Extension 1: Integer Operations


Extensions provide a more in-depth overview of various topics. Here, I present a more in-depth discussion behind primitive data types.

Type int

As mentioned before, integers consist of individual digits. You can prefix it with a minus sign (a hyphen) to make it negative. You cannot put spaces, commas, or periods inside ints.

When dividing an integer by another, integer division occurs. In this case, the remainder of the division is dropped. Therefore, the result of 8/5 is 1, not 1.6. A practical use of this is as follows:

int number = 1234;
int thousands = number / 1000;
NSLog ("@The thousands place of %d is %d.", number, thousands);

The output of the code above is

The thousands place of 1234 is 1.

In contrast, the modulus operator (%) is used to derive the exact remainder of integer division. Therefore, 8 % 5 = 3.

Extending the above example,

int number = 1234;
int thousands = number / 1000;
NSLog ("@The thousands place of %d is %d.", number, thousands);
int hundreds = (number % 1000) / 100;
NSLog ("@The hundreds place of %d is %d.", number, hundreds);
int tens = ((number % 1000) % 100) / 10;
NSLog (@"The tens place of %d is %d.", number, tens);
int ones = (((number % 1000) % 100) % 10);
NSLog (@"The ones place of %d is %d.", number, ones);

The output of the code is

The thousands place of 1234 is 1.
The hundreds place of 1234 is 2.
The tens place of 1234 is 3.
The ones place of 1234 is 4.

The code above is rather involved, but try to figure it out. Remember, the modulus operator extracts the remainder of the division that could have been placed in its place.

Objective-C Lesson 3: Object-Oriented Programming


When object-oriented programming arrived in the programming world, it was considered a savior of software by some and yet another foolish experiment by others. Regardless of the perspective, we need to define some basic concepts and terminology before we begin

A Thing

An object is a thing. In the real world, you have “things,” all of which have certain properties. For example, you probably have a car—a “thing.” The car has properties, such as paint color, total mileage, make, model, and year. You can also do things with the car, or have something done to it—for example, you can drive twenty miles in the car, or wash the car.

In an object-oriented program, your specific car is an instance of a class, generally called Car. A class is the definition of the object, and in most cases can be thought of as a “factory” for its instances. A class can create many instances of the same type of object, although they have to have different names. An object is simply a variable, but it can be (and usually is) of a custom type.

Instance Variables and Methods

As mentioned before, your car, and any other car, has certain properties that are potentially unique to that specific instance. These properties are known as instance variables, or ivars for short. Instance variables are variables, of either the standard data types, or of any other object, but they are private to the object—that is, every single instance of Car has its own identical set of instance variables; although the variables may be of the same type and number, they can (and usually do) contain different values across different instances.

A method is the proper term for a function, or simply a piece of code that is called upon to perform some action. In Objective-C, there are two types of methods—class methods and instance methods. Class methods are performed directly on the class that an object belongs to—for example, a class methods might ask the car factory how many cars it produced the previous year. Instance methods are performed on specific instances of a class—for example, you might ask one specific car to drive twenty miles, or you might wash one specific car; you wouldn’t do those things to the entire factory. Instance methods affect the state or condition of the object. Just like two identical cars adopt different characteristics over their lifetime, different instances of Car can take on different traits as well.

Message Objects

In Objective-C, there is a specific syntax for calling methods:

[ClassOrInstance method];

The method call begins with an open bracket. This is followed by the name of the class or instance, depending on whether the method belongs to a class or instance. This is followed by a space, and then the name of the method. This is followed by a closing bracket, and the all-important semicolon.
In more formal terminology, the following message call might be described as

[receiver message];

The meaning is the same.

Sample Program

Now, with the theory cleared up, we can move on to our first program involving classes.

Program 3.1

// A simple implementation of a Fraction
#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
	NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

	int numerator = 2;
	int denominator = 5;
	NSLog(@"This is a fraction with a value of %d/%d", numerator, denominator);

	[pool drain];
	return 0;
}

The output is

This is a fraction with a value of 2/5

In Program 3.1, we defined a numerator and denominator variable, and set them to integer values. We then printed the result of this simple fraction.

