Character Arrays

for letters and stuff

Overview

They say words are little more than horizontal vectors of characters. Well, I don't know if they say that, but I say that, and you should probably know that because, from the perspective of a computer, that is basically what they are.

In this module, we will learn about character arrays, which are useful for storing information such as names or other such unique identifiers.

Syntax Overview

Syntax	Special Character	Meaning
x='a'	' '	assign the character a to the variable x
x='cat'	' '	assign the characters 'c', 'a', and 't' to x

Learning Objectives

• Define a Character Array.
• Be able to assign values to character arrays using paired single quotes
• Be able to use the function sprintf to format character arrays
• Be able to use functions discussed in this module like [sort] and [unique] to parse characters in a character array
• Be able to use regular expressions to find and replace characters in character arrays

Special MATLAB Characters

• ' ' - paired single quotes (the straight kind) are used to concatenate characters into a character array

• [ ]- square brackets are used to concatenate character arrays

Important Terminology

• Character Arrays: An array of characters (letters, spaces, punctuation, etc). Sometimes called a string.

• ASCII: the American Standard Code for Information Interchange. A numeric code to indicate different characters.

Useful Mathworks Documentation

• Character Strings
• Text in String and Character Arrays

Important MATLAB Functions You should know

• char - Convert to a character array
• ischar: is the array a character array?
• upper and lower - Change case of letters
• isletter and isspace - returns a logical array that masks letters or spaces in a character array
• sprintf - Format data into character array
• regexp - Regular expression (super find function)
• regexprep - Replace text using regular expression

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Assignment and syntax

Each element in a character array contains a single letter or other such character (as opposed to a numeric or logical value). When creating a character array, MATLAB assumes that you probably don't want to separate each character by a space, so the syntax for creating a character array is different from creating a numeric array. Instead of using the paired square brackets, you use a pair of single quotes (' ') and you do not include any spaces in between the characters. In fact, the space character is a type of character known as a whitespace character. For example, the following is a very simple character array:

ch = 'hello'

Syntax Coloring

In the MATLAB editor, characters in character arrays are colored purple.

Anything you can type on a computer keyboard can be stored as a character array. Spaces are characters. Punctuation Marks are characters. Even numbers can be character arrays:

n = '1'

result

n =

    '1'

This can cause problems if you are not careful and accidentally try to do math with character arrays. MATLAB will display character array outputs in single quotes, as shown above '1' and color the character purple. However, it is always a good idea to check the class using whos or check the workspace. Make sure you know the class of your array or you may get an unexpected result.

whos n

result

Name      Size            Bytes  Class    Attributes
  n         1x1                 2  char               

Inspecting character arrays in the Variable Editor

If you double-click on the variable name of a character array in the workspace to bring up the "Variable Editor", all letters of the array appear to be contained in a single element, as shown in the following image:

…But, this is not the case. Just as in a numeric variable, each element in a character array contains a single character. This is even indicated right above the box: "1X5 char"

The whos function, which is a text version of the workspace, can further clarify the properties of ch:

  whos('ch')

whos output

   Name      Size            Bytes  Class    Attributes
     ch         1x5                10  char               

…As you can see, ch is a vector array with one row and 5 columns that contains the letters h,e,l,l, and o in the first five elements of the array. Also note that it has a 'char' class (character array class), and it requires 10 Bytes of memory. This means that a character array requires 2 Bytes of memory allocation (or is 16-bit) per element.

Indexing

Character Arrays can be indexed using parentheses, just like numeric arrays.

First ElementLast Element

ch(1)
ans = 
      'h'

This syntax returns the first element in a character array, which contains the letter h:

ch(end)

ans = 
      'o'

and this syntax returns the last element in the array, which contains the letter o:

Concatenation

You can use the paired square brackets to concatenate char arrays, just as you would concatenate numeric arrays. We can easily concatenate two character arrays using the following syntax:

c1 = 'together';
c2 = 'again';
ct = [c1 c2] % square brackets concatenate the contents of c1 and c2

result

ct =

    'togetheragain'

Note

The concatenation of character arrays takes no regard to grammar or spacing. The result is literally the two character arrays, smashed together.

To include a space between concatenated char arrays, you need to specify the space, as follows:

sp = ' ' % create space character
ct2 = [c1 sp c2] % concatenate all three character arrays

result

ct2 =
      'together again' % Now there is a space between the two words

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Character Matrices

What happens if you try to place two different words in separate rows of a character array using semicolon syntax?

