Basic C# programming theory

Types

Before you can write your first program, there is some theory that you must know. The first thing to know is that according to C#, everything has a type. For example, 2 is an integer. We can say that the type of 2 is integer.

We are going to use five of the C# primitive types, described below:

int
This is any whole number between approximately -2,000,000,000 and 2,000,000,000.
bool
This is either true or false.
double
This is any number with a decimal point in it.
char
This is any single character, digit, or punctuation mark. In C#, a char is always enclosed in single-quotation ( ‘ ) marks.
string
This is zero or more characters, digits, and/or punctuation marks. In C#, a string is always enclosed in quotation ( ” ) marks.

Important

We will be referring to these five types very often, and using one or more of them in every single program that we write! Therefore, become very familiar with them. When you hear the word string or bool, for example, you should immediately understand the Computer Science meaning of that term; and whenever you see any data, you should immediately understand what its type is.

Type Examples
int
  • 576
  • -873
  • 0
bool
  • true
  • false
double
  • 7.51954
  • 985.0
  • -33.614
char
  • ‘8’
  • ‘z’
  • ‘,’
  • ‘ ‘
string
  • “Pi, as an irrational number, cannot be represented accurately by any simple fraction.”
  • “z”
  • “”
  • ” “

Exercises

What is the type of:

  1. 89.9 ______________________________________
  2. false ______________________________________
  3. ‘p’ ______________________________________
  4. -0.0 ______________________________________
  5. 11 ______________________________________
  6. ‘0’ ______________________________________
  7. 0 ______________________________________
  8. “0” ______________________________________
  9. true ______________________________________
  10. “z” ______________________________________
  11. “775.21” ______________________________________

Which type would you use to represent:

  1. Your name? ______________________________________
  2. Your age in years? ______________________________________
  3. Whether you are alive? ______________________________________
  4. Your grade (A, B, C, D, or E) for an assignment? ______________________________________
  5. The length of a line in centimetres? ______________________________________
  6. The number of pages in a book? ______________________________________
  7. The exact price of a bottle of milk? ______________________________________
  8. Whether you have eaten anything in the last 6 hours? ______________________________________
  9. The second letter of your first name? ______________________________________
  10. The number of grains of sand in a jar? ______________________________________
  11. The square root of 2? ______________________________________
  12. Your reasons for reading this book? ______________________________________
  13. The colour of your eyes? ______________________________________

Working with time

There Is No “Right” Program

There are an infinite number of ways to write any particular program, just as there are an infinite number of ways to paint a picture of a person. The solutions that your instructors give you are just the way that they would write the program. Study them, and learn from them, but understand that their solutions are not the only way (or even the best way) to write a program.

The second (and last) bit of theory to know before we begin writing programs is that programming instructions are executed sequentially [1], one after the other. This means that the order in which you write programming statements matters. If you get the order incorrect, the program will do the incorrect thing.

The order in which statements could be executed is called the control flow of the program. Control flow consists of multiple paths, each of which could be taken during program execution. One of the most important skills of a programmer is the ability to “see” the control flow of a program [2]. The best way to get better at “seeing” control flow is to read other people’s programs, try to work out what they do without executing them, and then execute them to see if your reasoning was correct (or, if your reasoning was incorrect, where and why it was incorrect).

There are many techniques and tools that programmers have created to help themselves to visualise control flow: flowcharts, trace tables, call graphs, sequence diagrams, and so on. We may cover some of these in class. If you don’t find the standard tools for visualising control flow to be useful, you must invent your own notation or visualisation for understanding control flow.

[1]This is a slight simplification. You will learn about non-sequential control flows in your second or third year.
[2]This is not strictly true for all programming languages, but it is true for the vast majority of mainstream programming languages.

A first program

Syntax

Syntax is the collection of rules that define what a programming language looks like. Each programming language has its own syntax, just as each human language has its own grammar and spelling rules.