This program is short and condensed, and seems to be pretty simple—and it is. But what if we had a hundred fractions that we wanted to store? We’d have to keep track of two hundred variables. And performing operations on those variables would not be any easier.

As a matter of fact, Program 3.1 is not a true object-oriented program. It is still written using C-style code. We have not created any objects in the code; our Fraction object still exists only psychologically.

Program 3.2 presents the exact same functionality, except that we actually define this object in code. We then refer to the object as Fraction throughout the program. Don’t worry if it looks daunting; an explanation follows.

Program 3.2

// A Fraction class
#import <Foundation/Foundation.h>

//———————— Interface ————————

@interface Fraction : NSObject
{
	int numerator;
	int denominator;
}

- (void)showResults;
- (void)setNumerator:(int)n;
- (void)setDenominator:(int)d;

@end

//———————— Implementation ————————

@implementation Fraction

- (void)showResults {
	NSLog(@"This is a fraction with a value of %d/%d", numerator, denominator);
}

- (void)setNumerator:(int)n {
	numerator = n;
}

- (void)setDenominator:(int)d {
	denominator = d;
}

@end

//———————— Main Program ————————

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // Create an instance of Fraction
	Fraction *myFraction;

	myFraction = [[Fraction alloc] init];

	// Set myFraction to 2/5
	[myFraction setNumerator:2];
	[myFraction setDenominator:5];

	// Display the value of myFraction
	[myFraction showResults];

	// Clear up memory
	[myFraction release];

    [pool drain];
    return 0;
}

The output is

This is a fraction with a value of 2/5

Code, Demystified

Program 3.2 is divided into three logical sections. These are known as the interface, implementation, and program section, respectively.

The interface section (delineated by @interface in the code) provides basic program-wide definitions, the implementation section (delineated by @implementation) provides the implementation of the methods, and the program section contains the main code that does something.

The @interface Section

Every class in Objective-C has a single parent, or superclass, from which the class inherits. Next, you have to declare the instance variables that belong to the class. Finally, you declare the methods, or actions, that the class can perform. This is all done in the @interface section, which looks like this:

@interface ClassName : ParentClassName
{
	instanceVariableDeclarations;
}
methodDeclarations
@end

Remember that the compiler ignores spaces, so you may put line breaks and spaces anywhere within the code above.
By convention, the name of a class begins with a capital letter, as in the Fraction class.

You can tell that our Fraction class is a subclass of NSObject, which is the root object that almost everything else inherits from, directly or indirectly. NSObject is declared in the Foundation files which you imported.

Fraction contains two instance variables; both are ints. They are called instance variables because every instance of Fraction creates a new set of these variables, unique to that specific instance. So, FractionA’s numerator is a different variable than FractionB’s numerator. Objective-C automatically keeps track of this for you.

Currently, our Fraction class declares (and later implements) three methods. All are instance methods, as noted by the minus sign () preceding each method name. A class method is delineated by a plus sign (+).

Before we discuss the method declaration, we must remember than an object is considered a “black box.” Ideally, objects encapsulate, or hide, their instance variables, from anything except itself. The object exists as a single entity, and anything that works with the object does not need to know, and should not know, about the object’s inner workings. Therefore, we need to declare specific methods to set the variables, because the program should not directly manipulate them. We also need a separate method to print the values, because, once again, the program should not be able to directly access the variables. This is a fundamental concept in Objective-C, and is observed throughout the libraries and code.

The Fraction class defines three methods, all of which return nothing. The value in parentheses before the method name specifies the return type of the method. void is programming parlance for “nothing,” or nil. Any data type or object can be a return value; a method might return a float, int, or another Fraction, as illustrated by the following methods:

- (int)numerator;
- (float)divide;
- (Fraction *)randomFraction;

The asterisk after the Fraction is used to delineate a pointer; this is not an important concept at the moment. For now, simply remember that anytime you create an instance of a custom object, the name of the instance should be prefixed with an asterisk.

