Concatenating values to the same key in an NSMutableDictionary - ios

Question

I am getting data from my database and the data is being retrieved using a while loop.
success = [db executeQuery:#"SELECT * FROM apidataTwo;"];
while([success next]){
int first = [success intForColumn:#"id"];
NSString *id = [NSString stringWithFormat:#"%d",first];
[_tempArray addObject:id];
NSString *country_name = [success stringForColumn:#"country_name"];
[_tempArray addObject:country_name];
NSString *breezometer_description = [success stringForColumn:#"breezometer_description"];
[_tempArray addObject:breezometer_description];
NSString *country_description = [success stringForColumn:#"country_description"];
[_tempArray addObject:country_description];
NSString *dateString= [success stringForColumn:#"dateString"];
[_dateSectionArray addObject:dateString];
[_dataDictionary setObject:_tempArray forKey:dateString];
}
Suppose we get the same key in different iterations of the loop. When I pass the array to the NSMutableDictionary, the previous values will be replaced and lost.
And if I keep updating the NSMutableArray, then the values of a previous key will also be added to a different key.
So in situations like this when we want to concatenate the values to the same key, then what should be our approach.
The dictionary should look like this:
{
2016-10-05" = (
5,
"United States",
"Fair Air Quality",
"Good air quality"
);
"2016-10-06" = (
5,
"United States",
"Fair Air Quality",
"Good air quality"
);
}

Answer 1

Once you have figured out the key for this batch of data, try to retrieve an object from the dictionary for that key. If objectForKey: returns nil, then create a new mutable array. Then set that array as the dictionary's object for that key.
Every new batch of data is then added to the array, not to the dictionary. Here's a sketch of the structure:
while( /* processing data */){
// Collect this batch
NSArray * entry = ...;
// Figure out the dictionary key for the batch.
// (it doesn't have to be a string, this is just for example)
NSString * key = ...;
// Try to retrieve the object for that key
NSMutableArray * entries = _dataDictionary[key];
// If the result is `nil`, the key is not in the dictionary yet.
if( !entries ){
// Create a new mutable array
entries = [NSMutableArray array];
// Add that to the dictionary as the value for the given key
_dataDictionary[key] = entries;
}
// Now `entries` is a valid `NSMutableArray`, whether it already
// existed or was just created. Add this batch.
[entries addObject:entry];
// Move on to the next batch.
}