[cocoa myNotes];

Obviously is a technique that is not heavily used, but it comes handy some times. Methods in objective-C can return up to one argument. Double pointers can help you “return” more than one.

How is this useful,

For example, two methods:

// interface
- (UIImage*)logoForTag:(int)aTag;
- (NSString*)titleForTag:(int)aTag;
 
// implementation
- (UIImage*)logoForTag:(int)aTag
{
    switch (aTag) {
        case 0:
            return [UIImage imageNamed:@"0"];
            break;
        case 1:
            return [UIImage imageNamed:@"1"];
            break;
//..    
        default:
            break;
    }    
    return nil;
}
- (NSString*)titleForTag:(int)aTag
{
    switch (aTag) {
        case 0:
            return [NSString stringWithFormat:@"0"];
            break;
        case 1:
            return [NSString stringWithFormat:@"1"];
            break;
            //..    
        default:
            break;
    }    
    return nil;
}

We passing the same tag and we are doing the same comparison with the switch statement.
It already obvious that things could be different, THIS IS A HINT. you see duplicated code.

So, we want one method that does that:

- (void)setTitle:(NSString*)title andImage:(UIImage*)image forTag:(int)aTag
 
// calling the method
    NSString *string = nil;
    UIImage *image = nil;
 
    [singlePointerMethod setTitle:string andImage:image forTag:0];
 
    NSLog(@"%@,%@",string ,image);
    // NULL , NULL

But wait, this does not work Why? Well, passing a pointer to your function it can only modify the pointer that is pointing to. In this case nil, the objects have not ben allocated yet. We cannot return the change to the caller.

So the solution of double pointers or “pointer to pointer”. Passing a pointer to a pointer to the function we can modify the pointer to point to another object. This allows us to pass a pointer to a function and modify it.

// interface
- (void)setTitle:(NSString**)title andImage:(UIImage**)image forTag:(int)aTag
 
// implementation
- (void)setTitle:(NSString**)title andImage:(UIImage**)image forTag:(int)aTag
{
    switch (aTag) {
        case 0:
            *image = [UIImage imageNamed:@"0"];
            *title = [NSString stringWithFormat:@"0"];
            break;
        case 1:
            *image = [UIImage imageNamed:@"1"];
            *title = [NSString stringWithFormat:@"1"];
            break;
            //..    
        default:
            break;
    }        
}
 
// usage
    NSString *string = nil;
    UIImage *image = nil;
 
    [singlePointerMethod setTitle:&string andImage:&image forTag:0];
 
    NSLog(@"%@,%@",string ,image);
    // log 0,<UIImage: 0x685dc10>

You can do so much with regular expressions that it becomes one of the most attractive programming tools. I guess you can avoid them (I am not sure how) and extract your information from a text without them, but it will be a painful procedure.

Well this article is not about explaining them, is more like… hmm… Beat it and I will reconsider:

The most common examples where are I use them are:

1. Extracting information from a long piece of text.
2. Validating an input field.

The first one is fairly straight forward, you have a text and you want find something in particular, such as matching a word, a value and so on…

The other one, is the main reason of the article.

You often have a textfield for the user to enter a value. How do you validate the input? You cannot iterate through the characters and it would be very complex. Consider an example where a textfield accepts only postcodes (it is like the MOST sensible paradigm I can give)

Here is the format of the british postcodes:
A9 9AA, A99 9AA, AA9 9AA, AA99 9AA, A9A 9AA, AA9A 9AA

How will you do that? If you can do it without regular expressions I would be amazed… I have not checked the processing time, but I am happy to do so if you think you can do it faster.

1. Yeah clearly the first one needs to be a letter
2. We also know that it ends with a number followed by two letters. Cool. (9AA)

What about the rest:

1. Then it could be a letter or a number (AA9 9AA , A9 9AA)
2. Then it could be a space, or a letter or a number (A9_9AA, A9A 9AA , AA99 9AA)
3. Then it could be a space, or a letter or a number (AA9_9AA , AA9A 9AA , AA99 9AA)
4. Then it could be a space (AA9A_9AA)

Think about it so many combinations.

The following expression does it for you…

NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"^([A-Z])([A-Z]|[0-9])?([0-9]|[A-Z])?([0-9]|[A-Z])([\\s]*)[0-9][A-Z][A-Z]$"
    options:NSRegularExpressionSearch | NSRegularExpressionCaseInsensitive
       error:nil];
 
// search for matches
NSUInteger numberOfMatches = [regex numberOfMatchesInString:[textField text]
       options:0 range:NSMakeRange(0, [[textField text] length])];
 
// if it's a postcode enable the button
// else disable it
if (numberOfMatches>0)    
{
// found a match do something
// maybe enable the done button
}
else
{               
// did not found a match do something else
// maybe disable the done button
}

So much power... formal definitions, seriously you cannot beat that…