Computing with C#and the .NET Framework

Chapter 2

C# Programming Basics

©2003, 2011 Art Gittleman

Variables

A variable represents a storage location.

Every variable has a name and a type.

int age = 19;

Name is “age” Type is “int”

Value is 19.

age 19

Identifiers

An identifier names program elements.

C# is case sensitive.

Digits may occur, but cannot be the first character.

Identifiers may use the underscore character.

Keywords are identifiers reserved for special uses.

abstract as base bool break byte case catch char checkedclass const continue decimal defaultdelegate do double else enumevent explicit extern false finallyfixed float for foreach gotoif implicit in int interfaceinternal is lock long namespacenew null object operator outoverride params private protected publicreadonly ref return sbyte sealedshort sizeof stackalloc static stringstruct switch this throw truetry typeof uint ulong uncheckedunsafe ushort using virtual voidvolatile while

Some valid C# identifiers are: savings textLabel rest_stop_12 B3 _test My____my

Some invalid identifiers are: 4you // Starts with a number x<y // Includes an illegal character, < top-gun // Includes an illegal character, - int // Reserved keyword

The character set

The ASCII character set contain 128 printing and nonprinting characters including upper and lower case letters, digits, and punctuation.

C# uses Unicode with contains thousands of characters including ASCII.

For English ASCII is sufficient

Type int

A type defines the values a variable can have.

Type int represents a range of integer values from

int.MaxValue -2,147,483,648

to

int.MinValue 2,147,483,647.

Assignment

Initialize a variable once

int age = 19;

Assign values during runtime

The assignment operator is the equals sign, =

age = 10;

The value is stored in the memory location allocated for that variable.

19

10

20

a. int age =19; b. age = 10; c. age = 20;

Constructing a program

Define a class (used later to define object types)

Include a method that contains executable code

The Main method starts execution. It must have the static modifier to be accessible before any objects are created.

public class AssignIt { public static void Main( ) { int number1 = 25; int number2 = 12; number2 = number1 + 15; System.Console.WriteLine ("Number2 is now {0}", number2); }}// Output -- Number2 is now 40

Example 2.3

UML (Unified Modeling Language)

Standard notation for object-oriented design

Class diagram has three parts

Upper section – class name

Middle section – data fields

Lower section – methods

Underline static methods

AssignIt

Main

Constants

const int FIXED = 25;

Cannot assign to a constant. We use upper case names for constants to remind us.

Input from the keyboard

System.Console.Write("Enter your name: "); // displays a prompt to the userString name = System.Console.ReadLine(); // reads the line the user enters // saves it as a string of charactersint number = int.Parse

(System.Console.ReadLine()); // converts input to an integer

Using

The statement

using System;

Allows the code to refer to classes in the System namespace with the explicit prefix. We can replace

System.Console.ReadLine();

with

Console.ReadLine();

Formatting output

{0:C} formats output as currency, dollars and cents in the US.

{0, 10} right-aligns the output in a 10 character field

{0, -10} left-aligns the output in a 10 character field

Arithmetic Operators

Integer data produces integer results.

Binary operators, +, -, *, /, %

two operands

Unary operators +, -

one operand

Operation Math notation C# (constants) C# (variables)

Addition a + b 3 + 4 score1 + score2 Subtraction a - b 3 - 4 bats - gloves Multiplication ab 12 * 17 twelve * dozens Division a/b 7 / 3 total / quantity Remainder r in a=qb+r 43 % 5 cookies % people Negation -a -6 -amount

Figure 2.9 C# arithmetic operations

Operator Precedence

Evaluate 3 + 4 * 5Which operator gets its operands first?

has higher precedence than +so 3 + 4 * 5 = 3 + 20 = 23Avoid parenthesesWith parentheses 3 + (4 * 5) = 23 but (3 + 4) * 5 = 7 * 5 = 35

3 + 4 * 5

Figure 2.10 Multiplication gets its operands first

* 5(3 + 4)

Figure 2.11 Compute within parentheses first

Precedence Table

Higher precedence-, + Unary Negation and Plus*, /, % Multiplication, Division, Remainder+, - Binary Addition and Subtraction= Assignment

Lower precedence

See Appendix for full table

Combining Assign and Increment

Postincrement – evaluate then add 1 x=5 x++ evaluates to 5 and x becomes 6Preincrement – add 1 then evaluate x=5 ++x, x becomes 6 then ++x value is 6 Postdecrement – evaluate then subtract 1 x=5 x-- evaluates to 5 and x becomes 4Predecrement – subtract 1 then evaluate x=5 --x, x becomes 4 and --x value is 4

Figure 2.12 Expression a) and equivalent expression b)

then

3 + x

x++

a. b.

3 + x++

Figure 2.13 Expression a) and equivalent expression b)

then

++x

3 + x

a. b.

3 + ++x

Methods

A method can contain the code for an operation. public static int MultiplyBy4(int aNumber) {

return 4*aNumber;}Parameters pass data to the methodreturn statement specifies the return value, if

any

Argument 5

multiplyBy4

Return value 20

Pass By Value

C#passes arguments by value by default

The called method receives the value of the argument not the location

The value from the caller gets copied to the parameter which is like a local variable of the method

PassByValue.cs illustrates

value of argument x copied to aNumber

Figure 2.17 Memory usage

cube aNumber

12x

value

0

After the call

12

17280

12 then 17

During the call

12

1728

main

result

Before the call to cube(x)