Q:

Loop the code in @implementation section

I have written the program, which chooses a random element from the array.  
Once I press the button "Start", how do I loop that code? 
I would like, that once a button "Start" is pressed, a new element from the array is chosen and written in the text field every 5 seconds.... Thanks for the answer.
@implementation MARandom

- (IBAction)Start:(id)sender {

    NSArray *tones;

    tones = [NSArray arrayWithObjects: @"F#0", @"Gb0", @"G0", @"G#0",@"Ab0",@"A0",@"A#0",@"Bb0",@"B0",
            @"C1",@"C#1",@"Db1",@"D1",@"D#1",@"Eb1",@"E1",@"F1",@"F#1",@"Gb1",@"G1",@"G#1",@"Ab1",@"A1",@"A#1",@"Bb1",@"B1",
            @"C2",@"C#2",@"Db2",@"D2",@"D#2",@"Eb2",@"E2",@"F2",@"F#2",@"Gb2",@"G2",@"G#2",@"Ab2",@"A2",@"A#2",@"Bb2",@"B2",
            @"C3",@"C#3",@"Db3",@"D3",@"D#3",@"Eb3",nil];

    i= (arc4random() % 48);

        NSString *Tone; 
        Tone = [tones objectAtIndex: i];

        [TextField setStringValue:(NSString *)Tone];
    }

A:

Please try this:
- (IBAction)Start:(id)sender {
[NSTimer scheduledTimerWithTimeInterval:5.0
                                 target:self
                               selector:@selector(updateTextFieldWithRandomNumber)
                               userInfo:nil
                                repeats:YES];
}

-(void)updateTextFieldWithRandomNumber{
NSArray *tones;

tones = [NSArray arrayWithObjects: @"F#0", @"Gb0", @"G0", @"G#0",@"Ab0",@"A0",@"A#0",@"Bb0",@"B0",
         @"C1",@"C#1",@"Db1",@"D1",@"D#1",@"Eb1",@"E1",@"F1",@"F#1",@"Gb1",@"G1",@"G#1",@"Ab1",@"A1",@"A#1",@"Bb1",@"B1",
         @"C2",@"C#2",@"Db2",@"D2",@"D#2",@"Eb2",@"E2",@"F2",@"F#2",@"Gb2",@"G2",@"G#2",@"Ab2",@"A2",@"A#2",@"Bb2",@"B2",
         @"C3",@"C#3",@"Db3",@"D3",@"D#3",@"Eb3",nil];

i= (arc4random() % 48);

NSString *Tone; 
Tone = [tones objectAtIndex: i];

[TextField setStringValue:(NSString *)Tone];
}

and also consider preparing the array at one place rather than the code that will be repeatedly called, perhaps at viewDidLoad.
Note: untested code, but should work.