With that, you are ready to begin your first program!

int a;
int b;
int c;

This simply says that an int called a exists, an int called b exists, and an int called c exists. Notice that each statement ends with a ; (semicolon) character. A shorthand, equivalent way of saying exactly the same thing is

int a, b, c;

a, b, and c are variables (or identifiers, or symbols). A variable has a name and a type. It holds a value. The value of a variable can be changed, but its name and type cannot. The programming instructions above are called variable definitions, since they define that something exists. Identifiers should start with a letter and contain no spaces or punctuation.

Important

A variable in Computer Science is not the same as a variable in Statistics, or a variable in Mathematics. Understand the specific Computer Science meaning, and do not confuse it with your other subjects!

Merely saying that something exists doesn’t make for an exciting program. Let’s try this program instead:

int a, b, c;
a = 5;
b = 10;
c = a + b;

Why not one way to do things?

Programming languages are designed for humans as well as computers, and language designers have tried to make languages that let you express yourself in the way that you find to be most natural. Some people like writing very short, very concise code. Others like to write many lines, believing that this leads to clearer, more readable code. Most people find a balance between very-concise and very-verbose. It doesn’t matter which style you prefer to write in. What matters is that you are able to read programs written in any style.

Let’s understand what it does. First, we define the a, b, and c variables. Then we assign the value 5 to a. Then we give the value 10 to b. Lastly, we assign the value 15 to c. These statements, of the form X = Y;, are called assignment statements. Assignment statements are used to change the value of a variable. All assignment statements have a right-hand-side (on the right of the = symbol) and a left-hand-side (on the left of the = symbol). There is always a variable name on the left-hand-side. An assignment statement executes the right-hand-side first, and then assigns the resulting value to the variable on the left-hand-side. This is why the variable c is given the value 15, rather than the value a + b.

Remember that control flow matters ...

Programs do their work one statement at a time. So we would be wrong if we looked at the program above and tried to think about all four statements at the same time. Each statement in a program changes something from how it was before. (We say the program has a state, and statements change the state of the program.) So the first key skill we’ll need is to realize that statements are executed in a specific order, top-to-bottom, and that each one makes a change to the state. It is a bit like baking a cake from a recipe — you need to do things one step at a time, in some definite time order.

We can tighten this code up a bit, by combining definitions and assignment statements, and reduce it from 4 lines to 3 lines in length.

int a = 5;
int b = 10;
int c = a + b;

And if we’d like to, we can use the shorthand form of definitions to reduce it to a single line:

int a = 5, b = 10, c = a + b;

Putting things together

If you’re not yet convinced that Computer Science isn’t Mathematics, think about this: in Mathematics, there are common equivalents for int, double, and bool types. There are no common equivalents for char and string types. That might be because Computer Science is a more practical and pragmatic discipline, and one of the common things that people want to be able to do is read and write text. So it makes sense for Computer Science to have char and string, and it makes sense for Mathematics to not use those types. It also makes sense for programming languages, such as C#, to use those types in a non-mathematical way. For example, if you wrote this:

string a = "super";
string b = "man";
string c = a + b;

That would be perfectly OK in C# (and c would end up with the value superman). In Computer Science, + commonly means put these things together. If you put two ints together, you get the sum of the ints. If you put two strings together, you get the combined value of both strings. This makes absolutely no sense, mathematically. But this is Computer Science – not Mathematics!

Exercises

There is something wrong with each of these lines of code. For each line, write a sentence describing what is wrong.

  1. in a;

  1. string a b;

  1. bool a

  1. int a, string b;

  1. var a;

  1. int 3d;

  1. int a = 3.0;

  1. double a, b = 5.0, c = “6.0”;

  1. var a = 9, b = 8;

  1. char zoology = “”;

  1. char biology = “E”;

  1. bool botany = True;

  1. INT CHEMISTRY = 100;

  1. bool able, able;

  1. int articulate = 8; bool ampersand = articulate;

  1. string o’brien = “Connor”;

What is the type of x?

