Introduction of Kotlin

MD.

UNIT-1

INTRODUCTION OF KOTLIN:

Compilation is a process that translates the Kotlin program code into a form that the system can run. If compilation completes successfully, there are no errors in the program that would keep it from running. If there are problems, they will appear in pane at the bottom.

How does it work?

How is this done? This seems like a lot of code to just print something!

Well, if you wanted a friend to write "Hello, world!" on a piece of paper, there is a lot of implied information. If you just tell them, "Write 'Hello world!' on this piece of paper", they are going to make assumptions about the information you left out. For example, they are going to assume they need to use a pen, and that you want them to write it using letters! The computer does not make these assumptions, so you have to give precise instructions that include every step.

Just like the English language has structure, so does a programming language. If you've ever learned another language, you know the challenge of learning the grammar, the spelling, perhaps a new alphabet of symbols, and the vocabulary. Learning to program has similar challenges, but fortunately, it is less complex and a lot more logical than learning, for example English.

Understand the parts of the program

Now, take a look at the code. Each piece of this program serves a specific purpose, and you need all the pieces in order to be able to run the program. Let's start with the first word.

fun

  • fun is a word in the Kotlin programming language. fun stands for function. A function is a section of a program that performs a specific task.

Note: Kotlin has many special words with very specific meanings. As you learn to program in the Kotlin language, you will learn these words. They are often called keywords or reserved words.

fun main

  • main is the name of this function. Functions have names, so they can be distinguished from each other. This function is called main, because it is the first, or main, function that is called when you run the program. Every Kotlin program needs a function named main.

fun main()

  • The function name is always followed by () two parentheses.
  • Inside the parentheses, you can put information for the function to use. This input to the function is called "arguments" or args for short. You will learn more about arguments later.

fun main() {}

  • Notice the pair of curly braces {} after the parentheses. Inside a function is code that accomplishes a task. These curly braces surround those lines of code.

Look at the line of code between the curly braces:

println("Happy Birthday!")

This line of code prints the Happy Birthday! text.

  • println tells the system to print a line of text.
  • Inside the parentheses you put the text to be printed.
  • Notice that the text to be printed is surrounded by quotes. This tells the system that everything inside the quotation marks should be printed exactly as given.

To actually print the text, this whole println instruction has to be inside the main function.

So, there it is. The smallest Kotlin program.

fun main() {
    println("Happy Birthday!")
}

·        https://developer.android.com/training/kotlinplayground is an interactive code editor on the web where you can practice writing Kotlin programs.

·        All Kotlin programs need to have a main() function: fun main() {}

·        Use the println() function to print a line of text.

·        Place text you want to print between double quotes. For example "Hello".

·        Repeat the println() instruction to print multiple lines of text.

·        Errors are marked red in the program. There is an error message in the output pane to help you figure out where the error is and what might be causing it.

·        https://developer.android.com/training/kotlinplayground

·        Vocabulary for Android Basics in Kotlin

 

Note: Practice problems are optional. They provide an opportunity for you to practice what you've learned in this codelab.

Do the following:

  1. Change the println() instructions to print().
  2. Run your program. What happens?

Hint: The print() instruction just prints the text without adding a line break at the end of each string.

  1. Fix the text so that each part of the message is on its own line.

Hint: Use \n inside the text to add a line break. For example, "line \n break". Adding a line break changes the output as shown below.

Hint: You can print an empty line by supplying no text: println("").

Code:

fun main() {
    println("no line break")
    println("")
    println("with line \n break")
}

Output:

no line break

 

with line

 break

Check your work:

Here is one possible solution:

fun main() {
    print("Happy Birthday!\n")
    print("Jhansi\n")
    print("You are 25!")
}

 

 

Create a birthday message in Kotlin

What you will learn

  • How to print more complex text from your program.
  • How to do basic math in Kotlin and store the results in variables for later use.
  • How to create a function to print the same string several times.
  • How to create a loop that prints a text snippet multiple times.

