BMICalculatorViewController.m

#import "BMICalculatorViewController.h"

@interface BMICalculatorViewController ()

@property (weak, nonatomic) IBOutlet UITextField *height_TextField;
@property (weak, nonatomic) IBOutlet UITextField *weight_TextField;
@property (weak, nonatomic) IBOutlet UILabel *resultLabel;
- (IBAction)calculateBMI:(id)sender;

@end

@implementation BMICalculatorViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // Gesture for tap anywhere to dissmiss keyboard.
    UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(dismissKeyboard)];
    [self.view addGestureRecognizer:tap];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)dismissKeyboard
{ // Method for tap anywhere to dismiss keyboard
    [_height_TextField resignFirstResponder];
    [_weight_TextField resignFirstResponder];
}

- (IBAction)calculateBMI:(id)sender {
    // BMI formula: BMI = ( Weight(Kg) / ( Height(M) x Height(M) ) )
    double height = [self.height_TextField.text doubleValue];
    double weight = [self.weight_TextField.text doubleValue];

    NSString *bmi = [NSString stringWithFormat:@"%.2lf", (weight/(height*height))];
    NSString *resultString = [NSString stringWithFormat:@"Your BMI is:\n%@", bmi];

    // Declaring UIAlertView *resultAlert
    UIAlertView *resultAlert = [[UIAlertView alloc] initWithTitle:@"Invalid"
                                                          message:nil
                                                         delegate:self
                                                cancelButtonTitle:@"Okay"
                                                otherButtonTitles:nil];

    if ([self.weight_TextField.text length] == 0 && [self.height_TextField.text length] == 0)
    {
        resultString = @"Please fill in your Height and Weight!";
        resultAlert.message = resultString; // Update resultString with error message.
        [resultAlert show];
    }
    else if ([self.height_TextField.text length] == 0)
    {
        resultString = @"Please fill in your Height!";
        resultAlert.message = resultString;
        [resultAlert show];


    }
    else if ([self.weight_TextField.text length] == 0)
    {
        resultString = @"Please fill in your Weight!";
        resultAlert.message = resultString;
        [resultAlert show];
    }
    else
    {
        self.resultLabel.text = resultString;

        // Clear text fields after results are showned.
        self.height_TextField.text = nil;
        self.weight_TextField.text = nil;
    }
}

- (BOOL)textFieldShouldReturn:(UITextField *)textField
{
    [textField resignFirstResponder];
    return YES;
}

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{

}

@end