All of these methods must return a value somewhere in the method. Just as main returns 0 at the end, these methods must return an int value, a float value, and another instance of Fraction, respectively. The compiler will complain if you do not return a value.

The latter two methods declared in Fraction return no value, but they do each take an integer value as an argument. In the case of setNumerator:, the name of the argument is n. This name is arbitrary, as long as it follows the standard naming conventions, and is used only within the method to refer to the argument. The argument itself exists only within the method; you cannot access n anywhere else in Fraction.

Arguments are values that are passed into a method, which the method would not be able to obtain otherwise. For example, setNumerator: requires a value to set to the numerator, but it would not receive a custom value without an argument.

Note the syntax of these method declarations. The method name begins with the minus or plus, signifying the method type. This is followed by the return type in parentheses, the name of the method, and a colon. This colon is followed by an argument type in parentheses, a name for the argument, and a semicolon. In a method that takes an argument, the colon is considered part of the method name.

A method can take multiple arguments; they are delineated by a space, a new section of the method name, another colon, an argument type, and name. This will be covered in a later chapter.

The @implementation Section

In the implementation section, you define the code that each method implements. The general appearance of the @implementation section is similar to this:

@implementation ClassName
	methodDefinitions;
@end

In the implementation of Fraction, you also declare the method types and names, followed by an open brace. Between the opening and closing brace, you insert the code that the method will execute when it is called.

The program Section

In this program, the main program section contains the all-important main function that is the first thing to be executed.

In Program 3.2, we first create an instance of Fraction:

 Fraction *myFraction;

Again, don’t worry about the asterisk. From a technical perspective, it says that myFraction is a pointer to a Fraction.

Now that you’ve created an object “container” to store a Fraction, you have to actually create a Fraction to fill that space. You do that in the next line,

myFraction = [[Fraction alloc] init];

In this line, you call the alloc method on Fraction; alloc is a class method. Although you never defined the alloc method, it is inherited from NSObject. Inheritance will be discussed in a later lesson.

After you allocate a Fraction, you have to actually initialize it. Therefore, you pass the value of the alloc method to NSObject’s init method, and the resulting Fraction is assigned to myFraction. This nested sequence of messages is used almost every time you create a new object.

Next, you set the values of myFraction to 2 and 5 by using the proper method calls.

[myFraction setNumerator:2];
[myFraction setDenominator:5];

You pass the integers 2 and 5 as arguments to the method. The method then takes these arguments and sets the numerator and denominator variables of myFraction.

You then call myFraction’s showResults method to print the value of the fraction.

We then free up the memory that Fraction uses by calling the release method:

[myFraction release];

This is a critical part of programming for iOS. Whenever you create a new object through alloc/init, you have to release it—in other words, every alloc/init call is balanced by a release call. Otherwise, the now-useless object will still sit in memory, taking up space; in a memory-constrained environment such as the iPhone, you want to use as little memory as you can.

Messaging Receivers

In every message, you send the message to a specific receiver—the object or class which implements the method. However, instance methods are sent to specific instances, so that the same method call to two different Fractions will result in two Fractions with potentially different values. For example,

[fraction1 setNumerator:2];
[fraction1 setDenominator:5];

[fraction2 setNumerator:3];
[fraction2 setDenominator:4];

Will result in two separate fractions, one with a value of 2/5 and another with a value of 3/4. Although you are calling the same methods on both, they are received by different objects, with their own set of numerator and denominator variables.

Data Encapsulation

As mentioned before, you shouldn’t directly access the instance variables of an object. Data encapsulation provides some security against people working with the class, to prevent them from tinkering with the inner workings. So in the main function, how would you access these values?

Generally, you access these values by writing special methods, known as getter methods (the set methods that we’ve already wrote for Fraction are known as setter methods; they are generally written in pairs).

The getter methods for the Fraction class look something like this:

- (int)numerator;
- (int)denominator;

- (int)numerator {
	return numerator;
}
- (int)denominator {
	return denominator;
}

It doesn’t matter if the method name is the same as the variable name; in fact, it is common practice.

Program 3.3 contains the main function that tests these new methods.