What you will build

  • You will create a short program that you can use to print birthday messages, a text-based picture of a cake, and a banner.

2. Create and use variables

  1. In your finished code so far, notice how you are repeating the same age number twice.

Instead of repeating this number, you could store it in one place, as a variable. It's like putting your number into a box and giving it a name. Then you can use this variable name any time you need the value. And, if the age changes, you only have to change your program in one place. By changing the variable, the correct value for the age is printed everywhere the variable is used.

  1. In your program, as the first line of code inside the main() function, add the following code to create a variable called age, with a value of 5, as shown below. (You must put this line before the println() statements).

val age = 5

This line means:

  • val is a special word used by Kotlin, called a keyword, indicating that what follows is the name of a variable.
  • age is the name of the variable.
  • = makes the value of age (on its left) be the same as the value on its right. In math, a single equal sign is used to assert that the values on each side are the same. In Kotlin, unlike in math, a single equal sign is used to assign the value on the right to the named variable on the left.

A developer would say it like this: This line declares a variable named age whose assigned value is 5.

Important: A variable declared using the val keyword can only be set once. You cannot change its value later in the program.

You can declare a changeable variable with the var keyword, which you will do in a later codelab.

To use a variable inside a print statement, you need to surround it with some symbols that tell the system that what comes next is not text, but a variable. Instead of printing text, the system needs to print the value of the variable. You do this by putting your variable inside curly braces preceded by a dollar sign, like in the example below.

${variable}

Put text into a variable

You can not only put numbers into variables, but also text.

  1. Below the variable for age, add a variable called name for the name of the birthday person and set its value to "Rover".

val name = "Rover"

  1. Replace the name Rover in the birthday message with the variable, as shown below.

println("Happy Birthday, ${name}!")

And you can have more than one variable in a print statement.

  1. Add Rover to the age message, using the name variable, as shown below.

println("You are already ${age} days old, ${name}!")

 

 

Your completed code should look similar to this.

fun main() {

    

    val age = 5 * 365

    val name = "Rover"

    

    println("Happy Birthday, ${name}!")

    

    // Let's print a cake!

    println("   ,,,,,   ")

    println("   |||||   ")

    println(" =========")

    println("@@@@@@@@@@@")

    println("{~@~@~@~@~}")

    println("@@@@@@@@@@@")

    

    // This prints an empty line.

    println("")



    println("You are already ${age} days old, ${name}!")

    println("${age} days old is the very best age to celebrate!")

}

 

Create a function for printing the border

 

Recap

In a previous codelab, you learned that:

  • A function is a section of a program that performs a specific task.
  • The fun keyword marks some code as a function.
  • After the fun keyword follows the name of the function, parentheses for optional input to the function (arguments), and curly braces.
  • Your code for printing text always went inside those curly braces.

Your Kotlin program always has to have a main() function. In addition, you can create and use functions of your own. Just like variables help you avoid duplicating work, functions can help you avoid writing the same code multiple times. In your code, the print statements for the top and bottom of the banner are exactly the same. Let's create and use a function for printing those borders.

  1. In the editor, below the main() function, insert an empty line, just to give you some room to work. The system ignores empty lines, and you can insert them wherever they are helpful for organizing your code.
  2. Create a function. Start with the fun keyword, followed by a name, printBorder, a pair of parentheses (), and a pair of curly braces {}, as shown below.

fun printBorder() {}

 

A word about naming functions.

  • Notice how the name of the function printBorder starts with a lower-case letter and a verb. Function names almost always start with a lowercase letter, and a verb, and the name should describe what the function does. Like: print() or here, printBorder().
  • Notice also that the second word inside the name starts with uppercase. This style is called "camel case", and makes names much easier to read. Some more name examples are drawReallyCoolFancyBorder and printBirthdayMessage.

Note: Naming functions like this is a "coding convention", an agreement among developers about how to format code. Formatting all the code similarly makes it easier to read and learn from code that other programmers have written. When you see code from other Android developers, it will usually be formatted using these conventions.

