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Printing and reading

The basic structure of a program is following:

using System;

public class Program 
{  
    public static void Main(string[] args) 
    {  
        // Add your statements here  
    }  
}

The program execution starts from the first line after public static void Main(string[] args) { and ends at the closing } bracket. Everything in between is run one row at a time. For example, the most common first program of any programmer, the Hello World, would go like this:

public class Program {  
    public static void Main(string[] args) 
    {  
        Console.WriteLine("Hello World");  
    }  
}

In this example, the only runnable statement is Console.WriteLine(“Hello World”);, which prints out

Hello World!

We will later focus on the terms public class and public static void, no need to worry about them yet.

In the material the whole structure might not be shown, unless purposefully needed. The example above could be represented also as

Console.WriteLine("Hello World");

in the future. In the exercises, for the first weeks the basic structure is given, so you do not have to worry about memorising it quite yet.

Printing

As mentioned earlier, programming languages have statements built in them. Console.WriteLine is one of them. The statement is quite self-explanatory. It tells the computer to write a line to the console. You can change the Hello World to any text you wish, as long as the command itself is not changed, and it will work.

The requirements in the exercises are very precise. If for example the line needs to end with an exclamation mark, it cannot be left out.

Programs are created (and read) command by command, where every command has to be on their own line. In the next example, we are calling Console.WriteLine twice, which means the print command is executed twice.

public class Program {  
    public static void Main(string[] args) 
    {  
        Console.WriteLine("Hello World!");
        Console.WriteLine("... and Pietarsaari!"); 
    }  
}

This would print

Hello World!
... and Pietarsaari!

To be precise, the command Console.WriteLine(“example text”); prints the text example text and a line change. The line change can also be handled with special character \n, which is written as a part of the printable text. The example above could also be written as

public class Program {  
    public static void Main(string[] args) 
    {  
        Console.WriteLine("Hello World!\n... and Pietarsaari!"); 
    }  
}

Notice, there is no empty space around the \n. In a text, every character, even the empty ones, are part of the text. If you would put space around the line change, the first line would end with an empty character and the second one would start with one.

Sometimes the texts can be quite long, and reading them from a single line can become quite difficult. It is possible to do split the string into multiple pieces and then concatenate them together with the + operator. The example above could be

public class Program {  
    public static void Main(string[] args) 
    {  
        Console.WriteLine("Hello World!\n" + 
        "... and Pietarsaari!"); 
    }  
}

This will print exactly the same information as previously. Compared to the first solution this is more efficient, as we have to call the print command only once. Compared to the second solution, this is easier to read.

So far, everything we have printed have ended in a line change. If would want to print something but not change the line in the end, we would use Console.Write(“I do not change line”);

Thus, there are two statements for printing:

  • Console.WriteLine() prints the text and changes line
  • Console.Write() prints the text but keeps on the same line

In the printed text, there can be special characters, such as \n. There are also other special characters (here) you might want to familiarise yourself with.

Parameters for a command

When we want to print something, we have to give that information as a parameter to our print command. Parameters are given to the command inside (). For example, to print I like coding, we give the parameter inside quotes as follows: Console.Write(“I like coding”).

Semicolon separates commands

With a semicolon ; statements are separated from each other. We could write our examples on a one line, but they would not be very readable.

Console.Write("Hello "); Console.Write("World!"); Console.Write("\n");

This would print out

Hello World!

Code blocks

Code consists of Code blocks. Code block means part of the code, separated with {}. There are usually multiple of these in a single program, as could already be seen from our basic structure.

For example, the line public static void Main(string[] args) that defines where to start the program, defines a block of what to run when the program is started.

public class Program 
{  // Here starts block for the class

  public static void Main(string[] args) 
  {  // Here starts block for the main

  // Your code would be here

  } // The main block ends
 
} // The class block ends

The example shows a block inside another block. Blocks can be used to define the structure of a program. The block for the class contains the whole program structure, whereas the block for main contains the source code that is run when the program is started.

The block is always opened with { and closed with }. If either of them is missing, the code will not compile, and thus will not run.

Comments

As you might have noticed, we already have comments in our code. Comments are text that are not compiled and thus are not run. Comments can be used for example to comment certain elements in the code, or to temporarily comment out part of a code for debugging purposes. There are two types of comments:

  • // single line comment
  • /* multiple line comment */

As you can see, the single line comment is started with // but does not have an ending delimeter. Multiline comment starts with /* and ends with */. Everything between these are considered as a comment by the compiler.

Code style

Even though there is no style restraint from computer or the language we have chosen, there is great advantage in keeping the code neat and easy to read. For every language, there are widely spread coding conventions (here). Indentation and variable naming conventions are the most useful ones to keep in mind.

We could write our program as

public class Program 
{  
    public static void Main(string[] args) 
    {  
        Console.WriteLine("Hello World!\n" + 
        "... and Pietarsaari!"); 
    }  
}

As well as

public class Program { public static void Main(string[] args) { Console.WriteLine("Hello World!\n" + 
"... and Pietarsaari!");  }  
                                        }

But as you can see, the latter is not as easy to read, and understanding for example the different code blocks is harder. Keep your code neat and clean!

Printing a string

Now that we have the tools for programming, let’s go a bit deeper. so far we have printed simple lines of text. These texts are actually string literals. These literals can be saved into string variables. When we introduce a variable into a program, we usually give it a value. A value is given by following the variable with a = sign, the value and ending the line again with a semicolon, ;. For example, if we want a string variable called message with a value of I am learning, we would assign it with

string message = "I am learning";

Creating a variable gives us an opportunity to reference to the variable inside the program. We could use our variable for printing:

string message = "Print me";
Console.WriteLine(message);

And would get

Print me

If we now would use quotes around our variable name, we would print it as a string literal.

string message = "Print me";
Console.WriteLine("message");

message

As we earlier combined multiple lines of text, the string variables can also be concatenated as a part of printing.

string name = "Doctor Octopus";
Console.WriteLine("We meet again, " + name);

We meet again, Doctor Octopus

Same can be done with multiple parts

string name = "Doctor Octopus";
string greeting = "We meet again, ";
Console.WriteLine(greeting + name + "!");

We meet again, Doctor Octopus!

We could also create a string variable from multiple literals:

string counting = "One" + "\n" + "Two" + "\n" + "Three";
Console.WriteLine(counting);

One
Two
Three

Reading strings from user

So far we have used strings that are written directly to the source code. It would be nice if we could tell our program, what we want to print each time. This can be done with another built in command, ReadLine.

public class Program 
{
    public static void Main() 
    {  
    // Print a request for message.
    Console.Write("Give a message: ");

    // Assign a new string variable. The value is the command we want to use.
    string message = Console.ReadLine();

    // Print out the message we gave.
    Console.WriteLine(message);
    }
}

This will look something like this with imput I want to print this:

Give a message: I want to print this
I want to print this

This is the same example, but with concatenating the input message with Your message was:

public class Program 
{
   public static void Main() 
   {  
    // Print a request for message.
    Console.Write("Give a message: ");
    
    // Assign a new string variable. The value is the command we want to use.
    string message = Console.ReadLine();

    // Print out the message we gave.
    Console.WriteLine("Your message was: " + message);
   }
}

This will look something like this with imput I want to print this:

Give a message: I want to print this
Your message was: I want to print this

You can now do the exercises for printing and reading

Copyright © 2020 Heikki Ahonen and Tero Niemi. More information about licensing here.