# 2.1. A Sample C# Program¶

As a start let us consider a ridiculously simple problem and a program to solve it. Suppose you paint the walls of rooms in one color and the ceiling in another, and you want to calculate the size of the areas to cover with paint. For simplicity ignore doors. What data do you need to start with? Clearly the dimensions of the room. Suppose we consider modern houses where the height of the room is predictably 8 feet, so the new starting data is just the length and width of the room.

You need to

1. Obtain the length and width from the user.
2. Calculate the wall area and ceiling area.
3. Let the user know the results.

This is a very simple programming pattern: data in, calculate results, output results. In this case the calculations in the middle are very easy.

In the examples that you should have downloaded is a first simple program, painting/painting.cs.

Here is what it looks like when it runs, with the user typing the 20.5 and the 10:

Calculation of Room Paint Requirements
Enter room length: 20.5
Enter room width: 10
The wall area is 488 square feet.
The ceiling area is 205 square feet.


This is not very exciting, but it is a simple place to start seeing basic program features. We will refer back to this sample run while discussing the program. Here is the text of the program:

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 using System; class Painting { static void Main() { double width, length, wallArea, ceilingArea; string widthString, lengthString; double HEIGHT = 8; Console.WriteLine ( "Calculation of Room Paint Requirements"); Console.Write ( "Enter room length: "); lengthString = Console.ReadLine(); length = double.Parse(lengthString); Console.Write( "Enter room width: "); widthString = Console.ReadLine(); width = double.Parse(widthString); wallArea = 2 * (length + width) * HEIGHT; // ignore doors ceilingArea = length * width; Console.WriteLine("The wall area is " + wallArea + " square feet."); Console.WriteLine("The ceiling area is " + ceilingArea + " square feet."); } } 

This section gives an overview of a working program, even if all the explanations do not make total sense yet. This is a first introduction of concepts and syntax that gets fully explained in further sections.

Do not worry if you not totally understand the explanations! Try to get the gist now and the details later.

The different colors are used in modern program editors to emphasize the different uses of the parts of the program.

We give a line by line explanation:

using System;


The C# environment supplies an enormous number of parts that you can reference. Nobody is familiar with all of them. If you had to make sure you always used names that did not conflict with other names supplied, you would be in trouble. To avoid this C# has namespaces. The same name can be used in different namespaces without conflict. The central standard namespace is System. We will always include this first line, using System;.

Lines 2, 10, 18, and 21 are blank. This is merely for the human reader to separate sections visually. The computer ignores them.

class Painting
{


A basic unit in C# is a class. Our code sits inside a class. Each class has a heading with class followed by a name. This class is Painting. After the heading comes a body delimited by braces. The opening brace { in line 4, is matched by the closing brace } on the last line of the program.

static void Main()
{


A class is broken up with chunks called functions or methods. Each has a heading. C# allows the currently popular programming paradigm called object-oriented programming, where classes generally describe new kinds of objects. This is useful in complicated situations, but we start more simply with the older procedural programming. Unfortunately for now, the more common situation is with objects, so a function that does not involve such new objects must be marked specially as static.

Functions can be like in math, where they produce a function value for later use. In C# they can also just do something (like write to the screen), and not produce a value for later use in the program. To show that no function value is produced, the word void is used.

Every program must start running somewhere. In C# that is at a function with name Main. So our program starts running here. This syntax for this function needs to start just like here, with static void Main.

Even though this is not a mathematical function producing a value, a function in C# must be followed by parentheses ( ).

After the function heading comes a body. Like with a class, a function body is delimited by braces. The opening brace here is matched by the closing brace on the second to last line of the program.

double width, length, wallArea, ceilingArea;


A program works with data of many different possible types. One type is double. A double can hold an approximate numerical value, including a possible fractional part.

To refer to data in a program we use names called variables. This line says that width, length, wallArea, and ceilingArea are all the names for variables that can hold a double value. We will assign values to these variables later.

This line is a declaration statement. Most statements in C#, like this one, end with ; - a semicolon.

string widthString, lengthString;


This is another declaration. This time the type of the variables is string, which means a sequence of characters, like a line you might type at the keyboard.

double HEIGHT = 8;


Here is another declaration for a double, looking slightly different. In this case we follow a convention, using all capital letters, to suggest that the value of HEIGHT will be constant (unchanging), and we assign its value at the same time with = 8. This naming of constants is not strictly necessary, but it makes the program’s intention easier to follow.

Console.WriteLine ( "Calculation of Room Paint Requirements");


Console refers to the terminal or console window where text output appears for the program. One of the things you can do with the Console is WriteLine, to write a line. The period between Console and WriteLine indicates WriteLine is a named part of the Console. This WriteLine is a function. Like in math, it can have a parameter in parentheses. While you are used to a parameter for a function in math being a number, functions in C# are much more general. A function can be defined with any type of parameters. Here the parameter is a string, "Calculation of Room Paint Requirements", delimited by the quotes at either end. Notice that the contents of this string appear at the start of the screen output displayed for this program. The program did write this line.

