FontysICT-sem1

Basics afterthought: Variables

This chapter is written as a naslag work, it is not specifically written to teach how to program with variables.

Types of variables

A variable is a piece of memory in which temporarily a value can be stored. The common types of variables are:

Content	Name	Examples
Stukje tekst	String	“abcde”
		“dit is een tekst”
		””
		etc.
Geheel getal	Int	12
		-1337
		0
		etc.
Komma getal	Double	10.2
		-12.3
		5.0
		etc.
Waar of niet waar	Bool	true, false

Create variable (declare)

Variables can be created in different ways, some examples are listed below. Note that:

• You can choose the variable name yourself
• The line must be ended with a ";"-character

There are several ways to create variables. Program on a blank line the type of the variable (see above), the name you want to give the variable (you choose it yourself) and an ";" character to end the programming command.

Example	Effect
String s;	Variabele met de naam s wordt aangemaakt.
	De default waarde is “”.
int i;	Variabele met de naam i wordt aangemaakt.
	De default waarde is 0.
double d;	Variabele met de naam “d” wordt aangemaakt.
	De default waarde is 0.0
Bool b;	Variabele met de naam “b” wordt aangemaakt.
	De default waarde is false
String mijnString;	Variabele met de naam “mijnString” wordt aangemaakt.
	De default waarde is “”
int getal;	Variabele met de naam “getal” wordt aangemaakt.
	De default waarde is 0
double straal;	Variabele met de naam “straal” wordt aangemaakt.
	De default waarde is 0.0

Immediately after creation, a variable has a value that we call the default value. This may vary slightly from one programming language to another different. Therefore, it is good practice to make variables that you want to have a specific value explicitly assign this value. assign this value.

Give value to variable (assignment or assignment)

Once a variable is created, a value can be assigned to it. Notice:

• Only valid values can be assigned (string values to strings, numbers to int, etc.), programming a non-valid assignment results in an error that prevents the program from executing.
• The variable to which a value is to be assigned is on the left side of the "=" sign, and the value to be put into the variable is on the right side of the "=" sign.
• The line of code is again terminated with the ";" sign.

Here are some examples. In comments it is explained what it means.

String s; // create a variable named "s".
s = "test"; // Variable named "s" gets the value "test".

int i;
i = 10; // create variable named "i" and give it value 10

double d;
d = 1.52; // New variable named "d" is given the value 1.52

bool b;
b = true; // New variable "b" gets the value true

String string1;
string 1 = "abc";
String string2;
string2 = string1; // Variable named "string2" gets
                    // the value of "string1", namely "abc"

int numberA;
numberA = 5;
int numberB;
numberB = numberA; // Variable named "numberB" gets
                  // the value of "numberA", namely 5

double commaGetalA;
commaGetalA = 1.32;
double commaGetalB;
commaGetalB = commaGetalA; // Variable named "commaGetalB" gets
                            // the value of "commaGetalA."
                            // namely 1.32

String s;
s = textBox1.Text;
    // Variable named "s" gets
    // as the value the text contained in the
    // TextBox named "textBox1".

This works because the Text property of the TextBox is also is of the type string.

Create variable and give it a value directly (declare and initialize)

Create variable named s and assign value "test":

String s = "test";

Create variable named i and assign value 10:

int i =10;

Create variable named d and assign value 1.52:

double d = 1.52;

Create variable named b and assign value true:

bool b = true;

Convert values to other types (convert)

Note: converting an int or double to a String always succeeds, the other way around does not always succeed and may produce an error message during program execution (crash or Unhandled Exception).

A bool variable cannot be converted.

Convert the value of i to a text with the same value. The result of the last line is that variable s is given the value 81.

int i = 81;
String s;
s = Convert.ToString(i);

Convert the value of d to text of the same value. The result of the last line is that variable s is given the value "12.33":

double d =12.33;
String s;
s = Convert.ToString(d);

Convert the value of s to an integer (integer) with the same value if it succeeds (otherwise you get an error message). The result of the last line is that variable i gets the value 7:

int i;
String s = "7";
i = Convert.ToInt32(s);

Convert the value of s to a comma number with the same value if you can (otherwise you will get an error message). The result of the last line is that variable d gets the value 12,129:

double d;
String s = "12.129."
d = Convert.ToDouble(s);

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