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StringsCharacters and Sentences

1. Overview2. Creating Strings3. Slicing Strings4. Searching for substrings

1. Strings, overview.

• Strings are arrays of charactersStr = 'This is 21 characters';

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Spaces count as 1 characters too!

T h i s i s 2 1 c h a r a c t e r s

But what about the 1’s and 0’s?

• Letters are stored internally as numbers.• The “letter to number coding” is a standardized encoding

called ASCII.• Used/supported in almost every other computer

programming language.• Other encodings generally start with ASCII as the basis.

2. Creating Strings

1. Defining a string by hard-codingstr = 'Fred Flintstone’;

% Stored as: [70 114 101 100 32 70 108 105 110 116 115 116 111 110 101]

2. Creating with sprintf()str = sprintf('Curve of f(x)=%d + %dx',b,m);

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Example: sprintf()

• Prompt the user for the slope and y-intercept of a line, then plot. The title of the plot must reflect the current equation of the line.

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Example: sprintf(), cont.

• FACT: The function title() requires 1 and only 1 string argument: title( )

• This would NOT work:title('Curve of %d+%dx.', b, m)

sprintf() is used to create 1 string variable. This string variable is used in the title() command.

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Example: sprintf(), cont.

%ask for slope (m) and y-intercept (b)… %create x and y data points, then plot…plot(x,y) %label plot properlystr = sprintf('Curve of f(x)=%d + %dx',b,m);title(str)xlabel('x')ylabel('y')

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2. Creating Strings, cont.

3. Array-buildingstr = 'Barney';str = [str, ' Rubble was here'];

4. Concatenatingname = 'Fred';lastName = 'Flintstone';str1 = [name, ' was', ' ' , 'here'];str2 = ['Hello Mr.', lastName, '. How are you?'];

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2. Creating Strings, cont.

5. strcat() is used to combine strings, removing trailing whitespaces (trailing = end)

str = strcat('Fred ','Flintstone',' was here ','! ');

Expected: Fred Flintstone was here !

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2. Creating Strings, cont.

5. strcat() is used to combine strings, removing trailing whitespaces (trailing = end)

str = strcat('Fred ','Flintstone',' was here ','! ');

Expected: Fred Flintstone was here !

Got: FredFlintstone was here!

• It does NOT get rid of leading spaces.

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3. Slicing Strings

Assume: s = 'abc defghi jklmnopqrst uvwxyz';

• T = s(3:6)

• X = s(1:3:10)

• U = s(end:-1:22)

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Answer?T = 'c de'

X = 'a fi'Answer?

Answer?

U = 'zyxwvu t'

Example: Extract Data

• From a string, “parse” the data, i.e. extract each word separately.

• Humans find the spaces first, then extract the 3 names in a split second.

• MATLAB can do exactly the same!

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Example: Extract Data

% prompt user for full namestr = input('What is your full name (first middle

last)? ', 's');

% Find the spacesindices = strfind(str, ' '); %indices =find(str==' ');

% Extract each separate namesFirst = str(1:indices(1)-1)Middle = str((indices(1)+1):(indices(2)-1))Last = str((indices(2)+1):end)

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strfind() returns a vector containing the indices (positions) of the desired substring ' '. Allows multiple letters as substrings.

Example: Extract Data

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First = str(1: indices(1)-1 )= str(1: 5-1)= str(1:4)

Middle = str(indices(1)+1 : indices(2)-1)= str( 5+1 : 10-1 )= str(6:9)

Last = str(indices(2)+1 : end)= str(10+1 : end)= str(11:end)

Example: Dates

• Using the MM/DD/YYYY format for a birthday parse out the:– Month– Day– Year

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StringsBuilt-In Functions

1. Comparing Strings2. Converting strings/numbers3. Additional String Functions

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1. Comparing Strings

• Curious: What would using == do?

%hardcode a string. Set in stone. We know what it is.str = 'Fred';

%test in command window, whether str is equal to 'Fred'%(note that it is, so we expect a true!)>> str == 'Fred' <enter>

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1. Comparing Strings

• Curious: What would using == do?

%hardcode a string. Set in stone. We know what it is.str = 'Fred';

%test in command window, whether str is equal to 'Fred'%(note that it is, so we expect a true!)>> str == 'Fred' <enter>ans = 1 1 1 1

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1. Comparing Strings

• Curious: What would using == do?

%hardcode a string. Set in stone. We know what it is.str = 'Fred';

%test in command window, whether str is equal to 'Fred'%(note that it is, so we expect a true!)>> str == 'Fred' <enter>ans = 1 1 1 1

• MATLAB evaluates equality letter by letter, assigning a 0 (for false) or a 1 (for true)

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1. Comparing Strings, cont.

• Curious: What would using == do?

%hardcode a string. Set in stone. We know what it is.str = 'Fred';

%test in command window, whether str is equal to 'Frog'%(note that it is not, so we expect a false!)>> str == 'Frog' <enter>ans = 1 1 0 0

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1. Comparing Strings, cont.

• Curious: What would using == do?

