NSPredicate on NSDictionary

问题

I'm trying to make sections in a table view based on the alphabet and sort my entries alphabetically under these sections.

I have collected the first letter in every entry of bandsArray in bandsArrayIndex and I'm now trying to use NSPredicate to count how many of each letter there are.

I'm doing this by following this guide.

They are using an NSArray and I'm using an NSDictionary and can't seem to get it to work. Can anyone help me out?

The application crashes when trying to show the view with the table view in it. In Debugger Console the following error is shown:

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Can't do a substring operation with something that isn't a string

This is my code

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    NSString *alphabet = [bandsArrayIndex objectAtIndex:section];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SELF beginswith[c] %@", alphabet];
    NSArray *letters = [bandsArray filteredArrayUsingPredicate:predicate];

    return [letters count];
}

EDIT: This is my bandsArray.

The header

NSMutableArray *bandsArray;

@property (nonatomic, retain) NSMutableArray* bandsArray;

The implementation

// set array from plist
NSString *path = [[NSBundle mainBundle] pathForResource:@"Bands" ofType:@"plist"];
NSMutableArray* tmpArray = [[NSMutableArray alloc] initWithContentsOfFile:path];

self.bandsArray = tmpArray;

// sort array after name ascending
NSSortDescriptor *nameSorter = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES selector:@selector(caseInsensitiveCompare:)];
[bandsArray sortUsingDescriptors:[NSArray arrayWithObject:nameSorter]];

回答1:

Do you mean that bandsArray is an array of dictionaries? If so, and assuming each dictionary has a name key, you should be able to change the predicate to something like @"SELF.name beginswith[c] %@".

If, on the other hand, bandsArray is actually a dictionary itself, maybe you want to do [[bandsArray allKeys] filteredArrayUsingPredicate...].

回答2:

#import <Foundation/Foundation.h>
// clang -framework Foundation Siegfried.m 
    int
main() {
    NSArray *arr = @[
        @{@"1" : @"Fafner"},
        @{@"1" : @"Fasolt"}
    ];
    NSPredicate *p = [NSPredicate predicateWithFormat: @"SELF['1'] CONTAINS 'e'"];
    NSArray *res = [arr filteredArrayUsingPredicate:p];
    NSLog(@"Siegfried %@", res);
    return 0;
}