Parsing a JSON array into a NSDictionary

问题:

I'm working with the Weather Underground API to make an app and I've hit a snag while parsing the block relating to severe alerts. The JSON uses key-value pairs that have sub key value pairs -- which haven't been a problem for me, as I can make subsequent NSDictionaries out of those -- but the entry for severe alerts has proven problematic. See below:

"alerts": [
    {
    "type": "WAT",
    "description": "Flash Flood Watch",
    "date": "3:13 PM EDT on April 28, 2012",
    "date_epoch": "1335640380",
    "expires": "8:00 AM EDT on April 29, 2012",
    "expires_epoch": "1335700800",
    "message": "\u000A...Flash Flood Watch in effect through Sunday morning...\u000A\u000AThe National Weather Service in Charleston has issued a\u000A\u000A* Flash Flood Watch for portions of northeast Kentucky... (Note: I trimmed this for length's sake),
    "phenomena": "FF",
    "significance": "A"
    }
]

The "alerts" pair differs from others I've been able to parse because it has this [ ] bracket surrounding the sub-values and I'm not sure how to clear it so I can access the subvalues. In the other examples I've been able to parse, it only has the { } brackets, and not both the { } and [ ] brackets. For reference, the brackets are always present -- even when there are no severe weather alerts... in that instance the "alerts" pair returns the brackets [ ] with no sub-pairs present.

Is there a way I can remove the [ ] brackets from the NSDictionary, or otherwise ignore them? Any advice would be appreciated!

---

For reference and troubleshooting help, here's how I'm parsing the rest of the JSON document successfully:

1) Create an NSDictionary from the raw JSON

//Process Weather Call
NSError* error;
NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData options:kNilOptions error:&error];

2) Create subsequent dictionaries for nested json pairs

NSDictionary *current_observation = [json objectForKey:@"current_observation"];

3) Assign values

NSString* weather;
weather = [current_observation objectForKey:@"weather"];

So the end result would be a string that says "Partly Cloudy" or something, along with numerous related weather values that I haven't shown. These parse successfully because they only have the scope brackets { }, and not the [ ] brackets.

回答1:

The brackets means the Json data there are in an array. You can parse it as following

NSArray *alertArray = [json objectForKey:@"alerts"];

now you should loop through all alerts and parse them (in your case it's only 1, but it could be more in another json string):

//parse each alert
for (NSDictionary *alert in alertArray ){
     NSString* description = [alert  objectForKey:@"description"];
    //etc...
}

回答2:

Okay, I got it working -- and I wanted to provide an example here because I ended up having to build on the advice @Lefteris gave to get it working.

I ended up having to pass the json array first as an NSArray, and then I converted that into an NSDictionary with the first element of the array. Everything afterwards worked as @Lefteris described.

So, in the end, here's what I've got:

NSArray *alerts = [json objectForKey:@"alerts"];
NSDictionary *alertDict = [[NSDictionary alloc] init];

//Check that no alerts exist to prevent crashing
if([alerts count] < 1) {
    NSLog(@"No Alerts Here!");
    type = nil;
    ...
}
else  //Populate fields
{
    alertDict = [alerts objectAtIndex:0];
    for (NSDictionary *alert in alertDict)
    {
        NSLog(@"Printing alert!");
        type = [alertDict objectForKey:@"type"];
        ...
    }
} 

This got me up and running with a single array iterate -- going on I expect I can simply iterate through the array since I know the count and handle any additional alerts. Thanks again for the help!