NSMutableDictionary setObject:forKey: fails to add key - nsmutabledictionary

Question

I'm sure I'm missing something in a small iPhone program I'm trying to write, but the code is simple and it compiles without any errors and so I fails to see where the error is.
I've set up a NSMutableDictionary to store students' attributes, each with a unique key. In the header file, I declare the NSMutableDictonary studentStore:
#interface School : NSObject
{
#private
NSMutableDictionary* studentStore;
}
#property (nonatomic, retain) NSMutableDictionary *studentStore;
And of course in the implementation file:
#implementation School
#synthesize studentStore;
And I want to add an object into the dictionary:
- (BOOL)addStudent:(Student *)newStudent
{
NSLog(#"adding new student");
[studentStore setObject:newStudent forKey:newStudent.adminNo];
return YES;
}
class Student has the attributes:
#interface Student : NSObject
{
#private
NSString* name; //attributes
NSString* gender;
int age;
NSString* adminNo;
}
where newStudent has the values:
Student *newStudent = [[Student alloc] initWithName:#"jane" gender:#"female" age:16 adminNo:#"123"];
But when I look up the dictionary:
- (void)printStudents
{
Student *student;
for (NSString* key in studentStore)
{
student = [studentStore objectForKey:key];
NSLog(#" Admin No: %#", student.adminNo);
NSLog(#" Name: %#", student.name);
NSLog(#"Gender: %#", student.gender);
}
NSLog(#"printStudents failed");
}
It fails to print the values in the table. Instead, it prints the line "printStudents failed".
I guess this's quite basic, but since I'm new to iOS programming I'm a bit stumped. Any help will be appreciated. Thanks.

Answer 1

Your studentStore instance variable is a pointer to an NSMutableDictionary. By default, it points to nil, meaning it doesn't point to any object. You need to set it to point to an instance of NSMutableDictionary.
- (BOOL)addStudent:(Student *)newStudent
{
NSLog(#"adding new student");
if (studentStore == nil) {
studentStore = [[NSMutableDictionary alloc] init];
}
[studentStore setObject:newStudent forKey:newStudent.adminNo];
return YES;
}