Characters

English language speakers are familiar with characters such as the letters of the alphabet (A, B, C, …), numerals, and common punctuation. These characters are standardized and mapped to integer values between 0 and 127 by the ASCII standard (American Standard Code for Information Interchange).

There are, of course, many other characters used in non-English languages, including variants of the ASCII characters with accents and other modifications, related scripts such as Cyrillic and Greek, and scripts completely unrelated to ASCII and English, including Arabic, Chinese, Hebrew, Hindi, Japanese, and Korean.

The Unicode standard tackles the complexities of what exactly a character is, and is generally accepted as the definitive standard addressing this problem. It provides a unique number for every character on a worldwide scale.

A Char value represents a single character and is surrounded by single quotes:

julia> 'x'
'x': ASCII/Unicode U+0078 (category Ll: Letter, lowercase)
julia> '?'
'?': Unicode U+01f34c (category So: Symbol, other)
julia> typeof('x')
Char

Even emojis are part of the Unicode standard (:banana: TAB).

A String Is a Sequence

A string is a sequence of characters. You can access the characters one at a time with the bracket operator ([]):

julia> fruit = "banana"
"banana"
julia> letter = fruit[1]
'b': ASCII/Unicode U+0062 (category Ll: Letter, lowercase)

The second statement selects character number 1 from fruit and assigns it to letter.

The expression in brackets is called an index. The index indicates which character in the sequence you want (hence the name).

All indexing in Julia is 1-based—the first element of any integer-indexed object is found at index 1 and the last element at index end:

julia> fruit[end]
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)

As an index, you can use an expression that contains variables and operators:

julia> i = 1
1
julia> fruit[i+1]
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)
julia> fruit[end-1]
'n': ASCII/Unicode U+006e (category Ll: Letter, lowercase)

But the value of the index has to be an integer. Otherwise, you get:

julia> letter = fruit[1.5]
ERROR: MethodError: no method matching getindex(::String, ::Float64)

length

length is a built-in function that returns the number of characters in a string:

julia> fruits = "? ? ?"
"? ? ?"
julia> len = length(fruits)
5

To get the last letter of a string, you might be tempted to try something like this:

julia> last = fruits[len]
' ': ASCII/Unicode U+0020 (category Zs: Separator, space)

But you might not get what you expect.

Strings are encoded using UTF-8 encoding. UTF-8 is a variable-width encoding, meaning that not all characters are encoded in the same number of bytes.

The function sizeof gives the number of bytes in a string:

julia> sizeof("?")
4

Because an emoji is encoded in 4 bytes and string indexing is byte-based, the fifth element of fruits is a SPACE.

This also means that not every byte index into a UTF-8 string is necessarily a valid index for a character. If you index into a string at an invalid byte index, an error is thrown:

julia> fruits[2]
ERROR: StringIndexError("? ? ?", 2)

In the case of fruits, the character ? is a 4-byte character, so the indices 2, 3, and 4 are invalid and the next character’s index is 5; this next valid index can be computed by nextind(fruits, 1), the next index after that by nextind(fruits, 5) and so on.

Traversal

A lot of computations involve processing a string one character at a time. Often they start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal. One way to write a traversal is with a while loop:

index = firstindex(fruits)
while index <= sizeof(fruits)
    letter = fruits[index]
    println(letter)
    global index = nextind(fruits, index)
end

This loop traverses the string and displays each letter on a line by itself. The loop condition is index <= sizeof(fruit), so when index is larger than the number of bytes in the string, the condition is false and the body of the loop doesn’t run.

The function firstindex returns the first valid byte index. The keyword global before index indicates that we want to reassign the variable index defined in Main (see “Global Variables”).

for letter in fruits
    println(letter)
end

prefixes = "JKLMNOPQ"
suffix = "ack"

for letter in prefixes
    println(letter * suffix)
end

Jack
Kack
Lack
Mack
Nack
Oack
Pack
Qack

String Slices

A segment of a string is called a slice. Selecting a slice is similar to selecting a character:

julia> str = "Julius Caesar";

julia> str[1:6]
"Julius"

The operator [n:m] returns the part of the string from the nth byte to the mth byte, so the same caution is needed as for simple indexing.