To learn more about formatting code, you can find all the conventions in an official style guide at https://developer.android.com/kotlin/style-guide. There is a lot in that guide, but if you are curious, go take a look.

3.     Put the closing brace } of the printBorder function on a new line and add an empty line in between the two curly braces, to give you room to add more code. Having the closing brace } on its own line makes it easier to see where the function ends.

4.     lnside the main() function, copy the print statement for the border and paste it between the curly braces of the printBorder() function.

Your finished printBorder() function should look like this.

fun printBorder() {
    println("=======================")
}

To use or call, a function, use its name with parentheses. Notice that this is how you have been using println()! So to use the printBorder function, call printBorder()anywhere in your code that you need to.

  1. In the main() function, replace the lines of code that print a border line using println() with calls to the printBorder() function. Your finished code should look like this.

fun main() {
    printBorder()
    println("Happy Birthday, Jhansi!")
    printBorder()
}

fun printBorder() {
    println("=======================")
}

  1. Run your code to make sure all is working as before.

Note that changing to code to make it better or easier to work with without changing the output is called "refactoring".

Repeat a border pattern

Looking at the border line, it's really the same symbol all over again. So, instead of saying:

"Print this string of 23 symbols"

you could say,

"Print this 1 symbol 23 times".

In code, you do this by using a repeat() statement.

  1. In the printBorder() method, use a repeat() statement to print the equal sign 23 times.
  2. Instead of using println(), use print(), so that you do not jump to a new line after printing each "=".

Here is the code. You now have a single instruction to print the equal sign, and to repeat that instruction 23 times, you use a repeat() statement.

fun printBorder() {

    repeat(23) {

        print("=")

    }

}
  • The repeat() statement starts with the word repeat, followed by (). This kind of statement is referred to as a ‘loop' because you are repeating or looping over the same code multiple times. You will learn about other ways of creating loops later.
  • Inside the parentheses () is the number of repetitions,
  • followed by curly braces {},
  • and inside the curly braces {}, is the code to repeat.
  1. Within the printBorder() method, after the closing curly brace } of the repeat() statement, which is after you are done printing the border line, add a println() statement to print a newline.

Your code should now be like this.

fun printBorder() {
    repeat(23) {
        print("=")
    }
    println()
}

The code in the main() function does not change, and your entire program should look like this.

fun main() {
    printBorder()
    println("Happy Birthday, Jhansi!")
    printBorder()
}

fun printBorder() {
    repeat(23) {
        print("=")
    }
    println()
}

  1. Run your code. The output should be the same as before, but this time, you were able to create the border by specifying the "=" symbol only once!

=======================

Happy Birthday, Jhansi!

=======================

 

Use arguments to change the border

What if you wanted to create borders that use different symbols, such as the ones below?

%%%%%%%%%%%%%%%%%%%%%%%

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

You could define a separate function for each of these different symbols. However, there is a more efficient way to do this. You can reuse the function you already wrote and make it more flexible, so that it works for different kinds of symbols..

A cool thing about functions is that you can give them input using arguments. You encountered this briefly in a previous codelab, when you got acquainted with main(). In this step, you will add an argument to the printBorder() function, so that it can print any border pattern that you supply.

  1. In main(), at the top, create a variable called border for the border pattern. This will hold text to repeat for the border.
val border = "%"
  1. Now, pass that border variable into both calls to the printBorder() function as an argument. You do this by placing border inside the parentheses (), just like when you supplied text for println() to print.

Your main() function should look like the code below.

fun main() {

    val border = "%"

    printBorder(border)

    println("Happy Birthday, Jhansi!")

    printBorder(border)

}

The printBorder() function is going to take the value of this border as an input and figure out how to print out the full border.

  1. Run your code.Your code does not execute, and instead, you see error icons next to your code. 
  2. Look at the output panel, and there is an error message.

As before, the message indicates where the error is, and gives you a hint as to what it might be. The important part is: Too many arguments for public fun printBorder(). You are calling the printBorder() function and passing a border as input. However, the printBorder() function definition does not accept any input at the moment.