  1. var x = “Yes”; _______________
  2. var x = 213; _______________
  3. var x = 0.0; _______________
  4. var x = ‘y’; _______________
  5. var x = “y”; _______________
  6. var x = true; _______________
  7. var x = 5 + 3; _______________
  8. var x = “false”; _______________
  9. var x = 3.2/5.1; _______________
  10. var x = false; _______________
  11. var x = “20/5”; _______________

Basic Arithmetic

There are five arithmetic operators that are commonly used in C#. The first four should be familiar to you from school:

  • X + Y adds X and Y
  • X - Y subtracts Y from X
  • X * Y multiplies X and Y
  • X / Y divides X by Y

The last operator is the modulus operator:

  • X % Y gives the remainder that would result from dividing X by Y

The first time you did division at school, you probably expressed your answer as “A remainder B”. For example, you would say that 7 divided by 4 was “1 remainder 3”, or that 24 divided by 6 was “4 remainder 0”. The modulus operation gives you the “remainder” part of the answer; so 7 % 4 gives 3, and 24 % 6 gives 0. The sign of the answer is the same as the sign of the dividend (so, for example, -7 % 4 gives -3).

Negative numbers and spacing

You usually don’t need to put spaces between the parts of an arithmetic expression: 4-5+3 is the same thing as 4 - 5 + 3. The only exception to this is when you have a negative term in your expression. C# doesn’t understand the arithmetic expression 7--4, but it does understand 7 - -4, so be sure to put a space before negative numbers.

You can group arithmetic terms using brackets, just as you would in ordinary mathematics. The usual precedence of operations (brackets, division & multiplication, addition & subtraction) applies to arithmetic expressions in C#. When operators have the same precedence, arithmetic expressions are evaluated from left to right.

Arithmetic that is done exclusively with ints always results in integer answers. This means that 7 / 4 = 1, and not 1.75 or 2. Arithmetic that is done exclusively with doubles results in double answers. This means that 7.0 / 4.0 = 1.75. Arithmetic that mixes doubles and ints results in double answers. This means that 7.0 / 4 = 1.75, and 7 / 4.0 = 1.75.

Exercises

What is the answer to these arithmetic expressions? When the answer is a double, be sure to show a decimal point in your answer.

  1. 6 * 4 - 1 ________
  2. 6 * (4 - 1) ________
  3. 3.2 * 2.0 ________
  4. 3.2 * 2 ________
  5. (3 * (5 - 2) / 4) + 1 ________
  6. ((5 - 2) / 4 * 3) + 1 ________
  7. 1 + 1.0 ________
  8. (-(-2) + (-2 * -2 - 4 * 3 * 2)) / 2 * 3 ________
  9. (-(-2) + (-2 * -2 - 4.0 * 3 * 2)) / 2 * 3 ________
  10. 8 - -8 ________
  11. (10 * 7) % 3 ________
  12. (5 % 2) + (71553 % 2) ________
  13. (19 % 4) + (-72 % 10) ________

Debugging

Programming is a complex process, and because programs are written by human beings, they often contain errors. Some of these errors are simple, like messing up the grammar of the programming language, and the compiler can tell you there’s a problem and (usually) also tell you where it is, so you can fix it. These kinds of simple errors are called syntax errors. Syntax refers to the structure of a program and the rules about that structure. Mis-spelling a word in a programming language, or putting it in the wrong order, or not including necessary punctuation, or including too much punctuation, or not capitalising a word that should be capitalised (or vice versa!) are all examples of syntax errors.

During the first few weeks of your programming career, you will probably spend a lot of time tracking down syntax errors. As you gain experience, you will make fewer syntax errors and find them faster. After a few weeks, you won’t make any syntax errors at all! This is just like the process of learning a new spoken language. While you are learning, you will make silly errors that a native speaker wouldn’t make. But after a while, you don’t make any errors. And, just like learning a new spoken language, the more you practise, the better you’ll get ... and if you don’t practice enough, you’ll keep making mistakes. So practice: it really does make a big difference.