The end keyword can be used to indicate the last byte of the string:

julia> str[8:end]
"Caesar"

If the first index is greater than the second the result is an empty string, represented by two quotation marks:

julia> str[8:7]
""

An empty string contains no characters and has length 0, but other than that, it is the same as any other string.

Strings Are Immutable

It is tempting to use the [] operator on the left side of an assignment, with the intention of changing a character in a string. For example:

julia> greeting = "Hello, world!"
"Hello, world!"
julia> greeting[1] = 'J'
ERROR: MethodError: no method matching setindex!(::String, ::Char, ::Int64)

The reason for the error is that strings are immutable, which means you can’t change an existing string. The best you can do is create a new string that is a variation on the original:

julia> greeting = "J" * greeting[2:end]
"Jello, world!"

This example concatenates a new first letter onto a slice of greeting. It has no effect on the original string.

String Interpolation

Constructing strings using concatenation can become a bit cumbersome. To reduce the need for these verbose calls to string or repeated multiplications, Julia allows string interpolation using $:

julia> greet = "Hello"
"Hello"
julia> whom = "World"
"World"
julia> "$greet, $(whom)!"
"Hello, World!"

This is more readable and convenient than string concatenation:

greet * ", " * whom * "!"

The shortest complete expression after the $ is taken as the expression whose value is to be interpolated into the string. Thus, you can interpolate any expression into a string using parentheses:

julia> "1 + 2 = $(1 + 2)"
"1 + 2 = 3"

Searching

What does the following function do?

function find(word, letter)
    index = firstindex(word)
    while index <= sizeof(word)
        if word[index] == letter
            return index
        end
        index = nextind(word, index)
    end
    -1
end

In a sense, find is the inverse of the [] operator. Instead of taking an index and extracting the corresponding character, it takes a character and finds the index where that character appears. If the character is not found, the function returns -1.

This is the first example we have seen of a return statement inside a loop. If word[index] == letter, the function breaks out of the loop and returns immediately.

If the character doesn’t appear in the string, the program exits the loop normally and returns -1.

This pattern of computation—traversing a sequence and returning when we find what we are looking for—is called a search.

Looping and Counting

The following program counts the number of times the letter a appears in a string:

word = "banana"
counter = 0
for letter in word
    if letter == 'a'
        global counter = counter + 1
    end
end
println(counter)

This program demonstrates another pattern of computation called a counter. The variable counter is initialized to 0 and then incremented each time an a is found. When the loop exits, counter contains the result—the total number of a’s.

String Library

Julia provides functions that perform a variety of useful operations on strings. For example, the function uppercase takes a string and returns a new string with all uppercase letters:

julia> uppercase("Hello, World!")
"HELLO, WORLD!"

As it turns out, there is a function named findfirst that is remarkably similar to the find function we wrote:

julia> findfirst("a", "banana")
2:2

Actually, the findfirst function is more general than our function; it can find substrings, not just characters:

julia> findfirst("na", "banana")
3:4

By default, findfirst starts at the beginning of the string, but the function findnext takes a third argument, the index where it should start looking:

julia> findnext("na", "banana", 4)
5:6

The ∈ Operator

The operator ∈ (in TAB) is a Boolean operator that takes a character and a string and returns true if the first appears in the second:

julia> 'a' ∈ "banana"    # 'a' in "banana"
true

For example, the following function prints all the letters from word1 that also appear in word2:

function inboth(word1, word2)
    for letter in word1
        if letter ∈ word2
            print(letter, " ")
        end
    end
end

With well-chosen variable names, Julia sometimes reads like English. You could read this loop as “for (each) letter in (the first) word, if (the) letter is an element of (the second) word, print (the) letter.”

