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Course | Learn Objective-C by Learning C : Program Basics

Updated on January 14, 2012

Someone once asked me what was the best book or course for learning Objective-C. The answer that immediately came to mind was: Learn C. The reason is that Objective-C superset of the C programming language. In fact within a Objective-C source file (header or implementation) you can mix and match C code along with Objective-C. Of course in addition to C, Objective-C also includes elements of Smalltalk.

The purpose of this tutorial is threefold:

  1. To help you learn Objective-C, by
  2. Teaching C programming in an Objective-C context
  3. Use the C programming language within the XCode context


The first lesson is on using C with Objective-C in Xcode is on the basic program structure. C programs are organized into header files and source files. You define functions in the header files and you implement the functions and other code in the source files. These files are merged together through a process called preprocessing and linking and compiling.

Objective-C also uses header files where you define your variables and methods and instead of source files, Objective-C has implementation files. However the process is the same and you can C code in the Objective-C header and implementation files.

A C program usually follows this architecture:

Import libraries using the #include keyword followed by either the name of the library. For system libraries, you would place the name of the library header file between angle brackets, while user defined header files, i.e. the ones that you create in your program, would be placed in between double quotes.



A C library:

#include <stdio.h>

or

#include “myLib.h”

for Objective-C

#import <stdio.h>

or

#import <CoreFoundation/CoreFoundation.h>

or

#import “myLib”

next, you would declare and implement any functions (C code) or in the case of Objective-C, you would implement methods. These are either implemented outside the main program code block or in a separate source file (C) or implementation file (Objective-C). A more detailed tutorial functions and methods will be given in a next installment.

Finally, the main part of the program, which is called main. It is called the same in both languages. However in C, the main function can have no parameters or two parameters, like so:



main()
{
printf(“Hello World”);

}

or

main(int argc, const char * argv[])
{
printf(“Hello World”);

}


The second form is to be able pass in parameters arguments. The first parameter argc is the number of parameters you will supply; the second parameter or argument is the list of values. It is import to note that the number of values must match the number specified in the first parameter.

Also, if you specify that you are providing 2 parameter values. To use them, you reference them numerically as 0 and 1 as with any other array, the index starts at 0.

So if you program was called for example sake: exampleProgram, you would provide the parameters like this:



exampleProgram 1 “kevin”



and the program would look like this:


main(int argc, const char * argv[])
{
printf(“My name is : %s”, argv(0));
return 0;
}

The main function also returns a numerical value. In our example, 0 means successful, like most other programming languages.

For Objective-C, the syntax is somewhat different and you almost never have to edit the main program when working with a cocoa program. In Objective-C, the main program is located in the main.m file. If you open this file, which is located in the Supporting files sub folder in your project, you will see a somewhat different version which would something like this:



#import <UIKit/UIKit.h>
#import “NameOfProgramDelegate.h”

int main(int argc, char *argv[])
{
@autoreleasepool {

return UIApplicationMain(argc, argv, nil,

NSStringClass([NameOfProgramDelegate class]));

}


}


To reference the arguments, you could do it the same way as with a C program or you can do this way using the cocoa framework:


NSArray *arguments = [[NSProcessInfo processInfo] arguments];

The last piece of the program syntax, is the semi-colon ;. The semi-colon, ;, indicates the end of a statement or expression in C and likewise in Objective-C.

Test It

Now that we have got the basic rhetoric out the way, it is time to try out some code examples. Open Xcode and select to create a New Application. Under the Mac OS X Application, choose the CommandLine Application. On the next page, you will notice that the Type is automatically set to “C”. All you need to do is provide a name like HelloWorldFromC.


Select a CommandLine Project
Select a CommandLine Project
Name the project and keep the type as a C project
Name the project and keep the type as a C project
Main Project Configuration Page
Main Project Configuration Page

To add some code open the main.c file and you will notice that the template already added the Hello World print out for us. All you need to to do is click on the Run button and once the compiling and linking is complete, you will notice the Hello World text appear in the Console at the bottom of the screen.

Code already included
Code already included
Console Output
Console Output

Optionally but all good programmers do it, is add comments to your code. In C and Objective-C you can add comments two ways:

On a single line



//This is a comment on a single line



On multiple lines


/*
This is a multi line comment. You cannot add executable code between the opening and closing delimiters.
*/

In Summary

That is it for the first tutorial. It was pretty simple I agree but you got the overall look and feel of writing C programs and Objective-C programs. The next tutorials will focus on variables, constants and types.

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