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I have an iPhone application which gets a json string from a server and parses it. It contains some data e.g. school timetable information. The data is stored in an nsdictionary and then passed into a nsmuttablearray. e.g.

Could someone please point me in the right direction/tell me how i can access the different 'roots' of the array? I want to store the elements that have the field 'Day' as "Monday" (or maybe u can tell me how i can sort/do a search though them). Been spending quite some time trying to find some information on this (how to access/alter nested values) but no luck =/. thanks..

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Your best bet for accessing all of the Dictionary objects with Day = Monday would be to use a Predicate with a Key Path to what you're after...

NSPredicate* predicate = [NSPredicate predicateWithFormat:@"Day == 'Monday'"];
NSArray* mondayCourses = [rows filteredArrayUsingPredicate:predicate];

You can make the predicate dynamic too.

NSPredicate* predicate = [NSPredicate predicateWithFormat:@"Day == %@", dayOfTheWeek];

where dayOfTheWeek might be an NSString that holds the current day of the week in a loop etc.

If you want to sort the array, you can do so using Sort Descriptors.

NSSortDescriptor* idSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"id" 
                                                                   ascending:YES];
NSSortDescriptor* levelSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"Level" 
                                                                      ascending:YES];
NSArray* sortedMondayRows = [mondayRows sortedArrayUsingDescriptors:[NSArray arrayWithObjects:idSortDescriptor, levelSortDescriptor, nil]];
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3 Comments

Thanks for the reply. In regards to the predicate, when i attempt to pass value, e.g. rows = mondayCourses my program crashes and there isnt anything in console. rows is of nsarray format, u have any ideas?
If I'm looking at your example JSON structure correctly, your NSDictionary "rows" will hold only 1 key/value pair with the key "Array". So you will want to create an array like so: NSArray* courses = [rows objectForKey:@"Array"]; Then you can use my predicate code almost verbatim (just change [rows filteredArrayUsingPredicate:predicate]; to [courses filteredArrayUsingPredicate:predicate];)
You're welcome. I've been elbow deep in this sort of thing for months & it's almost become second nature!
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Check if your JSON is correct in the URL : http://jsonlint.com/. If its correct than read on how to Parse JSON file by googling. Are you using any specific framework for parsing JSON files ?

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the parsing of the json is fine i just dont understand how to manipulate the data once i have it into a muttablearray (what i posted above is inside the array)
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Start out by looking at the Apple documentation for NSArray and NSDictionary. In this case, it looks like the array (an NSArray) contains a number of NSDictionaries.

The data is stored in an nsdictionary and then passed into a nsmuttablearray.

First, I must note that variables in Objective-C are just pointers. When you have a line like this:

NSMutableArray *rows;

You are only saying, “I expect this variable to point to an NSMutableArray.” It is pretty much equivalent to,

id rows;

…and you will not be stopped, at runtime, from putting a regular array in a variable which was declared for mutable arrays (or putting a string in a variable which was declared for a dictionary!). It will work! But your program will crash when you try to do mutable things to a non-mutable array, or dictionary things to a string.

Declaring it as a pointer to an NSMutableArray, instead of id, has a few advantages:

  • It lets XCode help you out by autocompleting method names
  • It lets the compiler warn you when you try to call methods that NSMutableArray doesn’t have

So, even if your variable is declared as NSMutableArray *rows, on this line:

rows = [[dict objectForKey:@"courses"] retain];

…if the “courses” object was not mutable, what you have in rows is not either. Instead, you can do this:

rows = [[dict objectForKey:@"courses"] mutableCopy];

…to get your own NSMutableArray copy of the array (which you have to release, just as if you retained the original).

You explore the array and dictionaries with standard Cocoa code. Here would be one way to get all of the “Monday” courses:

NSMutableArray *mondayCourses = [NSMutableArray array];

for (NSDictionary* course in rows){
    if([[course objectForKey:@"Day"] isEqual:@"Monday"]){
        [mondayCourses addObject:course];
    }
}

(Warning, I have not run this code.)

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