Program 3.3

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // Create an instance of Fraction
	Fraction *myFraction;

	myFraction = [[Fraction alloc] init];

	// Set myFraction to 2/5
	[myFraction setNumerator:2];
	[myFraction setDenominator:5];

	// Display the value of myFraction
	NSLog (@"The value of myFraction is %d/%d", [myFraction numerator], [myFraction denominator]);

	// Clear up memory
	[myFraction release];    [pool drain];
    return 0;
}

The output is

The value of myFraction is 2/5

Objective-C 2.0 introduces properties, which provide an easy way to create getter and setter methods. This will also be covered in a later lesson.

Shameless Self-Promotion


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Programming for iOS Programming for iOS Programming for iOS Programming for iOS Programming for iOS

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Format Specifiers


Format specifiers are the percent character, followed by a letter, such as %d or %f that tell NSLog or printf() to print the value or result of a variable, value, and/or expression.

Here is a list of C format specifiers:

Format Specifiers in C

Just think of them as types of placeholders, each designed to represent a different value.

Escape Sequences


NSLog, and its corresponding C function printf(), use escape sequences to print certain characters that are “invisible.” For iOS, include:

Sequence Meaning
\b Backspace
\f Form Feed
\n Newline
\t Horizontal Tab
\v Vertical Tab
\\ Backslash
\’ Single Quote
\” Double Quote
\? Question Mark

There are others, such as \a for Alert, but they are rarely used, and don’t make an effect.

Objective-C Lesson 2: Basic Variables


In almost all programs, values may change; these values are stored in variables, which, as the name implies, are designed to have varying contents as the program is used. We’ll dive right in with a sample program.

Program 2.1

// Introducing variables

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

	int i;
	i = 1;
	NSLog (@"The variable i has a value of %d", i);

	i = 2;
	NSLog (@"The variable i now has a value of %d", i);

    [pool drain];
    return 0;
}

The output of this simple program is as follows:

The variable i has a value of 1
The variable i has a value of 2

Code, Demystified

Program 2.1 contains the same “boilerplate” code in the template that Xcode provides for you. It is still not necessary for now, so explanation will be deferred.
The first “real” line of the program introduces the concept of a variable.

int i;

This variable is declared to be of type int and with the name i. Before we talk about the variable type, we have to take about the choice of the variable’s name.

Variable Names

In C, all variable names (in fact, almost all names in general) must be made of lowercase and uppercase letters, digits, and the underscore character (_). The first character of the name must be a letter or underscore. Because Objective-C is a strict superset of C, the same rules apply.

Note that C and Objective-C are case-sensitive. A lowercase letter is not the same as a capital letter. Therefore, fraction, Fraction, FRACTION, and fractioN are considered four completely different names. As a result, programmers often use camel case to declare variable names of multiple words. Examples include aPairOfDice, lotsOfMoney, and someAddress.

Because of the wide variety of options available to you, make sure to choose descriptive variable names, such as taxRate instead of t. You can only thank yourself later on.

Reserved Names

The Objective-C language reservescertain words, so that you cannot use them as a name; in fact, some are reserved by C itself. For a complete list, check out this post.

Data Types

In program 2.1, the variable i was declared to be of type int. This means that i can only hold a whole-number value, without a decimal point—an integer. Integer values are one of four basic data types. The others are floating-point values, characters, and boolean values. Floating-point values are simply numbers that include a fractional portion, represented as a decimal, such as 5.2 and 10.0. Characters are any single character that you can type on your keyboard, such as ‘A’ and ‘@’ and few others. Boolean values are either true or false; in Objective-C they represented as YES and NO.

Beyond the basics, C also defines three “adjectives” to modify the size of variables. These are short, long, and unsigned. An integer declared to be short will take up less memory than a standard int, and a long will take up more space. You can also use the specifier long long to indicate even larger values, but in OS X and iOS the additional long makes no difference. unsigned is used for values that will never be negative; this effectively doubles the maximum (positive) value of a variable without doubling its storage requirement (because every single negative value can now be “mapped” to a larger positive value).

short and long cannot be used in conjunction with each other in the same variable, but unsigned, or its optional counterpart signed, may be used on any variable, short, regular, or long.