  1. Fix this error by adding an argument for the border to the printBorder() function definition. See the first line of code as shown below.

fun printBorder(border: String) {
    repeat(23) {
        print("=")
    }
    println()
}

  • Notice that the name of the argument is border.
  • The name is followed by a colon :
  • and the word String, which is a description of what kind, or type, of argument this is.

A String is a piece of text made up of characters surrounded by quotes. You can think of it as beads lined up on a string to form a necklace, like, characters lined up to form words and text. Specifying that the argument has to be a String helps the system enforce that your argument is text, and not, for example, a number.

  1. Run your code. The printBorder() function now accepts a border String as input. And the code in main()calls printBorder(border) with border as the argument. Your code should run without errors.
  2. Look at the output of your program in the Console and it still shows the same border as before?

This is not the intended behavior! You tried to make a border with the "%" symbol, but the program is still printing a border with the "=" symbol. In the next steps, you will investigate why this is happening.

  1. In the editor, notice a grayed exclamation mark. This icon indicates a warning. Warnings are about issues with your code that you need to pay attention to, but they don't keep your code from running.

 

  1. Hover the mouse over the exclamation mark, and a message appears. It says "Parameter 'border' is never used." This warning explains the problem with the output. You are passing a new string for the border to the function, but you are not using it for printing.
  2. Change the printBorder() function to use the passed in border instead of printing the "=". This works exactly the same as if border was a variable that you defined inside the function!

Modify a function to have two arguments

What if you wanted to use a different pattern that was longer than 1 character, say "-._,-'". You would not repeat this pattern 23 times as it would be way too long. You could repeat it maybe 4 times. To accomplish this, you could change the number of repetitions in the repeat()statement ofprintBorder()`. However, you can do something better!

You can define a fancier border based on two things:

  • The pattern to repeat (which you already have done)
  • The number of times you want to repeat the pattern

You can create variables for each, the pattern and the number of repetitions, and then pass both pieces of information into the printBorder() function.

  1. In main(), change the border to be the "-._,-'"` pattern.

val border = "`-._,-'"

  1. Run your code, and notice that the pattern is now way too long.
  2. In main(), below the definition of border, create a new variable named timesToRepeat for the number of repetitions. Set its value to 4.

val timesToRepeat = 4

  1. In main(), when calling printBorder(), add the number of repetitions as a second argument. Separate the two arguments with a comma.

printBorder(border, timesToRepeat)

The main() function should now look like this:

fun main() {
    val border = "`-._,-'"
    val timesToRepeat = 4
    printBorder(border, timesToRepeat)
    println("Happy Birthday, Jhansi!")
    printBorder(border, timesToRepeat)
}

As before, this code gives you an error, because you have more arguments calling printBorder() than in the definition of printBorder().

  1. Fix printBorder() to also accept the number of repetitions as input. Add a comma after the argument, followed by the additional argument: timesToRepeat: Int. The first line of your function definition now looks like shown below.

fun printBorder(border: String, timesToRepeat: Int) {

Notice:

  • The comma separates the two arguments.
  • timesToRepeat is the name of the argument,
  • followed by a colon : symbol,
  • and the type: Int. timesToRepeat is a number, so instead of making it of type String, you need to make it of type Int, which is short for an integer, a whole number.
  1. Inside printBorder(), change repeat to use the timesToRepeat argument (instead of the number 23). Your printBorder() code should look like this.

fun printBorder(border: String, timesToRepeat: Int) {
    repeat(timesToRepeat) {
        print(border)
    }
    println()
}

  1. Run your code. And the output looks as shown below.

`-._,-'`-._,-'`-._,-'`-._,-'

Happy Birthday, Jhansi!

`-._,-'`-._,-'`-._,-'`-._,-'

 

Here is the final code for your banner:

fun main() {
    val border = "`-._,-'"
    val timesToRepeat = 4
    printBorder(border, timesToRepeat)
    println("  Happy Birthday, Jhansi!")
    printBorder(border, timesToRepeat)
}

fun printBorder(border: String, timesToRepeat: Int) {
    repeat(timesToRepeat) {
        print(border)
    }
    println()
}

Congratulations! With functions, arguments, variables, and a repeat loop, you have learned fundamental building blocks that are used in almost all programs.

