NSMutableDictionary don't set value for key - objective-c

Question

I have a code in which the existing handle dates in an array, separates the day of the month and will put inside a NSMutableDictionary with their respective day of the month, the array has the following structure:
"12/01/2014" //Structure of my date is -> dd-mm-yyyy
"16/01/2014"
"30/01/2014"
"02/02/2014"
"08/02/2014"
I use this code to put this values inside a NSMutableDictionary:
dictionary = [NSMutableDictionary dictionary];
for(int x=0;x<[list count];x++){
NSMutableArray *lstaInfo2 = [[list[x] componentsSeparatedByString:#"/"] mutableCopy];
if([lstaInfo2[1] isEqual: #"01"]){
[dictionary setValue:list[x] forKey:[NSString stringWithFormat:#"January of %#",lstaInfo2[2]]];
}
if([lstaInfo2[1] isEqual: #"02"]){
[dictionary setValue:list[x] forKey:[NSString stringWithFormat:#"February of %#",lstaInfo2[2]]];
}
}
The values ​​that I hoped to return within the variable dictionary is:
January of 2014 =>
"12/01/2014"
"16/01/2014"
"30/01/2014"
February of 2014 =>
"02/02/2014"
"08/02/2014"
But the variable dictionary, returns only the last values, like this:
January of 2014 =>
"30/01/2014"
February of 2014 =>
"08/02/2014"
why? How can I solve this problem?

Answer 1

dictionary = [NSMutableDictionary dictionary];
for(int x=0;x<[list count];x++){
NSString* key = [self getKeyForDate:list[x]];
NSMutableArray* listForMonth = dictionary[key];
if (key == nil) {
listForMonth = [NSMutableArray array];
[dictionary setValue:listForMonth forKey:key];
}
[listForMonth addObject:list[x]];
}
.....
In init
monthArray = #[#"January", #"February", #"March" ...
Separate method:
-(NSString*) getKeyForDate:(NSString*)date {
NSMutableArray *lstaInfo2 = [date componentsSeparatedByString:#"/"] mutableCopy];
NSInteger monthNum = lstaInfo2[0].integerValue;
NSString* result = [NSString stringWithFormat;#"%# of %#", monthArray[monthNum-1], lstaInfo2[2]];
return result;
}
You can also use NSDateFormatter to parse the date, and NSCalendar to serve up the month names, but that's getting too deep for one lesson.

Answer 2

When you try to add new value to the dictionary check whether value for this key is actually in the dictionary. If no, create and set NSMutableArray object for this key and add values to this array.
Try this:
```objc
dictionary = [NSMutableDictionary dictionary];
for(int x=0; x < [list count]; x++){
NSArray *lstaInfo2 = [list[x] componentsSeparatedByString:#"/"];
NSString *key;
if([lstaInfo2[1] isEqual: #"01"]){
key = [NSString stringWithFormat:#"January of %#",lstaInfo2[2]];
}
if([lstaInfo2[1] isEqual: #"02"]){
key = [NSString stringWithFormat:#"February of %#",lstaInfo2[2]];
}
if (key) {
//if there is no value for the specified key create and set
//NSMutableArray object for this key, otherwise keep value for
//the key without modyfing it.
dictionary[key] = dictionary[key] ?: [NSMutableArray array];
[dictionary[key] addObject:list[x]];
}
}
```

Answer 3

It is because setValue:forKey: doesn't append to the current value. So in the first iteration through the loop you set "January of 2014" => ["12", "01", "2014"] and in the next iteration you set "January of 2014" => ["16", "01", "2014"], and so on. You want to map from a string to a NSMutableArray.
I would really suggest you use the NSDateFormatter class for this: https://developer.apple.com/library/ios/documentation/cocoa/reference/foundation/classes/nsdateformatter_class/Reference/Reference.html#//apple_ref/occ/instm/NSDateFormatter/. It will be easier and much more flexible than manually parsing the dates.