Initialization

After declaring a variable, you have to initialize it with a value; otherwise, the variable would be useless. You can do this in a single statement, along with the declaration:

int variable = 1;

Alternatively, you can declare the variable, and then initialize it sometime later (it does not have to be the next statement):

int variable;
// Some code
variable = 1;

Printing Variables

Program 2.1 introduces a few new concepts in the NSLog statements.

NSLog (@"The variable i has a value of %d", i);

The real magic happens with the %d. This is a format specifier, and tells NSLog that it should expect a variable of a certain data type, and to print the value of that variable. In this case, the type %d tells NSLog to expect an integer value, and it does receive i as an argument, which is an integer value. Therefore, NSLog looks for the value of i, and prints the integer value. This explains the output:

The variable i has a value of 1

In the next few lines, we set i to the value 2, and then use NSLog to print it.

Note that the specifier %i is also used to print integer values.

Program 2.2

// Arithmetic operations with variables
#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    int result;
	result = 10 + 20;
	NSLog(@"The sum of 10 and 20 is %i", result);

	result = 25 - 30;
	NSLog(@"The difference between 25 and 30 is %i", result);

	result = 8 * 9;
	NSLog(@"The product of 8 and 9 is %i", result);

	NSLog(@"The quotient of 8 and 5 is %i", (8 / 5));
	NSLog(@"The quotient of 8.0 and 5.0 is %.1f", (8.0 / 5.0));	[pool drain];
    return 0;
}

The output is:

The sum of 10 and 20 is 30
The difference between 25 and 30 is -5
The product of 8 and 9 is 72
The quotient of 8 and 5 is 1
The quotient of 8.0 and 5.0 is 1.6

Program 2.2 presents some interesting situations. To begin, we define an integer value, and name it result. We set result equal to 10 + 20. Then we have the NSLog statement print out the value of the variable result, and we use the integer specifier because the result is an integer. The next few lines does the same, but note that negative values are supported. Multiplication works as expected, but the last two NSLogs deserve some attention.

The first thing to note is that you can specify an expression as an argument to NSLog, not simply a variable. As long as the result is of the proper type, NSLog doesn’t care whether you specify an expression or a variable.

Next, note that the first division results in a value of 1. Obviously, 8 ÷ 5 ≠ 1, but this is not a mathematical error. Remember that the format specifier %i is responsible for printing integers. Also note that the values 8 and 5 are both integers. This is an example of integer division. Through integer division, only the whole-number portion of the result is reported—any fractional portions are discarded.

To get the accurate result, we have to use floating-point numbers. These numbers are only syntactically different from ints in that they include a .0 portion at the end—as in 8.0 and 5.0. To print a floating-point value, we use the format specifier %f. The .1 portion of the specifier tells NSLog to only print the first decimal point, and disregard any others that may follow. Here, we can see that the result is indeed 1.6.

Reserved Keywords


Objective-C reserves certain words, so that you can’t, or shouldn’t, use them in your own code. At best, you’ll get a compiler warning or error; at worst, you’ll get a nasty bug that you won’t discover until millions of people are already using your app. Here is a list of reserved keywords, to help you avoid that pitfall.

Words You Can’t Use Words You Shouldn’t Use
auto BOOL
break Class
case bycopy
char byref
const id
continue IMP
default in
do inout
double nil
else NO
enum NULL
extern oneway
float out
for Protocol
goto SEL
if self
inline super
int YES
long @interface
register @end
restrict @implementation
return @protocol
short @class
signed @public
sizeof @protected
static @private
struct @property
switch @try
typedef @throw
union @catch()
unsigned @finally
void @synthesize
volatile @dynamic
while @selector
_Bool atomic
_Complex nonatomic
_Imaginery retain

Extending “Hello, World!”


Now that you understand how the “Hello World” program works, let’s add the phrase “I’m ready to program in Objective-C!” to the output. Simply add another NSLog statement to the program. The solution is below, but try to figure out the problem for yourself.


