Lesson 8 - Strings in Kotlin - Working with single characters
In the previous exercise, Solved tasks for Kotlin lesson 7, we've practiced our knowledge from previous lessons.
Lesson highlights
Are you looking for a quick reference on Kotlin characters and ASCII codes instead of a thorough-full lesson? Here it is:
Getting the character at a given
position using the []
operator:
{KOTLIN_CONSOLE}
val s = "Hello ICT.social"
println(s[2])
{/KOTLIN_CONSOLE}
Converting between characters and their ASCII value:
{KOTLIN_CONSOLE}
var c: Char // character
var i: Int // ordinal (ASCII) value of the character
// conversion from text to ASCII value
c = 'a'
i = c.toInt()
println("The character '$c' was converted to its ASCII value of $i")
// conversion from an ASCII value to text
i = 98
c = i.toChar()
println("The ASCII value of $i was converted to its textual value of '$c'")
{/KOTLIN_CONSOLE}
Would you like to learn more? A complete lesson on this topic follows.
In the previous lesson, Solved tasks for Kotlin lesson 7, we learned to work with arrays. If you
noticed some similarities between arrays and strings, you were absolutely onto
something. For the others, it may be a surprise that a
String
is essentially an array of characters
(Char
s) and we can work with it like so.
First, we'll check out how it works by simply printing the character at a given position:
val s = "Hello ICT.social" println(s) println(s[2])
The output:
Hello ICT.social l
We can see that we can access characters of a string through the brackets as it was with the array. It may be disappointing that characters at the given positions are read-only in Kotlin, so we can't write the following:
// This code won't work var s = "Hello, ICT.social" s[2] = "l" println(s)
Of course, there is a way to do it, but we'll go over it later. For now, we'll be reading characters.
Character occurrence in a sentence analysis
Let's write a simple program that analyzes a given sentence for us. We'll
search for the number of vowels, consonants and non-alphanumeric characters
(e.g. space or !
).
We'll hard-code the input string in our code, so we won't have to write it
again every time. Once the program is complete, we'll replace the string with
readLine()
. We'll iterate over characters using a loop. I should
start out by saying that we won't focus as much on program speed here, we'll
choose practical and simple solutions.
First, let's define vowels and consonants. We don't have to count non-alphanumeric characters since it'll be the string length minus the number of vowels and consonants. Since we don't want to deal with the case, uppercase/lowercase, of the letters, we'll convert the entire string to lowercase at the start. Let's set up variables for the individual counters, also, because it is a more complex code, we'll add comments.
// the string that we want to analyze var s = "A programmer gets stuck in the shower because the instructions on the shampoo were: Lather, Wash, and Repeat." println(s) s = s.toLowerCase() // counters initialization var vowelsCount = 0 var consonantsCount = 0 // definition of character groups val vowels = "aeiouy" val consonants = "bcdfghjklmnpqrstvwxz" // main loop for (c in s) { }
First of all, we prepare the string and convert it to lowercase. Then, we
reset the counters. For the definition of characters groups, we only need
ordinary String
s. The main loop iterates over each character in the
String s
. In each iteration of the loop the variable c
will contain the current character.
Now let's increment the counters. For simplicity's sake, I'll focus on the loop instead of rewriting the code repeatedly:
for (c in s) { if (vowels.contains(c)) { vowelsCount++ } else if (consonants.contains(c)) { consonantsCount++ } }
We already know the contains()
method on a string. As a
parameter, it can take both a substring or a character. Firstly, we try to find
the character c
from our sentence in the string vowels
and possibly increase their counter. If it's not included in vowels, we look in
consonants and possibly increase their counter.
Now the last thing left is to print it at the end:
var s = "A programmer got stuck in shower because the label said: apply, wash, repeat." println(s) s = s.toLowerCase() // counters initialization var vowelCount = 0 var consonantCount = 0 // definition of character groups val vowels = "aeiouy" val consonants = "bcdfghjklmnpqrstvwxyz" // main loop for (c in s) { if (vowels.contains(c)) { vowelCount++ } else if (consonants.contains(c)) { consonantCount++ } } println("Vowels: $vowelCount") println("Consonants: $consonantCount") println("Other characters: ${s.length - (vowelCount + consonantCount)}")
The output:
A programmer gets stuck in the shower because the instructions on the shampoo were: Lather, Wash, and Repeat. Vowels: 33 Consonants: 55 Non-alphanumeric characters: 21
That's it, we're done!
