ECE 102 Engineering Computation

Transcription

1 ECE 102 Engineering Computation Phillip Wong MATLAB Introduction to Strings String Operations Converting Between Strings & Numbers

2 Introduction to Strings A string is a collection of characters bounded by single quotes. Example: 'Hello' 'time t (s)' Characters include letters, digits, punctuation, MATLAB stores string data in arrays (vectors). Characters in the string occupy consecutive elements in the array. Example: s = 'Hello'; s H e l l o

3 length() function finds the length of a string. Example: length('ece102') 6 s=' '; s=' '; s=''; Example: Methods for checking if a string is empty: 1. if length(str) == 0 2. if ~length(str) 3. if isempty(str) length(s) 7 length(s) 1 length(s) 0 2

4 Access individual characters within a string by using a vector index. Example: s = 'Hello, world!'; s(2) Access substrings with the colon operator. Example: s = 'Hello, world!'; s(1:5) s(6:7) 'e' 'Hello' ', ' s(8:13) 'world!' s(8:length(s)) 'world!' s(8:end) 'world!' 3

5 String Operations Concatenation Vector approach Example: first = 'ECE'; last = '102'; s = [first last] 'ECE102' Note: This preserves all blanks. strcat(s1,s2) Example: a='cat'; b=' Dog'; c='dog '; s = strcat(a,b); 'Cat Dog' s = strcat(a,c); 'CatDog' Note: strcat removes trailing blanks. 4

6 Creating Customized Strings blanks(n) Creates a string consisting of n blank chars. Example: ['Ho' blanks(2) 'ho'] 'Ho ho' sprintf( ) Works like fprintf, except the output is returned as a string. Example: w = 512; xv = sprintf('v= %d',w); 'v= 512' 5

7 Removing Blank Characters deblank(s) Removes blank spaces from the end of a string. Example: str = ' Hello, world '; s = deblank(str); ' Hello, world' strtrim(s) Removes both leading and trailing blanks. Example: str = ' Hello, world '; s = strtrim(str); 'Hello, world' 6

8 Changing Case lower(s) Converts string to all lowercase characters. Example: str = 'abcdefghi%123'; s = lower(str); 'abcdefghi%123' upper(s) Converts string to all uppercase characters. Example: str = 'abcdefghi%123'; s = upper(str); 'ABCDEFGHI%123' 7

9 Comparing Strings strcmp(s1,s2) Performs a character-by-character comparison of one string with another string. Returns true if the strings are identical. Otherwise, it returns false. Example: x='abc'; y='abc'; z='abc'; w='abcd'; strcmp(x,'abc') true (1) strcmp(x,y) true (1) strcmp(x,z) false (0) if strcmp(x,w) disp('strings are equal'); end 8

10 strncmp(s1,s2,n) Compares only the first n characters of strings. Example: s1='abc'; s2='abc'; s3='abcd' strncmp(s1,'abc',3) true (1) strncmp(s1,s2,2) true (1) strncmp(s1,s2,3) false (0) strncmp(s1,s3,3) true (1) strncmpi(s1,s2,n) Works like strncmp, but case is ignored. Example: s1='abc'; s2='abc'; strncmpi(s1,s2,3) true (1) 9

11 To compare strings, use the strcmp, strncmp, or strncmpi functions. Do not use == to compare strings since it does a logical test of each vector element separately. Example: s1 = 'abcde'; s2 = 'abcxe'; s1 == s Use == to test for equality of single characters. Example: ch == 'Q' 10

12 Example: % Determine instrument type based on model number s = input('enter model# ', 's'); if strcmp(s,'tds3034a') strcmp(s,'tds3034b') disp('general-purpose oscilloscope') elseif strncmp(s,'mso4054',7 ) disp('mixed-signal oscilloscope') else disp('unknown instrument') end 11

13 findstr(s1,s2) Given two strings, it searches for all occurrences of the shorter string within the longer string. It returns a vector with each occurrence's index. Example: x='man'; y='woman'; z='woman2woman' findstr(x,'5') (empty vector) findstr(x,'a') 2 findstr(x,y) findstr(y,x) findstr(x,z)

14 strfind(string, substring) Works like findstr, except the order of the arguments makes a difference. Example: x='man'; y='woman'; strfind(x,y) (empty vector) strfind(y,x) 3 strrep(str, oldstr, newstr) Finds all occurrences of oldstrin strand replaces them with newstr. Example: x='mississippi'; strrep(x,'s','z') 'Mizzizzippi' 13

15 Converting Between Strings & Numbers MATLAB has functions to convert a number into a string, and a string into a number. int2str(v) Returns the string version of the integer argument Example: x = 72; s = int2str(123); '123' s = int2str(x); '72' fprintf('int: %d str: %s', x, s); str=['ram= ' x 'GB']; 'RAM= HGB' str=['ram= ' s 'GB']; 'RAM= 72GB' 14

16 num2str(v) Returns the string version of the real argument Example: x = ; s = num2str(x); ' ' num2str(v, N) Limits result to N digits with rounding Example: x = ; s = num2str(x,3); '16.3' num2str(v, format_str) Uses a format string to control the conversion Example: x = ; s = num2str(x,'%6.2f'); '16.25' 15

17 str2num(s) Returns the numeric equivalent of the string argument. The result is of type double. Example: s = '-3.5'; v = str2num(s); -3.5 v = str2num('4.2e3'); 4200 v = str2num('7'); 7 v = str2num(' '); v = str2num('a 3'); (empty vector) 16