In this blog, we’ll see how to use a UIPicker control to implement a simple lottery application. The user will choose a two digit number using a picker control, then press a button to see if their number matches a random number generated by the app. Let’s see how it works!

Start Xcode, choose “Create a new Xcode project,” then select the Single View Application template. Name the project “PickerLotto,” and choose options as shown:

Click Next, choose a location to save the project, and Click Create.

Open ViewController.h and make the following changes:

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
<UIPickerViewDataSource, UIPickerViewDelegate>

@property (nonatomic, weak) IBOutlet UILabel *prompt;
@property (nonatomic, weak) IBOutlet UIPickerView *picker;

- (IBAction)checkEntry:(UIButton *)sender;

@end

As you can see, we’ve added two properties: a UILabel control named prompt, and a UIPickerView control called picker. We’ve also added a single action method called checkEntry, which will check the value in the picker view against a random number. The ViewController class has also been made to adopt two protocols: UIPickerViewDataSource and UIPickerViewDelegate.

Now open ViewController.xib, and drag controls to the main view as shown below:

Wire up the label to the prompt outlet and the picker view to the picker outlet. The button is wired to the checkEntry action method. Finally, right click the picker view and drag from both the delegate and datasource to File’s Owner. Save your work at this point. (The File’s Owner property sheet is shown here):

Now it’s time to write the implementation for the app. Open ViewController.m, and make the changes shown in the listing below:

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

@synthesize prompt, picker;

#pragma mark – PickerView delegate methods

- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView
{
    return 2;
}

- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component
{
    return 10;
}

- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component
{
    return [NSString stringWithFormat:@"%d", row];
}

#pragma mark – Action methods

- (IBAction)checkEntry:(UIButton *)sender
{
    int winningNumber = (arc4random() % 100) ;
    int chosenNumber;
    int tens = [self.picker selectedRowInComponent:0] * 10;
    int ones = [self.picker selectedRowInComponent:1];
    chosenNumber = tens + ones;
    if (chosenNumber == winningNumber) {
        self.prompt.text = @"You Win!";
    } else {
        self.prompt.text = [NSString stringWithFormat:@"Sorry, the winning number was: %d", winningNumber];
    }
}

- (void)viewDidLoad
{
    [super viewDidLoad];
        // Do any additional setup after loading the view, typically from a nib.
    self.prompt.text = @"Choose a number…";
}

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

@end

After synthesizing the properties, we proceed to implement three delegate methods of the UIPickerView control: numberOfComponentsInPickerView:, numberOfRowsInComponent:, and pickerView: titleForRow: forComponent:. Since each component of the picker view only needs to contain the digits 0 through 9, these implementations are every straightforward. NumberOfComponentsInPickerView returns 2 (because there are two “dials” in the picker, one for each digit), numberOfRowsInComponent returns 10 (for all components) because each component will list 10 digits, and pickerView: titleForRow: forComponent: simply formats each row number in each component as a string before returning it.

In the checkEntry: method, we first generate the random number by calling arc4random(), which generates a random int. We mod this with 100, which will return an int in the range 0..99. Next, we get the value of the selected row in each component of the picker into two int variables named tens and ones. We must multiply the tens row by 10 before adding the two together and placing the result in chosenNumber. Finally, we compare chosenNumber with winningNumber and display the result in the prompt control.

We could have set up a property to hold the random number in the ViewController class, and just done the comparison with the picker values in checkEntry:. But we would have had to initialize the random number property in ViewDidLoad:, and reset it somewhere after the comparison was made. In this implementation, we take advantage of the fact that it doesn’t matter when the random number is generated, as long as the code is “honest” (doesn’t influence the choice of random number depending on the values in the picker). Implementing the random number generation this way also has another advantage: since winningNumber is entirely within the scope of checkEntry, we are certain to get a new winning number each time the button is touched.