Console.Write ( "Enter room length: ");


This statement is similar to the last one, except that it uses Write rather than WriteLine. The WriteLine function wrote a whole line - see that the output next after the WriteLine statement started on the next line. Here Write does not advance the printing position to the next line after it.

This statement serves as a prompt: letting the user know that information is being requested (a room length).

lengthString = Console.ReadLine();


Here is where the program takes in the information requested from the user. Its action is actually right to left: Console.ReadLine is another function available with the Console, that reads a line typed in by the user on the keyboard. Here in the sample run, on the same line as the prompt string (because of the previous Write, not WriteLine), the user types 20.5 and the Enter or Return key.

In the sample run, the value produced by the Console.ReadLine function is these four characters 20.5.

Recall that lengthString was declared as a variable to hold a string. The = indicates an assignment statement. It is an assignment of the value on the right of the equal sign to be the current value of the variable on the left of the equal sign. In the sample run, this would mean that the variable lengthString would end up holding the value "20.5". Though these characters happen to look like a number, any sequence of characters can be typed. The Console.ReadLine() function produces this sequence of characters as a string type.

length = double.Parse(lengthString);


Of course we want to interpret the user’s input as a number in order to do our arithmetic. This line makes the conversion between the types.

It is another assignment statement (with the =). We are assigning to the variable length, which we declared as a double. The value assigned comes from the expression on the right of the =, double.Parse(lengthString). The function double.Parse, is just the one we want, it takes a string parameter lengthString containing the string from the user input, and the value produced is the corresponding double number. In the sample run that assigns to length the value 20.5.

Console.Write( "Enter room width: ");
width = double.Parse(widthString);


These lines are analogous to the previous three lines: give a prompt for the user; get the user response; convert it to a double, and assign to a variable (width in this case). In the sample run the variable width is assigned the value 10.

At this point we have all the data we need from the user. The next part is the brief calculations of results:

wallArea = 2 * (length + width) * HEIGHT; // ignore doors
ceilingArea = length * width;


At the end of the first line is a comment. It starts with // and ends at the end of the same line. It is ignored by the compiler. It is there for humans, hopefully to add something that helps understanding of the program.

We have two assignment statements. The values to assign are given by arithmetic expressions on the right side of the equal signs. It looks pretty much like regular math, except in math class you may be used to only having one letter names for variables, unlike length, width, and HEIGHT.

The tradeoff for allowing multiple character names is that multiplication must have an explicit operation symbol. The symbol used for multiplication in C# is * an asterisk. The + and parentheses serve their normal mathematical purpose. In the sample run, the value of 2 * (length + width) * HEIGHT is

2 * (20.5 + 10) * 8


which simplifies to 488.

With the sample run, ceilingArea would get the value 20.5 * 10, or 205.

Console.WriteLine("The wall area is " + wallArea
+ " square feet.");


This is a single statement. Line endings act just like a space in C#. The statement ends with the semicolon on the second line.

Again Console.WriteLine will print something to the computer console. This time the string printed is more complicated: It starts off with the literal string "The wall area is ", but then we want to print out the calculated result. The + wallArea allows that. The + sign after the string is not a mathematical operator here. Coming after a string, it has a special string meaning: It converts the next part wallArea to be a string. In the sample run that would be converting the double value 488 to be the string "488". The plus sign then “adds” the strings in a manner appropriate for strings, concatenating them. That means joining them together, end to end.

The + " square feet." then tacks on the last part to the string. In the sample output you see what is printed:

The wall area is 488 square feet.


sandwiching the value taken from the variable wallArea between two literal string, given in quotes.

Console.WriteLine("The ceiling area is " + ceilingArea
+ " square feet.");


This statement behave like the previous one, except with different quoted strings and the value of a different variable. See the sample output.

   }
}


Finally we have the matching closing braces marking the end of the body of the Main function and the end of the body of the Painting class.

Of course the display would look different if the user entered different data. Here is what is displayed when the user enters length 15 and width 6.5:

Calculation of Room Paint Requirements
Enter room length: 15
Enter room width: 6.5
The wall area is 344 square feet.
The ceiling area is 97.5 square feet.


The blank space in the program was there to aid human understanding. In a C# program whitespace is any consecutive combination of spaces, newlines, and tabs. C# treats any amount of whitespace just the same as a single space, except inside quoted strings, where every character is important.

Also the compiler does not require whitespace around special symbols like {};().=*+,. Hence the painting/painting.cs program above would be just as well translated by the compiler if it were written as:

using System;class Painting{static void Main(){double width,length
,wallArea,ceilingArea;string widthString,lengthString;double HEIGHT
=8;Console.WriteLine("Calculation of Room Paint Requirements");Console.