is [7 0 5 2 4 6]
Strings Assignment s= ‘hello’; Indexing c= s(2); % c is ‘e’ s(1)= ‘J’; % s is ‘Jello’ t= s(2:4); % t is ‘ell’ : notation s= ‘a’:‘g’; % s is ‘abcdefg’ Appending s= ‘duck’; s(5)= ‘s’; % s is ‘ducks’ Concatenation s= [s ‘ quack’]; % s is ‘ducks quack’ October 28, 2008 Lecture 17 8
Some useful string functions str= ‘Cs 1112’; length(str) % 7 isletter(str) % [1 1 0 0 0 0 0] isspace(str) % [0 0 1 0 0 0 0] lower(str) % ‘cs 1112’ upper(str) % ‘CS 1112’ ischar(str) % Is str a char array? True (1) strcmp(str(1:2),‘cs’) % Compare strings str(1:2) & ‘cs’. False (0) strcmp(str(1:3),‘Cs’) % False (0)
Example: capitalize 1st^ letter Write a function to capitalize the first letter of each word in a string. Assume that the string has lower case letters and blanks only.
function [str, nCaps] = caps(str) % Post: Capitalize first letter of each word. % str = partially capitalized string % nCaps = no. of capital letters % Pre: str = string with lower case letters & blanks only look for the spaces Look For The Spaces (^) October 28, 2008 Lecture 17 11
Character vs ASCII code
str= ‘Age 19’ %a 1-d array of characters code= double(str) %convert chars to ascii values str1= char(code) %convert ascii values to chars
CS1112 Lecture 17 Oct 28, 2008
Lecture slides 2
October 28, 2008 Lecture 17 12
Arithmetic and relational ops on characters ‘c’-‘a’ gives 2 ‘6’-‘5’ gives 1 letter1=‘e’; letter2=‘f’; letter1-letter2 gives -
‘c’>’a’ gives true letter1==letter2 gives false
‘A’ + 2 gives 67 char(‘A’+2) gives ‘C’ October 28, 2008 Lecture 17 13
What is in variable g (if it gets created)? d1= ‘Oct 3’; d2= ‘Oct 9’; x1= d1(5); x2= d2(5); g= x2-x1; A: the character ‘ 6 ’ B: the number 6 C: Error in the subtraction operation D: Error in assigning variables x1, x E: Some other value or error
October 28, 2008 Lecture 17 14
What is in variable g (if it gets created)? d1= ‘Oct 13’; d2= ‘Oct 29’; x1= d1(5:6); x2= d2(5:6); g= x2-x1; A: the string ‘ 16 ’ B: the number 16 C: Error in the subtraction operation D: Error in assigning variables x1, x E: Some other value or error October 28, 2008 Lecture 17 15
Example: toUpper Write a function toUpper(cha) to convert character cha to upper case if cha is a lower case letter. Return the converted letter. If cha is not a lower case letter, simply return the character cha. Hint: Think about the distance between a letter and the base letter ‘a’ (or ‘A’). E.g., a b c d e f g h …
A B C D E F G H … Of course, do not use Matlab’s function upper!
distance = ‘g’-‘a’ = 6 = ‘G’-‘A’
October 28, 2008 Lecture 17 16
Example: removing all occurrences of a character From a genome bank we get a sequence ATTG CCG TA GCTA CGTACGC AACTGG AAATGGC CGTAT… First step is to “clean it up” by removing all the blanks. Write this function:
function s = removeChar(c, s) % Return string s with all occurrences % of character c removed
October 28, 2008 Lecture 17 17
Example: removing all occurrences of a character Can solve this problem using iteration—check one character (one component of the vector) at a time New strategy: recursion Possible when result can be accumulated iteratively E.g., remove all the blanks in string s Same as remove blank in s(1) and remove blanks in s(2:length(s)) E.g., capitalize first letter of all words in a sentence Same as capitalize 1 st^ letter of first word and capitalize 1 st^ letter of the rest of the words