/* Add the phrase
"I'm ready to program in Objective-C!"
to the output */

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        NSLog (@"Hello, World!");
        NSLog (@"I'm ready to program in Objective-C!");

        [pool drain];
        return 0;
}

Build and Run the program. The output should be the same as below:

Hello, World!
I'm ready to program in Objective-C!

The NSLog calls automatically create a paragraph break, or a new line, between subsequent calls. It does this automatically, but you can also force a new line in the middle of an NSLog call by using \n anywhere in the line of output. Therefore,

NSLog (@"Hello, World! \nI'm ready to program in Objective-C!");

Produces the same output as above.

Objective-C Lesson 1: Hello World!


Note: Click on any image to see a larger version.

Since Dennis Ritchie demoed the C programming language, it has been conventional to begin a programming course with a program that writes the words “Hello world!” to the computer screen. Diving right in, the code is shown below.

Program 1.1

// First program example

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        NSLog (@"Hello, World!");
        [pool drain];
        return 0;
}

Using Xcode

So what does all this gibberish mean? Before we can get to that, we’ll run the program first.

Download a copy of Xcode, if you don’t have it already, and install it. Navigation to your boot drive (usually Macintosh HD), and the folder called Developer. Inside, there will be a folder called Applications. Xcode, and some other programs you will be using, is in there.

Xcode is located at /Developer/Applications

Location of Xcode.app

Drag it to your dock, or create an alias for it in your Applications folder. Launch Xcode. The first thing you’ll see is the launch window:

Xcode Launch Window (With Highlighting)

Xcode Launch Window (New Project Highligted)

Click on “Create a new Xcode project,” or select File > New Project (⇧⌘N). The button is highlighted in the image above. You will be greeted by the following window:

Xcode's New Image Window

Xcode's New Image Window

Select Application under Mac OS X on the left, and choose Command Line Tool on the right. Make sure the Type drop-down menu in the middle is set to Foundation. Click Choose…

Name your project, and save it somewhere on your hard drive. In a few moments, you’ll see Xcode’s main interface:

Xcode

Xcode's Main Window

We’ll take more about the parts of the window in later sections. Click on the file called Hello World.m, or something similar. This should bring up the editor window. Replace the code in the editor view with the code above, so that it looks like this:

Hello, World entered in Xcode

The Code in Xcode's Editor

Don’t worry about all the colors—we’ll discuss that later. When you have entered the code, save the file, and click the “Build and Run” or “Build and Go” button in the toolbar.

If you’ve made a mistake when typing it in, the toolbar at the bottom of Xcode’s window will change, and a message bubble will appear:

Xcode tells you when you've made a mistake

Xcode Tells You About Errors

Go back, and check the code.

When everything is working, you’ll see yet another window:

Xcode Displays the Result of the Code in the Console

Xcode Displays the Result of the Program

The output is as follows:

2010-09-03 10:47:32.414 Hello World[25379:a0f] Hello, World!

The beginning of the line shows the date and time, followed by the name of the program, and bunch of gibberish. But at the end of the line is the text that you expected to see: The program has written the words “Hello, World!” onto the screen.

Throughout the rest of these tutorials, the text that appears before that actual text that is outputted will be omitted for brevity.

Code, Demystified

Now that you’ve created a working program, let’s talk about how it works.

In Objective-C, a capital and lowercase letter mean different things. So, main, Main, and mAin are three completely different things, and cannot be used interchangeably. Also note that white space—spaces, tabs, and blank lines are ignored. You can use this to format your code properly. Take advantage of this freedom to properly format your code.

The first line of the program is a comment:

// First program example

Comments exist solely for the benefit of the programmer. Comments are important, because they allow the developer to explain a thought process. They help demystify the code, and make documenting the code sometime down the road easier. Comments help during the debugging process, or when you revisit your code some time later. It also helps other people who may use your code understand why things are the way the are. The compiler ignores comments, so you can type anything into a comment.

There are two types of comments in Objective-C. One is shown above, in which every line of the comment is preceded by two slashes. This type of comment ends at the end of the line.

