fast enumeration for NSDictionary instance ordered by the key

问题

Overview

I am using fast enumeration to iterate through an NSDictionary instance
I expected the NSDictionary instance to be enumerated based on the ascending order of the key but that doesn't seem to be the case

What I want to do:

I want to be able iterate through the NSDictionary instance in the ascending order of the key using fast enumeration

Note: Pls see expected output vs actual output

Questions

Am i making a mistake with my implementation ?
Does NSDictionary's fast enumeration guarantee ordering based on keys ?
If not then is there a work around for this and yet use fast enumeration ?

Example

#import<Foundation/Foundation.h>

int main()
{
    system("clear");

    NSDictionary *d1 = nil;

    @autoreleasepool
    {   

        d1 = [[NSDictionary alloc] initWithObjectsAndKeys: @"AAA", [NSNumber numberWithInt:10], 
              @"BBB", [NSNumber numberWithInt:20],               
              @"CCC", [NSNumber numberWithInt:30],               
              nil];
    }   

    for(NSNumber* n1 in d1)     //I expected fast enumeration for NSDictionary to be based on the 
        //ascending order of the key but that doesn't seem to be the case
    {
        printf("key = %p"
               "\t [key intValue] = %i"
               "\t value = %s\n", 
               n1, 
               [n1 intValue], 
               [[d1 objectForKey:n1] UTF8String]);
    }   

    return(0);
}

Expected Output

key = 0xa83      [key intValue] = 10     value = AAA
key = 0x1483     [key intValue] = 20     value = BBB
key = 0x1e83     [key intValue] = 30     value = CCC

Actual Output

key = 0x1e83     [key intValue] = 30     value = CCC
key = 0xa83      [key intValue] = 10     value = AAA
key = 0x1483     [key intValue] = 20     value = BBB

回答1:

No your implementation is correct.
NSDictionary fast enumeration does not guarantee sorting (and it will not output anything in order because of implementation as hashed container).
No, you have to sort it yourself.

回答2:

for (NSString *key in [[d1 allKeys] sortedArrayUsingSelector:@selector(compare:)])
{
    id value = [d1 valueForKey:key];
    ...
}

回答3:

There is no guaranties about the order in which you will receive your object.

allKeys
Returns a new array containing the dictionary’s keys.
- (NSArray *)allKeys
Return Value
A new array containing the dictionary’s keys, or an empty array if the dictionary has no entries.
Discussion
The order of the elements in the array is not defined.

So my suggestion is, if your dictionary doesn't change often, cache an NSArray with the key in the order you want them.
If your dictionary often change, you may have to sort allKeys when you need them.

来源：https://stackoverflow.com/questions/8529660/fast-enumeration-for-nsdictionary-instance-ordered-by-the-key

标签

objective-c

nsdictionary

fast-enumeration