Take a break, and then tackle the next task below, where you will create more functions and loops, and you will gain the power to build a gigantic cake with the right number of candles with just a few lines of programming.

5. Create a cake with the layers and candles

Create the main() function

  1. Reset your code in the editor to the Hello, world! program.
  2. You can remove the argument to main(), because you won't be using it.
  3. In main(), create a variable age and set it to 24.
  4. In main(), create a second variable layers and set it to 5.
  5. In main(), call a function printCakeCandles() and pass in the age. This will leave you with an error, because you have not created that function yet.
  6. Samewise, call a function printCakeTop() and also pass in the age.
  7. Finally, call a function printCakeBottom() and pass in the age and also the number of layers.
  8. To get rid of the errors, comment out the three function calls by adding // at the beginning of each line, as shown below. This technique allows you to draft your code without triggering errors.
  9. Run your program, and it should have no errors and do nothing.

Your main() function should look like the code below.

fun main() {
    val age = 24
    val layers = 5
    // printCakeCandles(age)
    // printCakeTop(age)
    // printCakeBottom(age, layers)
}

Create printCakeTop()

The printCakeTop() function to print the top of the cake, a line of equal signs, is almost the same as the printBorder() function you created earlier in this codelab.

==========================
  1. Below the main() function, add a blank line, and then create a function, printCakeTop() that takes one argument, age, of type Int.
  2. Inside, use a repeat() statement to print one equal sign age times plus 2. The extra two equals signs are so that the candles won't fall off the side of the cake.
  3. At the end, when the repeat() is done, print an empty line.
  4. In main(), remove the two // symbols from the beginning of the line of code for printCakeTop(), because the function now exists.
printCakeTop(age)

Here is your finished function.

fun printCakeTop(age: Int) {

    repeat(age + 2) {

        print("=")

    }

    println()

}
  1. Run your code to see the top of the cake.

Create printCakeCandles()

Each candle is made up of two symbols: a comma (,) for the flame, and a vertical line (|) for the candle body.

,,,,,,,,,,,,,,,,,,,,,,,,

||||||||||||||||||||||||

To accomplish this in one function, put two repeat() statements, one for the flames and one for the bodies, into your function.

  1. Below the main() function and the printCakeTop() function, create a new function, printCakeCandles() that takes one argument, age, of type Int.
  2. Inside, use a repeat() statement to print one comma , for the flame.
  3. Repeat this age times.
  4. At the end, print an empty line.
  5. Add a print() statement to print one space for insetting the candles.
  6. Below, repeat the steps to create a second repeat() statement to print the candle bodies with a vertical line |.
  7. In main(), remove the two // symbols from the beginning of the line of code for printCakeCandles().
printCakeCandles(age)
  1. Run your code to see the top of the cake and the candles

Solution:

fun printCakeCandles(age: Int) {

    print (" ")

    repeat(age) {

        print(",")

    }    

    println() // Print an empty line   

 

    print(" ") // Print the inset of the candles on the cake

    repeat(age) {

        print("|")

    }    

    println()

}

Create printCakeBottom()

In this function, you are drawing a cake bottom that is as wide as age + 2, and you draw it for a height of a given number of layers.

@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
  • This means your function needs two arguments, one for the width (age) and one for the height (layers).
  • To print the bottom of the cake, you first repeat the ‘at' @ symbol age + 2 times to print one layer. Then, you repeat printing one layer layers times.

