NSDictionary: convert NSString keys to lowercase to search a string on them - iphone

Question

I'm developing an iPhone application.
I use a NSDictionary to store city's names as key, and population as value. I want to search the keys using lowercase.
I've using this:
NSDictionary *dict;
[dict objectForKey:[[city stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] lowercaseString]];
But, it doesn't work.
I know, I can do a for, convert keys to lowercase and compare with city.
Is there any other way to do that? Maybe, with a NSDictionary method.
UPDATE The NSDictionary is loaded from a property list.
Thank you.

Answer 1

I use this method in an NSDictionary category.
#implementation NSDictionary (MyCategory)
- (NSDictionary *)dictionaryWithLowercaseKeys {
NSMutableDictionary *result = [NSMutableDictionary dictionaryWithCapacity:0];
NSString *key;
for (key in self) {
[result setObject:[self objectForKey:key] forKey:[key lowercaseString]];
}
return result;
}
#end

Answer 2

Although I'm still not clear on what you want, this loop with search for keys case insensitively. Getting the value of that key is then trivial.
for key in dict
{
if ([key caseInsensitiveCompare: #"Whatever"] == NSOrderedSame)
NSLog(#"They are equal.");
}

Answer 3

I'd say create a second dictionary. After you load from the property set, loop through that dictionary and insert objects into the second dictionary. Convert the keys to lowercase as you go. Then release the first dictionary.

Answer 4

The above answers only work for a flat dictionary without any nesting whatsoever. I created a category that creates a copy of an existing NSDictionary with every key converted to lower case.
The header: NSDictionary+LowercaseKeys.h
#import <Foundation/Foundation.h>
#interface NSDictionary (LowercaseKeys)
/*
Recursive algorithm to find all nested dictionary keys and create an NSMutableDictionary copy with all keys converted to lowercase.
Returns an NSMutableDictionary with all keys and nested keys converted to lowercase.
*/
+ (NSMutableDictionary *)dictionaryWithLowercaseKeysFromDictionary:(NSDictionary *)dictionary;
/*
Convienience method to create a new lowercase dictionary object an existing NSDictionary instance
Returns an NSMutableDictionary with all keys and nested keys converted to lowercase.
*/
- (NSMutableDictionary *)dictionaryWithLowercaseKeys;
#end
The Implementation: NSDictionary+LowercaseKeys.m
#import "NSDictionary+LowercaseKeys.h"
#implementation NSDictionary (LowercaseKeys)
/*
Recursive algorithm to find all nested dictionary keys and create an NSMutableDictionary copy with all keys converted to lowercase
Returns an NSMutableDictionary with all keys and nested keys converted to lowercase.
*/
+ (NSMutableDictionary *)dictionaryWithLowercaseKeysFromDictionary:(NSDictionary *)dictionary
{
NSMutableDictionary *resultDict = [NSMutableDictionary dictionaryWithCapacity:[dictionary count]];
[dictionary enumerateKeysAndObjectsUsingBlock:^(NSString *key, id obj, BOOL *stop) {
// There are 3 types of objects to consider, NSDictionary, NSArray and everything else
id resultObj;
if ([obj isKindOfClass:NSDictionary.class])
{
// Recursively dig deeper into this nested dictionary
resultObj = [NSMutableDictionary dictionaryWithLowercaseKeysFromDictionary:obj];
}
else if ([obj isKindOfClass:NSArray.class])
{
/*
Iterate over this nested NSArray. Recursively convert any NSDictionary objects to the lowercase version.
If the array contains another array then continue to recursively dig deeper.
*/
resultObj = [NSMutableArray arrayWithCapacity:[obj count]];
for (id arrayObj in obj)
{
if ([arrayObj isKindOfClass:NSDictionary.class])
[resultObj addObject:[NSMutableDictionary dictionaryWithLowercaseKeysFromDictionary:arrayObj]];
else if ([arrayObj isKindOfClass:NSArray.class])
[resultObj addObject:[NSMutableDictionary arrayWithLowercaseKeysForDictionaryArray:arrayObj]];
else
[resultObj addObject:arrayObj];
}
}
else
{
// The object is not an NSDictionary or NSArray so keep the object as is
resultObj = obj;
}
// The result object has been converted and can be added to the dictionary. Note this object may be nested inside a larger dictionary.
[resultDict setObject:resultObj forKey:[key lowercaseString]];
}];
return resultDict;
}
/*
Convienience method to create a new dictionary object with all lowercase keys from an existing instance
*/
- (NSMutableDictionary *)dictionaryWithLowercaseKeys
{
return [NSMutableDictionary dictionaryWithLowercaseKeysFromDictionary:self];
}
#pragma mark - Private helpers
/*
Convert NSDictionary keys to lower case when embedded in an NSArray
*/
+ (NSMutableArray *)arrayWithLowercaseKeysForDictionaryArray:(NSArray *)dictionaryArray
{
NSMutableArray *resultArray = [NSMutableArray arrayWithCapacity:[dictionaryArray count]];
for (id eachObj in dictionaryArray)
{
if ([eachObj isKindOfClass:NSDictionary.class])
[resultArray addObject:[NSMutableDictionary dictionaryWithLowercaseKeysFromDictionary:eachObj]];
else if ([eachObj isKindOfClass:NSArray.class])
[resultArray addObject:[NSMutableDictionary arrayWithLowercaseKeysForDictionaryArray:eachObj]];
}
return resultArray;
}
#end