How do I add .m files to Xcode without conflicts - objective-c

Question

I'm currently learning Objective-C and working my way through a book which contains lots of exercises. At the beginning of the book I set up an OS X project command line project and have been working in the main.m file, block commenting prior exercises, but it's becoming tiresome working in such a long file, constantly commenting and un-commenting the exercise code I want to build. I'd like to create separate files in the Navigator for each exercise so they are easy to find. How would I go about doing this without causing issues with the file in the Products folder?
I'm currently getting this red alert when I try to build after adding a new .m file:
Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

This might not be a direct answer to the question (multiple files...) and a somewhat unorthodox approach to it, but it is a fun and convenient way to achieve what you describe by utilizing unit testing.
I understand that you are just starting to learn the language and the tools so I will make it as easy as possible. Let's get started!
1. Add a test target to your project
You just select the project in the navigator, and press the + button under 'targets' to add a new one.
select OS X > Other > Cocoa Testing Bundle and click 'Next'
add a product name (something like 'Exercises' would make sense in your case) and click 'Finish'
Now you should be able to see your new target in the list
2. Configuring the test target
Click and hold on the 'Run' button, select 'Test' from the popup menu
Now Xcode will ask you if you'd like to configure the project for testing. Click 'Edit Scheme...'
Select 'Test' from the left and click on the + button
select the test target we have created in previous step and click 'Close'
3. The Fun part
Now if you click on the 'Test Navigator' you'll see a list of your tests (your 'exercises'). Adding a test is as simple as adding a method starting with test - for example -(void)test_Exercise_x.
Furthermore you will now be able to verify that your answer to each exercise is what should be by using assertions and selectively run a test (exercise) on its own by clicking on the icon in the left of each test (the circled one in the screenshot) or of course, all of them by clicking on the 'Test' button or selecting Product > Test from the menu

Answer 2

Objective-C, C and C++ programs must include precisely one definition of main().
I expect it would work for you do something like:
int main( int argc, char *argv[] )
{
printf( "Solution to exercise #%d\n", getExerciseNumber() );
mySolution();
return 0;
}
you could reuse this file in each of the Xcode projects for your exercises. Alternatively you could have just one project that contained multiple targets, one for each exercise.
Does that help? I'm not completely clear that I understand your question.

Answer 3

Adding multiple files to my Xcode project looks like it's beyond my current abilities. I'll try and attempt to break the project down at a later date, when I'm more confident with Objective-C and Xcode. Thanks for your comments and help.