Question

NSDictionary *story = [stories objectAtIndex: indexPath.row];
cell.text=[NSString stringwithFormat:[story objectForKey@"message];

i dont knw what exaclty “message ” contains (what is the meaning of objectForKey@”message“)

EDIT CODE

NSString *key =[appDelegate.books objectAtIndex:indexPath.row];
    //dict y=@"Name";

    NSArray *nameSection = [dict objectForKey:key];


    NSDictionary *story = [nameSection objectAtIndex: indexPath.row];
    cell.text=[NSString stringwithFormat:[story objectForKey:key]];

               NSLog(@"Value Of message: %@", [dict objectForKey:key]);

why my code crashes

Answer 1

If you are more familiar with Java or C# the code is equivalent to something like this:

// Assuming stories is declared as: List<Dictionary<string, string> stories;

Dictionary<string, string> story = stories[indexPath.row];
cell.Text = String.Format(story["message"]);

In Smalltalk-style (and therefore Objective-C too) Object Oriented programming, methods are more like messages to other objects. So a good Objective-C method name should read like an English sentence (Subject-Verb-Object). Because of this working with dictionaries (hash tables) looks like this:

[myDictionary setObject:@"Value" forKey:@"someKey"];
[myDictionary objectForKey:@"someKey"]; // == @"Value"

In Java it would be:

myDictionary.put("someKey", "Value");
myDictionary.get("someKey"); // == "Value"

Notice how the key (“someKey”) was the first argument in the Java example. In Objective-C you name your arguments with the method name, hence setObject: forKey:. Also notice that in Objective-C strings start with an @ symbol. That’s because Objective-C strings are different from regular C strings. When using Objective-C you almost always use Objective-C’s @ strings.

In C# there is a special syntax for Dictionaries so it becomes:

myDictionary["someKey"] = "Value";
myDictionary["someKey"]; // == "Value"

One important problem that you might encounter if you’re new is the problem of native types.

In Java to add an int to a Dictionary you used to have to do:

myDictionary.put("someKey", new Integer(10));

Because the primitive types (int, char/short, byte, boolean) aren’t real Objects. Objective-C has this problem too. So if you want to put an int into a dictionary you must use NSNumber like so:

[myDictionary setObject:[NSNumber numberForInt:10] 
                 forKey:@"someKey"];

And you pull out the integer like so:

NSNumber *number = [myDictionary objectForKey:@"someKey"];
[number intValue]; // == 10

EDIT:

Your code might be crashing if you have a ‘%’ character in your string, since stringWithFormat is just like NSLog in that it takes many arguments. So if story[“message”] is “Hello” then it’ll work fine without extra arguments but if it’s “Hello %@” you need to add one argument to stringWithFormat.

NSString *message = @"Hello %@";
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setObject:message forKey:@"message"];

NSString *output = [NSString stringWithFormat:[dict objectForKey:@"message"], @"World!"];
// output is now @"Hello World!".