['hello'; 'goodbye']

Error

Dimensions of arrays being concatenated are not consistent

The syntax fails because 'hello' has 5 characters, while 'goodbye' has 7. And as we remember from the Numeric Array section, we have to have an equal number of filled columns for each row in a column. You can't have any empty elements

So, how do you create character arrays with more than one row of characters? Just like in a numeric matrix, you need an equal number of columns for every character in a character array. If there are not enough characters in a given word, you can pad that word with spaces.

To properly concatenate the two words 'hello' and 'goodbye' into one matrix, you need to pad 'hello' with 2 trailing spaces, as follows:

['hello  '; 'goodbye']

result

ans =

      2×7 char array
      'hello  '
      'goodbye' % Now we have a proper character matrix

Note

Even though the spaces are not visible, they are occupying elements in the matrix.

You don't have to pad with spaces—you can use any character:

['hello**'; 'goodbye']

result

ans =
      2×7 char array
      'hello**'
      'goodbye' % this matrix is padded with asterisks

The char Function

If you don't want to worry about padding your character arrays, you can use the function char to automatically add the proper number of trailing spaces for you. Just plug in the character arrays that you want concatenated, and the function will do the rest

p = char('hello', 'goodbye','farewell')

result

p =
    3×8 char array

    'hello   '
    'goodbye '
    'farewell'

Note

Here, the function char automatically creates a 3X7 character array, padding both 'hello' and 'goodbye' with spaces at the end to match the length of 'farewell'.

Indexing Character Matrices

Remember, in character arrays, each element contains one character.

So, when indexing elements out of a character matrix, you get one character back. Consider the following examples of indexing p.

p(2,2) 

result

ans =
      'o' % you get the second 'o' in 'goodbye'

p(1,3)

result

ans =
      'l' % you get the first 'l' in 'hello'

ChallengeAnswerChallenge 2Answer 2

How would you index out the 'w' in 'farewell' from p?

 p(3,5)

 ans =
        'w'

You need to index the third row, fifth column.

What does the following syntax return?

 p(1,end)

p(1,end)

ans =
          ' '

This syntax returns a space.

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Character Array Generation

You can generate a sequential series of characters as you would a series of incremental numbers by using the colon operator. The following syntax generates a character array that contains all of the lowercase letters from 'a' to 'z', in alphabetical order.

lower_letters = 'a' : 'z'

result

lower_letters =

'abcdefghijklmnopqrstuvwxyz'

If you want every other letter, you could use the following syntax (just like with numeric arrays):

letter_subset = 'a':2:'z'

result

letter_subset =
                 'acegikmoqsuwy'

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Character Array Functions

The following functions are very useful for character arrays.

Case functions

The functions upper and lower change the case of a letter:

ch = 'a':'f'
CH = upper(ch) % change to uppercase
ch2 = lower(CH) % change to lowercase

result

ch =
      'abcdef'

CH =
      'ABCDEF'

ch2 =
      'abcdef'

is* functions

is* functions return logical arrays that mask specific types of characters in an array

• isletter: which elements contain letters (a-z, A-Z)
• isspace: which elements contain spaces?

Consider the following character array

ch = ['a':'c' ' ' '1':'c' ' !@#']

ch =
     'abc 123 !@#'

The function isletter returns a logical array masks the letters

laL = isletter(ch)

result

laL =
      1×11 logical array
      1   1   1   0   0   0   0   0   0   0   0

…Just the first three characters are letters

The function isspace masks the spaces

laS = isspace(ch)

laS =
      1×11 logical array
      0   0   0   1   0   0   0   1   0   0   0

…The spaces are after the letter c and after the number 3.

sprintf

The sprintf function allows you to add data to a character array, sort of like creating a template for form letter where you add the data when you create the letter.

To use sprintf, you first create a character array that has placeholders in them. These placeholders are prefaced by the % symbol. Some common placeholders include:

Placeholder	Data
%s	character array
%d	number
%f	floating point number

The basic syntax for sprintf is as follows:

formatted_char_array = sprintf(char_2_format,data)

Consider the following example.

input_array = 'The value of pi is %d' % a character array with placeholders
output_array = sprintf(input_array,pi) % second input is a function that returns the value for pi

For sprintf, the first input is the character array, and the second (and subsequent inputs) are the data that you want to add to the character array.

result

output_array = 
'The value of pi is 3.141593e+00'

In this example, input_array is the character array to be formatted. It has one placeholder: %d. This placeholder is replaced by the data found in the second input of sprintf, which in this case is the value of π. The value of π is returned by the MATLAB function pi. The use of the placeholder %d here returns the value of pi in the default format of MATLAB.