The more difficult errors are semantic errors, where you’ve given the computer a valid sequence of instructions, but the instructions don’t lead to the result that you think they lead to. You’ll run into plenty of these, and when you do you’ll need to understand exactly what you’re asking the computer to do, so that you can figure out where the flaw in your reasoning is.

Runtime errors, which cause a program to stop running, occur because of semantic errors. Usually, you’ve made the error of assuming something that you shouldn’t have assumed! These errors are also called exceptions because they indicate that something exceptional (and bad) has happened.

Programming errors are called bugs and the process of tracking them down and correcting them is called debugging. It is useful to distinguish between types of bugs in order to find the cause of the error and fix it more quickly. Here is how you can distinguish between types of bugs:

  1. Do you get errors when you compile the program? If so, you have a syntax error.
  2. Does the program fail to do what you expect it to do? If so, you have a semantic error.
  3. Does the program stop running before you expect it to? If so, you have a runtime error which is caused by a semantic error. You need to fix the semantic error so that the runtime error will go away.

What if ...?

For our second program, let’s make a simple password program. If you give it the right password, it will tell you some secret message. If you don’t give it the right password, it’ll tell you to go away.

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string password = Password.Text;
bool passwordIsOK = password == "top sekrit";
if (passwordIsOK)
{
   Result.Content = "My secret message is: Hi, how are you?";
}
else
{
   Result.Content = "Go away.  You don't know the password.";
}

The first line is easy: a combined definition and assignment. The second line is where we check whether the password is correct or not. We do this using the ==, which compares two things for equality. If the things are equal, the expression is true; otherwise, the expression is false. There are a few other symbol-combinations (called operators) which are useful for comparing things, and it’s useful to know what they are when you want to make your own conditions:

X == Y true if X is equal to Y
X >= Y true if X is greater than or equal to Y
X <= Y true if X is less than or equal to Y
X != Y true if X is not equal to Y
X > Y true if X is greater than Y
X < Y true if X is less than Y

X and Y may be any expressions, as long as the expressions have the same type. Memorise these operators. They will be used in almost every program that you will write.

Then we come to an if-statement. An if-statement is a way of doing something only if some expression (which is called the condition of the if-statement) is true. In this case, if passwordIsOK is true, then Result.Content will be given the value My secret message is: Hi, how are you?. If it’s not true, then Go away. You don’t know the password. will be assigned to Result.Content.

Conditions must result in a bool value. If your condition results in a string, int, double or anything except a bool value, your program will not compile.

The else part of an if-statement is optional. This means that if you only wanted to do something when the password was correct, you could have written

if (Password.Text == "top sekrit") {
   Result.Content = "My secret message is: Hi, how are you?";
}

Important

In class, you will already have seen brackets being used to group terms, just as you would do in Mathematics. In the if-statement, brackets are used to tell C# where the condition starts and ends. Within the brackets, you can have other brackets if you want to group terms.

Exercises

What is the answer to these expressions?

  1. 5 < 3 __________
  2. 5 > 3 __________
  3. 6 <= 6 __________
  4. 6 >= 6 __________
  5. 6 < 6 __________
  6. 6 == 6 __________
  7. 6 != 6 __________
  8. 8 != 6 __________
  9. 7 >= 6 __________
  10. 5 - 3 > 2 + 1 __________
  11. 3 - 5 < 1 - 2 __________
  12. “apple” != “orange” __________
  13. “banana” <= “candy” __________
  14. 3.14 * 2 > 6 __________

There’s something wrong with each of the following pieces of code. For each piece of code, write a sentence describing what is wrong.

if 5 > 3 {
   Result.Content = "Something";
}



if (5 <> 3) {
   Result.Content = "Something";
}



if ("true") {
   Result.Content = "Something";
}



if () {
   Result.Content = "Something";
}



If (5 + 3 < 7) {
   Result.Content = "Something";
}



if (5 + 3 < 7) {
   Result.Content = "Something";



if {
   Result.Content = "Something";
}



if (5 - 7 > 3)
   Result.Content = "Something";
}