Draw the at symbol age+2 times to create a layer

  1. Below the existing functions, create a function printCakeBottom() with two arguments, age and layers, both of type Int.
  2. Inside the function, use a repeat() statement to print one layer of ‘at' @ symbols age + 2 times. Finish by printing an empty line, as shown below.
fun printCakeBottom(age: Int, layers: Int) {

    repeat(age + 2) {

        print("@")

    }

    println()

}
  1. Run your code to verify that it prints one line of cake bottom.
 ,,,,,,,,,,,,,,,,,,,,,,,,
 ||||||||||||||||||||||||
==========================
@@@@@@@@@@@@@@@@@@@@@@@@@@

Nested repeat() statements

To print multiple identical layers of cake bottom, you could say:

For layer 1 repeat the symbol 12 times: @@@@@@@@@@@@

For layer 2 repeat the symbol 12 times: @@@@@@@@@@@@

For layer 3 repeat the symbol 12 times: @@@@@@@@@@@@

Or you can say this much more concisely as:

Repeat for three layers:

Repeat the symbol 12 times.

@@@@@@@@@@@@

@@@@@@@@@@@@

@@@@@@@@@@@@

Now, this is something neat you can do with repeat() statements. You can put one repeat() statement inside another repeat() statement. So you could create a repeat() statement within a repeat() statement to print the symbol a certain number of times for a certain number of layers.

Use a nested repeat() to print cake layers

  1. Put a second repeat() statement around all of the code inside the function. Repeat this loop layers times.
  2. In main(), remove only the two // from the line of code for printCakeBottom().
printCakeBottom(age, layers)
  1. Run your code to see the whole cake.

Solution for printCakeBottom().

fun printCakeBottom(age: Int, layers: Int) {

    repeat(layers) {

        repeat(age + 2) {

            print("@")

        }

        println()

    }    

}

Congratulations! You've just finished a pretty complex program with several functions and a nested repeat statement. And your cake will always have the right number of candles!

The final output of your program should be:

 ,,,,,,,,,,,,,,,,,,,,,,,,
 ||||||||||||||||||||||||
==========================
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@@@@@@@@@@@@

 

Code:

fun main() {
    val age = 24
    val layers = 5
    printCakeCandles(age)
    printCakeTop(age)
    printCakeBottom(age, layers)
}

fun printCakeCandles(age: Int) {
    print (" ")
    repeat(age) {
          print(",")
    }    
    println() // Print an empty line

    print(" ") // Print the inset of the candles on the cake
    repeat(age) {
        print("|")
    }    
    println()
}

fun printCakeTop(age: Int) {
    repeat(age + 2) {
        print("=")
    }
    println()
}

fun printCakeBottom(age: Int, layers: Int) {
    repeat(layers) {
        repeat(age + 2) {
            print("@")
        }
        println()
    }    
}

 

7. Troubleshooting

If the browser-based Kotlin programming tool is not executing your code or gives you an unexpected error not related to your code, you can try the following:

  • Reload the page using Shift + Reload.
  • Wait a minute, then try again.

8. Summary

  • Use ${} to surround variables and calculations in the text of print statements. For example: ${age} where age is a variable.
  • Create a variable using the val keyword and a name. Once set, this value cannot be changed. Assign a value to a variable using the equal sign. Examples of values are text and numbers.
  • A String is text surrounded by quotes, such as "Hello".
  • An Int is a whole positive or negative number, such as 0, 23, or -1024.
  • You can pass one or more arguments into a function for the function to use, for example: fun printCakeBottom(age:Int, layers:Int) {}
  • Use a repeat() {} statement to repeat a set of instructions several times. For example: repeat (23) { print("%") } or repeat (layers) { print("@@@@@@@@@@") }
  • A loop is an instruction to repeat instructions multiple times. A repeat() statement is an example of a loop.
  • You can nest loops, that is, put loops within loops. For example, you can create a repeat() statement within a repeat() statement to print a symbol a number of times for a number of rows, like you did for the cake layers.

Summary of using function arguments: To use arguments with a function, you need to do three things:

  • Add the argument and type to the function definition: printBorder(border: String)
  • Use the argument inside the function: println(border)
  • Supply the argument when you call the function: printBorder(border)

9. Learn more

Here is the official documentation for the Kotlin concepts you learned in this codelab.

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