If you would like to change the way π is displayed, such as the number of significant digits displayed or the field width, you can use the %f placeholder preceded by some formatting operators, as shown in this image:

input_array = 'pi to the 10th significant digit is: %1.10f' % here the .10 indicates the precision
output_array = sprintf(input_array,pi) % again, the second input the value for pi

result

output_array =
'pi to the 10th significant digit is: 3.1415926536'

Escape Characters

Sometimes you want to include a line return or a tab in your formatted string. There are special character combinations that allow you to do this. They are often preceded by the backslash. Here are a few:

Escape Character	Indicates
\n	new line
\t	tab
''	single quotation

The one I use the most is \n, which allows me to create a string with multiple lines.

More Placeholders, More data

In sprintf, the number of inputs depends on the number of placeholders that you have added to the input character array.

For example, The following character array has four placeholders (3 %d's and 1 %s's); therefore, you need four inputs after the input character array, as shown here:

input_array = 'The product of %d %s %d equals %d'; % four placeholders
x = 2;
y = 3;
result = sprintf(input_array, x, 'times', y, x*y) % (1)

1. Four inputs after the char array: x, 'times', y, x*y

result

result =
The product of 2 times 3 equals 6

Note

Notice that the last input into sprintf is actually the product of the two variables, x and y.

fprintf

Similar to the sprintf function, fprintf can format data into strings. In addition, fprintf can then output those strings to the command window (or even to files).

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ChallengeAnswer

What would you change in the previous example to get the following output?

result

    result =
      The sum of 2 plus 3 equals 5

input_array = 'The sum of %d %s %d equals %d'
x = 2
y = 3
result = sprintf(input_array, x, 'plus', y, x+y)

and this syntax returns the last element in the array, which contains the letter o:

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Regular Expressions

Now we're getting into the really complicated stuff. Don't sweat it if you don't understand this section.

Regular Expressions are like a super-charged search function. They are used widely—not just in MATLAB. A regular expression is nothing but a sequence of characters that match a pattern. Besides using literal characters (like 'abc'), there are some meta characters (*,+,? and so on) which have special purposes. Using regular expressions (sometimes called GREP), you can find patterns in strings, like all words in a paragraph that are capitalized but are not preceded by a period. Or, suppose you have a list of people’s names that you want to alphabetize. If the list is arranged first name first, but you want to alphabetize by last names, a simple grep pattern can be used to put the names in the proper order for sorting.

• A Practical Guide to Regular Expressions
• MATLAB Functions
• regexp - Regular expression (super find function)
• regexprep - Replace text using regular expression

regexp

The function regexp is MATLAB's version of this an incredibly powerful search function. regexp uses regular expressions to find these snippets of strings and performs some sort of operation on those characters / snippets.

For example, consider the following character array:

s = 'together at last';

We can use regexp to find the first letter in each word. To do this, we need to create a Regular Expression, or special code to search the character array. The easiest way to do that is to use MATLAB CoPilot or other AI. For example, you might ask

What is the regular expression to find the first letter in each word of a sentence?

In this case, the regular expression we need is '\<(\w)' (1). We enter that as the second input into regexp, as follows:

1. The regular expression '\<(\w)' matches the first letter of each word. Here, \< asserts the position at the start of a word, and (\w) captures the first word character.

idc = regexp(s,'\<(\w)')

The default output are the indices of the letters

The indices of the First letter in each word in s

idc =
      1   10    13

We can use these indices to modify our character array, as follows

s(idc) = upper(s(idc))

result

s = 
    'Together At Last'

…This syntax replace the lower case letters in s with the upper case version at only the idc locations in the character array

regexprep

We can use a variant of regexp, the function regexprep, to replace characters in a character array with other characters or with nothing at all.

For example, to replace the spaces in s with underscores, we would use the following syntax:

t = regexprep(s,' ','_')

result

T = 
    'Together_At_Last'

Notice that regexprep accepts three inputs. The second input (' ') is the regular expression to match (space in this case). The third input is the character ('_') used to replace the regular expression. In effect, we have replaced all of the spaces with the underscore character.

We can eliminate the underscores entirely using an empty pair of single quotes as the third input, as follows:

u = regexprep(s,'_', '')

result

u =
    'TogetherAtLast'

As you can see regular expressions are an incredibly powerful way to manipulate strings. However, sometimes they can be difficult to use because the search strings are not intuitive. Use MATLAB Copilot to help.

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MODULE Complete. Congrats, you made it to the end. High Five.