A comment can also be written across multiple lines, with the following syntax:

/*
A multi-line comment can be inserted
by using a slash
and then an asterisk.
The comment can be as long as you want,
but make sure you close it
with an asterisk, then a slash. */

Which style you choose is a matter of preference, or as the situation dictates.

The next line of the program imports, or brings into your project, a file called Foundation.h:

#import <Foundation/Foundation.h>

The file in question is a system file, and that is why the name is enclosed in the brackets < and >. If you were importing a local file , you would enclose the file name in double quotes “ and ”.  This file is imported because code later in the file requires information that is contained in this file;  you are telling the compiler to look up the information in that file as necessary.

The following line declares a function called main:

int main (int argc, const char * argv[])

main is a function that is where every C or C-based program begins. It is a reserved name, which means that you can’t have a function named main. The word int that precedes main is a declaration of the return type of the function. These topics will be discussed in further chapters.

After identifying main to the compiler, you tell the system what to do when it is called. These statements are enclosed in the braces { and } that surround the next few lines. Every opening brace has to be matched with a closing brace.

The first statement in main is as follows:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

This involves memory management, which is a topic thoroughly discussed in upcoming chapters.

The next line calls NSLog, which, along with the NSAutoreleasePool from above, is a function brought in by Foundation.h, which you imported.

NSLog (@"Hello, World!");

This line tells the function NSLog, which is designed to output text, to print the characters Hello, World! on the screen.

The leading @ in front of the string signifies to the compiler that this is an NSString (another object brought in from the Foundation.h file), not a C-style string. For the most part, you will be using NSStrings, rather than C-style strings.

All statements, or lines that indicate some action, must be terminated with a semicolon ; just like regular sentences are terminated with a period.

The next line is part of the NSAutoreleasePool, and its purpose and meaning will be discussed in a later chapter.
The final line tells the main method to return the value 0. Remember that the int the preceded main tells the system that this function will return a value. This value is 0. By convention, a return value of zero indicates that the function was successful.

return 0;

This line is followed by a closing brace, signifying the end of the main function.

If you look back at the window that displays the output of your program, you should see the line

Program exited with status value:0.

This is an indication of the return value of main.

Congratulations, you have created your first program in Objective-C!

How Programming Languages Work—A Brief Overview


Warning: Extremely technical information follows. Feel free to skip this section for now, but make sure you do read it sometime. It’s important enough to know, but not vital for you to begin programming.

A computer only understands a string of commands in a very basic format—machine language. At the lowest level, a computer can only process 0s and 1s—off, or on. In effect, computer is just a huge collection of billions of switches, rapidly turning on and off. A typical command that the computer understands may look like this:

00101011 11110111 10000100 01010110 00000110 10001001

Of course, having to program like this would not be very easy, and it is easy to loose track of what you’re doing. To fix this, chip makers created what is called assembly language—a language that chips understand, and will get translated to machine language, independently of the programmer. A typical command in assembly might look like this:

l_objc_msgSend_fixup_alloc:
.quad	_objc_msgSend_fixup
.ascii "long int"
.byte	0x4
.byte	0x8
.byte	0x7
.byte	0x2
.byte	0x1
.byte	0x6
.ascii "char"

Better than machine language, but still not too useful. In addition, the programmer is still interacting directly with the hardware, which increases the chances of mistakes.

Both machine language and assembly are specific to the hardware. Different hardware involves different machine and assembly code.

Higher-level languages were invented to solve of these problems. The “height” of these languages varies, but they all involve English-like syntax, and are hardware-independent. Objective-C is one such language.

The majority of these languages are passed through a compiler—a program that converts the code into assembly. Compilers are usually provided with a system; Xcode, Apple’s native development program, includes gcc, one of the most prominent compilers, as well as LLVM, an Apple-developed compiler that offers many benefits over gcc, all of which are beyond the scope of this book.

Higher-level languages allow programmers to focus on their task, and not on writing obscure code.

For anyone who’s interested in learning more, an even more technical and low-level approach will be coming. When? I don’t know.

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