Here’s what you get if you compare "apples" and "oranges":

julia> inboth("apples", "oranges")
a e s

String Comparison

The relational operators work on strings. To see if two strings are equal, use ==:

word = "Pineapple"
if word == "banana"
    println("All right, bananas.")
end

Other relational operations are useful for putting words in alphabetical order:

if word < "banana"
    println("Your word, $word, comes before banana.")
elseif word > "banana"
    println("Your word, $word, comes after banana.")
else
    println("All right, bananas.")
end

Julia does not handle uppercase and lowercase letters the same way people do. All the uppercase letters come before all the lowercase letters, so:

Your word, Pineapple, comes before banana.

Debugging

When you use indices to traverse the values in a sequence, it is tricky to get the beginning and end of the traversal right. Here is a function that is supposed to compare two words and return true if one of the words is the reverse of the other, but it contains two errors:

function isreverse(word1, word2)
    if length(word1) != length(word2)
        return false
    end
    i = firstindex(word1)
    j = lastindex(word2)
    while j >= 0
        j = prevind(word2, j)
        if word1[i] != word2[j]
            return false
        end
        i = nextind(word1, i)
    end
    true
end

The first if statement checks whether the words are the same length. If not, we can return false immediately. Otherwise, for the rest of the function, we can assume that the words are the same length. This is an example of the guardian pattern; see “Checking Types”.

i and j are indices: i traverses word1 forward while j traverses word2 backward. If we find two letters that don’t match, we can return false immediately. If we get through the whole loop and all the letters match, we return true.

The function lastindex returns the last valid byte index of a string and prevind finds the previous valid index of a character.

If we test this function with the words “pots” and “stop,” we expect the return value true, but we get false:

julia> isreverse("pots", "stop")
false

For debugging this kind of error, my first move is to print the values of the indices:

    while j >= 0
        j = prevind(word2, j)
        @show i j
        if word1[i] != word2[j]

Now when I run the program again, I get more information:

julia> isreverse("pots", "stop")
i = 1
j = 3
false

The first time through the loop, the value of j is 3, but it has to be 4. This can be fixed by moving j = prevind(word2, j) to the end of the while loop.

If I fix that error and run the program again, I get:

julia> isreverse("pots", "stop")
i = 1
j = 4
i = 2
j = 3
i = 3
j = 2
i = 4
j = 1
i = 5
j = 0
ERROR: BoundsError: attempt to access "pots"
  at index [5]

This time a BoundsError has been thrown. The value of i is 5, which is out of range for the string "pots".

Glossary

sequence

An ordered collection of values where each value is identified by an integer index.

ASCII standard

A character encoding standard for electronic communication specifying 128 characters.

Unicode standard

A computing industry standard for the consistent encoding, representation, and handling of text expressed in most of the world’s writing systems.

index

An integer value used to select an item in a sequence, such as a character in a string. In Julia indices start from 1.

UTF-8 encoding

A variable-width character encoding capable of encoding all 1,112,064 valid code points in Unicode using one to four 8-bit bytes.

traverse

To iterate through the items in a sequence, performing a similar operation on each.

slice

A part of a string specified by a range of indices.

empty string

A string with no characters and length 0, represented by two quotation marks.

immutable

The property of a sequence whose items cannot be changed.

string interpolation

The process of evaluating a string containing one or more placeholders, yielding a result in which the placeholders are replaced with their corresponding values.

search

A pattern of traversal that stops when it finds what it is looking for.

counter

A variable used to count something, usually initialized to zero and then incremented.

Exercises

julia> fruit = "banana"
"banana"
julia> fruit[1:2:6]
"bnn"

function anylowercase1(s)
    for c in s
        if islowercase(c)
            return true
        else
            return false
        end
    end
end

function anylowercase2(s)
    for c in s
        if islowercase('c')
            return "true"
        else
            return "false"
        end
    end
end

function anylowercase3(s)
    for c in s
        flag = islowercase(c)
    end
    flag
end

function anylowercase4(s)
    flag = false
    for c in s
        flag = flag || islowercase(c)
    end
    flag
end

function anylowercase5(s)
    for c in s
        if !islowercase(c)
            return false
        end
    end
    true
end

julia> Int('c') - Int('a')
2

julia> Char(Int('A') + 32)
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)