Finding minimum and maximum values in a nested NSDictionary - objective-c

Question

I have a Person NSDictionary, whose key is the Name of the person, and the object is an NSDictionary with two keys: his nickname (NSString) and his age (NSNumber).
I would like to end up with the Person dictionary sorted by the ascending order of their age, so that I could get the name of the youngest and the oldest person.
What is the best way to do it?
Thanks!

Answer 1

There are a few convenience methods defined in NSDictionary to sort items by values and get back the sorted keys.
See docs,
keysSortedByValueUsingComparator:
keysSortedByValueUsingSelector:
keysSortedByValueWithOptions:usingComparator:
I'm guessing you're using the modern Objective-C syntax and the age is actually represented as numbers. Here's how it looks:
[people keysSortedByValueUsingComparator:(NSDictionary *firstPerson, NSDictionary *secondPerson) {
return [firstPerson[#"age"] compare:secondPerson[#"age"]];
}];

Answer 2

Some languages offer sorted dictionaries, but the standard NSDictionary is inherently unsorted. You can get all the keys, sort the key array and then walk over the dictionary according to the sorted keys. (NSDictionary has several convenience methods for this use case that I didn’t know about, see Anurag’s answer.)
Your case is a bit more complex, one way to solve it is to introduce a temporary dictionary mapping ages to names. But if you’re only after the minimum and maximum ages, just iterate over all persons and keep track of the maximum & minimum ages and names:
NSString *oldestName = nil;
float maxAge = -1;
for (NSString *name in [persons allKeys]) {
NSDictionary *info = persons[name];
float age = [info[#"age"] floatValue];
if (age > maxAge) {
oldestName = info[#"nick"];
maxAge = age;
}
}
And if we get back to the idea of sorting the dictionary, this could work:
NSArray *peopleByAge = [people keysSortedByValueUsingComparator:^(id a, id b) {
// Again, see Anurag’s answer for a more concise
// solution using the compare: method on NSNumbers.
float ageA = [a objectForKey:#"age"];
float ageB = [b objectForKey:#"age"];
return (ageA > ageB) ? NSOrderedDescending
: (ageB > ageA) ? NSOrderedAscending
: NSOrderedSame;
}];

Answer 3

As #Zoul said the standard NSDictionary is unsorted.
To sort it you can use an array, and I do things like that
//the dictionary is called dict : in my case it is loaded from a plist file
NSDictionary *dict = [[NSDictionary alloc] initWithContentsOfFile:plistPath];
//make a dicoArray that is sorted so the results are sorted
NSArray *dicoArray = [[dict allKeys] sortedArrayUsingComparator:^(id firstObject, id secondObject) {
return [((NSString *)firstObject) compare:((NSString *)secondObject) options:NSNumericSearch];
}];
check the help for all the sort options. In the presented case the dictionary is sorted with keys treated as numeric value (which was the case for me).
If you need to sort another way the list of sort possibilities is
enum {
NSCaseInsensitiveSearch = 1,
NSLiteralSearch = 2,
NSBackwardsSearch = 4,
NSAnchoredSearch = 8,
NSNumericSearch = 64,
NSDiacriticInsensitiveSearch = 128,
NSWidthInsensitiveSearch = 256,
NSForcedOrderingSearch = 512,
NSRegularExpressionSearch = 1024
};

Answer 4

In iOS 9.2
// Dictionary of NSNumbers
NSDictionary * phoneNumbersDict = #{#"400-234-090":67,#"701-080-080":150};
// In Ascending Order
NSArray * keysArraySortedByValue = [phoneNumbersDict keysSortedByValueUsingComparator:^NSComparisonResult(id _Nonnull obj1, id _Nonnull obj2) {
return [obj1 compare:obj2];
}];
// In Descending Order
NSArray * keysArraySortedByValue = [phoneNumbersDict keysSortedByValueUsingComparator:^NSComparisonResult(id _Nonnull obj1, id _Nonnull obj2) {
return [obj2 compare:obj1];
}];
Here is the enum for NSComparisonResults.
enum {
NSOrderedAscending = -1,
NSOrderedSame,
NSOrderedDescending
};
typedef NSInteger NSComparisonResult;

Answer 5

Look at the NSDictionary's method that returns keys sorted by a selector. There are more than one such method. You get an array of sorted keys, then access the first and last and have your youngest and oldest person.