%hardcode a string. Set in stone. We know what it is.str = 'Fred';

%test in command window, whether str is equal to 'Frog'%(note that it is not, so we expect a false!)>> str == 'Frog' <enter>ans = 1 1 0 0

• 2 letters were identical, 2 letters were not. But.. this does not give an overall true or false. So, let’s find another way to compare strings.

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1. Comparing Strings, cont.

• Curious: What would using == do?

%hardcode a string. Set in stone. We know what it is.str = 'Fred';

%test in command window, is str equal to 'Flintstones'?%(note that it is not, so we expect a false!)>> str == 'Flintstone' <enter>??? Error using ==> eqMatrix dimensions must agree.

• It even gets worse when the length of each string does not match.. It creates an error. Definitely not the right method to compare strings..

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1. Comparing Strings, cont.

• Two built-in functions are commonly used to compare strings:

1. strcmp()– STRing CoMPare returns true if the two arguments (both strings) are identical

(CaSe sEnSItiVE)

Practice: strcmp('hi', 'hi') evaluates to ______Practice: strcmp('HI', 'hi') evaluates to ______Practice:

str = 'yes';strcmp(str, ‘yes') evaluates to _____strcmp(‘yes', str) evaluates to _____

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0

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1

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1. Comparing Strings, cont.

• Two built-in functions are commonly used to compare strings:

1. strcmpi()– STRing CoMPare Insensitivereturns true if the two arguments (both strings) are identical WITHOUT REGARD TO CASE

Practice: strcmpi('hi', 'hi') evaluates to ______Practice: strcmpi('HI', 'hi') evaluates to ______Practice:

str = 'yes';strcmpi(str, 'Yes') evaluates to _____strcmpi('YeS', str) evaluates to _____

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Example: Access Granted

% ask for usernameusername = input('Enter username: ', 's');

if %correct username% ask for a passwd

if %correct passwordgrant access…

elsequit/end code

endelse % quit/end codeend

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Example: Access Granted

% ask for usernameusername = input('Enter username: ', 's');

if strcmpi(username, 'John') % correct username%ask passwd pass = input('Enter password: ', 's');if strcmp(pass, 'u23!9s2') %if correct password

%grant access...else

%quit/end code...end

else % quit/end code...end

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The user name may not be case-sensitive…

…but a password is usually case-sensitive

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2. Converting strings

• Convert: string numbersstr2num()str2double() [will convert an entire cell arrays of strings]

• Convert: number stringint2str()num2str()

Example: Dynamic prompts

• Task: Introduce the sensor's number when prompting.

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Example: Dynamic prompts

• One way is as a combination of an fprintf() and an input() command:

%loop to prompt for each valuefor position = 1:nbSensors fprintf('Enter value of sensor #%d:',position); table(position) = input(' '); %leave a spaceend

• CAUTION: they cannot be combined. The input() command accepts only 1 string argument, or 2 when prompting for a string ('s').

input() never accepts placeholders and variable names.

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Example: Dynamic prompts

• Another way: using string concatenation

%loop to prompt for each valuefor position = 1:nbSensors

prompt = ['Enter value of sensor #', int2str(position) , ': ']; table(position) = input(prompt);

end

• CAUTION: the [] must be there to concatenate the 3 pieces (first part of the sentence, the number, and the colon) into 1 string.

Convert the integer to a string before concatenating all 3 pieces. The digit 1 becomes the string '1', the digit 2 becomes the string '2', etc

Example: Validating input

• “What happens when the user enters letters (instead of numbers)?”

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Example: Validating input

• To solve this problem, every input must now be considered as a string, even if they are numbers!

• Algorithms possible% Grab user's input as strings% Use str2double() to convert to numbers% Use isnan() to check if the conversion worked% Continue with calculations if it did

% Grab user's input as strings% Use str2double() to convert to numbers% while isnan() is true

% grab again, convert again% Continue with calculations if it did

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Example: Validating input

• What does str2double() and isnan() do?

• Example when string does look like a number:

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Example: Validating input

• What does str2double() and isnan() do?

• Example when string has an ERROR:

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Example: Validating input

• Now, prompt the user for a value, then put the str2double() and isnan() in a loop!

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EXTREMELY IMPORTANT:

isnan() MUST BE THE FIRST CONDITION.

3. Additional String Functions

• lower() – converts a string to lowercase

• upper() – converts a string to uppercase

• isletter() – which characters in the string are letters?

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Get Bounded Integer

• Write a function that accepts a prompt, a lower limit, and an upper limit and validate the input, re-prompting until the user enters a valid number.

• Algorithm:% Print prompt and collect input as a string% Identify if input is a number% Identify if input is between lower and upper limits% As long as input is invalid, print an error message and re-prompt

function ValidInt = GetBoundedInt( Prompt, LowerLimit, UpperLimit )% Prompts for an integer between 2 values %Print prompt and collect input as a stringValidInt = input(Prompt, 's'); %Identify if input is a number%Identify if input is between lower and upper limits%As long as input is invalidwhile( isnan( str2double( ValidInt )) || str2double(ValidInt) < LowerLimit || str2double(ValidInt) > UpperLimit ) %print an error message and re-prompt fprintf('Enter a valid integer between %d and %d\n', LowerLimit, UpperLimit ); ValidInt = input(Prompt, 's');end