How to format brackets of nested JSON - objective-c

Question

I'm trying to create a string which holds JSON data. I'm having an issue formatting the inner nested data, it currently has square brackets '[]' and I'm trying to change it to '{}', so it is valid JSON. Are there better approaches/solutions to parsing this data?
Actual Results:
[
[
"eventtime":"2019-09-20 09:52:47",
"keyid":"15",
],
[
"eventtime":"2019-09-20 09:52:47",
"keyid":"15",
]
]
Desired Results
[
{
"eventtime":"2019-09-20 09:52:47",
"keyid":"15",
},
{
"eventtime":"2019-09-20 09:52:47",
"keyid":"15",
}
]
Code
// Parse and loop through the JSON
for (NSDictionary * dataDict in eventArray) {
NSMutableArray *array = [[NSMutableArray alloc] init];
eventTime = [NSString stringWithFormat:#"%#%#",#"eventtime\":\"",[self eventTimeConversion:[dataDict objectForKey:#"event_time"]]];
keyID = [NSString stringWithFormat:#"%#%#",#"keyid\":\"",[dataDict objectForKey:#"key_id"]];
editTime = [NSString stringWithFormat:#"%#%#",#"edittime\":\"",[self eventTimeConversion:[dataDict objectForKey:#"event_time"]]];
projectID = [NSString stringWithFormat:#"%#%#",#"projectid\":\"",[gotHeaders objectAtIndex:0]];
lockID = [NSString stringWithFormat:#"%#%#",#"doorid\":\"",[dataDict objectForKey:#"lock_id"]];
eventType = [NSString stringWithFormat:#"%#%#",#"eventid\":\"",[dataDict objectForKey:#"event_type"]];
[array addObject:eventTime];
[array addObject:keyID];
[array addObject:editTime];
[array addObject:projectID];
[array addObject:lockID];
[array addObject:eventType];
[event_array addObject:array];
}
}
NSData *jsonnData = [NSJSONSerialization dataWithJSONObject:event_array options:NSJSONWritingPrettyPrinted error:&error];
NSString *eventsString = [[NSString alloc] initWithData:jsonnData encoding:NSUTF8StringEncoding];

Answer 1

Don't create a JSON string manually, create dictionaries, this is a simplified example for the keys eventtime and keyid
The code uses modern Objective-C syntax (introduced at least 6 years ago)
NSMutableArray *array = [[NSMutableArray alloc] init];
for (NSDictionary * dataDict in eventArray) {
NSString *eventTime = [self eventTimeConversion: dataDict[#"event_time"]];
NSString *keyID = dataDict[#"key_id"];
[array addObject:#{#"eventtime":eventTime, #"keyid":keyID}];
}
NSError *error;
NSData *jsonnData = [NSJSONSerialization dataWithJSONObject: array options:NSJSONWritingPrettyPrinted error:&error];
NSString *eventsString = [[NSString alloc] initWithData:jsonnData encoding:NSUTF8StringEncoding];

Answer 2

Instead of using an array to store the JSON id and value parameters, use a NSDictionary:
NSDictionary *d1 = [NSDictionary dictionaryWithObjectsAndKeys:
#"eventtime", #"2019-09-20 09:52:47",
#"keyid", #"15", nil];
[event_array addObject:d1];
...
NSData* jsonData = [NSJSONSerialization dataWithJSONObject:event_array
options:NSJSONWritingPrettyPrinted error:&error];
NSString *eventsString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
NSLog(#"%#", eventsString);
You will need to obviously replace the hardcoded values I have put on the dictionary in the example with your values.
This will output:
[
{
"eventtime":"2019-09-20 09:52:47",
"keyid":"15",
},
{
"eventtime":"2019-09-20 09:52:47",
"keyid":"15",
}
]