iOS Testing objective-c

//
//  main.m
//  test3
//
//  Created by Imran Ganiy on 7/8/12.
//  Copyright (c) 2012 __MyCompanyName__. All rights reserved.
//

#import <UIKit/UIKit.h>

#import "AppDelegate.h"

int main(int argc, char *argv[])
{
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
        
        NSLog(@"Hi.");
        return 0;    
        
    }
}[COLOR="#808080"]

----------

[/COLOR]Okay I deleted all the other code and just kept 

int main()
{
    NSLog(@"Hi.");
    return 0;
}

but now the app just immediately closes after I run it.[COLOR="#808080"]

----------

[/COLOR]I copy and pasted this code from the guide and the app doesn't run but it also says no issues.  I put it in main.m

#import <Foundation/Foundation.h>
float circleArea(float theRadius);
float rectangleArea(float width, float height);
int main()
{
    float pictureWidth, pictureHeight, pictureSurfaceArea,
    circleRadius, circleSurfaceArea;
    pictureWidth  = 8.0;
    pictureHeight = 4.5;
    circleRadius  = 5.0;
    pictureSurfaceArea = rectangleArea(pictureWidth, pictureHeight);
    circleSurfaceArea = circleArea(circleRadius);
    NSLog(@"Area of circle: %10.2f.", circleSurfaceArea);
    NSLog(@"Area of picture: %f. ", pictureSurfaceArea);
    return 0;
}
float circleArea(float theRadius)               // first custom function
{
    float theArea;
    theArea = 3.1416 * theRadius * theRadius;
    return theArea;
}
float rectangleArea(float width, float height) // second custom function
{
    return width*height;
}

#import <UIKit/UIKit.h>
#import "AppDelegate.h"
#import <Foundation/Foundation.h>

float circleArea(float theRadius);
float rectangleArea(float width, float height);

int main(int argc, char *argv[])
{
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
       
        float pictureWidth, pictureHeight, pictureSurfaceArea,
        circleRadius, circleSurfaceArea;
        pictureWidth  = 8.0;
        pictureHeight = 4.5;
        circleRadius  = 5.0;
        pictureSurfaceArea = rectangleArea(pictureWidth, pictureHeight);
        circleSurfaceArea = circleArea(circleRadius);
        NSLog(@"Area of circle: %10.2f.", circleSurfaceArea);
        NSLog(@"Area of picture: %f. ", pictureSurfaceArea);
        return 0;
        
    }
}

#include <stdio.h>

int main()
{
	int aligator;  /*Why does it fail to compile if I try to use float instead of int?*/
	int aligator2;
	
	/* prompts two inputs, then divides the first by the second */
	
	printf( "PLease enter a number: " );
	scanf ("%d", &aligator ); /*what is %d and why does it have to be there along with the variable?*/
	printf( "Now enter the number you wish to divide by: ");
	scanf ("%d", &aligator2 );
	printf( "%d", aligator/aligator2);
	
	/* Just a silly way of testing the if and else function */
	
	if (aligator/aligator2>100) 
	{printf("Number too big");}
	else {printf("Number too small");}
	
	getchar();
	/*Still not exactly sure what getchar does*/
	
	return 0; 
	/*What if I didn't include the return 0?*/
}

#include <stdio.h>

int main()

{
	int age;
	
	printf( "Enter your age: ");
	scanf( "%d", &age );
	
	if (age ! = 100) {
	printf ("You are not 100");
	}
	
	   return 0;
	}

Imrans-Mac-mini:Jasper imranganiy$  gcc elseif.c
elseif.c: In function ‘main’:
elseif.c:11: error: expected ‘)’ before ‘!’ token

#include <stdio.h>

int mult ( int x, int y);

int main()
{
	int x;
	int y;
	
	
	printf( "Please enter two numbers to be multiplied: " );
	scanf( "%d", &x );
	scanf( "%d", &y );
	
	/*There is something wrong with the below line.  How can I fix this?  I want it to check to see if x * y = 25 and then print a message
	if so */
	
	/*from here*/
	if (mult (x, y) = 25) 
	{ 
	printf( "You entered 5 times 5" ); }
	/*to here*/
	
	else {
	
	printf( "The product of your two numbers is %d\n", mult (x, y) );
	
	}
	
	
	getchar ();
	}
	
	int mult (int x, int y)
	{
	    return x * y;
	    }