ASCII value
Maybe you've already heard about the ASCII table. Especially, in the MS-DOS
era when there was practically no other way to store text. Individual characters
were stored as numbers of the byte datatype, so of a range from 0
to 255
. The system provided the ASCII table which had
256
characters and each ASCII code (numerical code) was assigned to
one character.
Perhaps you understand why this method is no longer as relevant. The table
simply could not contain all the characters of all international alphabets, now
we use Unicode (UTF-8) encoding where characters are represented in a different
way. In Kotlin, we have the option to work with ASCII values of individual
characters. The main advantage is that the characters are stored in the table
next to each other, alphabetically. For example, at the position 97
we can find "a"
, at 98
"b"
etc. It's the
same with numbers, but unfortunately, the accent characters are messed up.
Now, let's convert a character into its ASCII value and vice versa create the character according to its ASCII value.
var c: Char // character var i: Int // ordinal (ASCII) value of the character // conversion from text to ASCII value c = 'a' i = c.toInt() println("The character $c was converted to its ASCII value of $i") // conversion from an ASCII value to text i = 98 c = i.toChar() println("The ASCII value of $i was converted to its textual value of $c")
The output:
The character a was converted to its ASCII value of 97 The ASCII value of 98 was converted to its textual value of b
These conversions are called type casts, which we'll get into later on.
The Caesar cipher
Let's create a simple program to encrypt text. If you've ever heard of the
Caesar cipher, then you already know exactly what we're going to program. The
text encryption is based on shifting characters in the alphabet by a certain
fixed number of characters. For example, if we shift the word
"hello"
by 1
character forwards, we'd get
"ifmmp"
. The user will be allowed to select the number of character
shifts.
Let's get right into it! We need variables for the original text, the
encrypted message, and the shift. Then, we need a loop iterating over each
character and printing an encrypted message. We'll also hard-code the message
defined in the code, so we won't have to write it over and over during the
testing phase. After we finish the program, we'll replace the contents of the
variable with the readLine()
method. The cipher doesn't work with
accent characters, spaces and punctuation marks. We'll just assume the user will
not enter them. Ideally, we should remove accent characters before encryption,
as well as anything except letters.
// variable initialization val s = "blackholesarewheregoddividedbyzero" println("Original message: $s") var message = "" var shift = 1 // loop iterating over characters for (c in s) { } // printing println("Encrypted message: $message")
We'll now move into the loop. We'll cast the character in c
to
its ASCII value, its ordinal value, increase the value by however many
shifts
and cast it back to the character. This character will be
added to the final message:
var i = c.toInt() i += shift val c = i.toChar() message += c
Let's try it out! The result looks pretty good. However, we can see that the
characters after "z"
overflow to ASCII values of other characters
(e.g. "{"
). Therefore, the characters are no longer just
alphanumeric, but other nasty characters. Let's enclose our characters as a
cyclical pattern, so the shifting could flow smoothly from "z"
to
"a"
and so on. We'll get by with a simple condition that decreases
the ASCII value by the length of the alphabet so we'd end back up at
"a"
.
var i = c.toInt() i += shift if (i > 'z'.toInt()) { i -= 26 } val char = i.toChar() message += char
If i
exceeds the ASCII value of 'z'
, we reduce it
by 26 characters (the number of characters in the English alphabet). The
-=
operator does the same as we would do with
i = i - 26
. It's simple and our program is now operational. Notice
that we don't use direct character codes anywhere. There's a
'z'.toInt()
in the condition even though we could write
122
there directly. We did it this way so that our program is fully
encapsulated from explicit ASCII values, so it'd be clearer how it works. Try to
code the decryption program as practice for yourself.
In the next lesson, Solved tasks for Kotlin lesson 8, we'll see that there are still a couple more things we haven't touched base on that strings can do. Spoiler: We'll learn how to decode "Morse code".
In the following exercise, Solved tasks for Kotlin lesson 8, we're gonna practice our knowledge from previous lessons.
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