Question

I am reading Apple’s Key Value Programming Guide.
In the topic, Simple Collection Operators, I read:

The following example returns the average value of the transaction amount for the objects in transactions:
NSNumber *transactionAverage=[transactions valueForKeyPath:”@avg.amount”];

I attempt to write a program in which I can use the above. Below are the files of that program. I am unable to understand why the above does not work for me.

Transaction.h

    #import <Foundation/Foundation.h>

@interface Transaction : NSObject
{
NSString *payee;
NSNumber *amount;
NSDate *date;
}
- (void) setPayee: (NSString *) aP 
    setAmount: (NSNumber *) aN
      setDate: (NSDate *) aD;

@end

Transaction.m
#import “Transaction.h”

@implementation Transaction
- (void) setPayee: (NSString *) aP 
    setAmount: (NSNumber *) aN
      setDate: (NSDate *) aD 
{
payee = aP;
amount = aN;
date = aD;
} 
@end

TransactionDataBase.h

#import <Foundation/Foundation.h>
#import "Transaction.h"
@interface TransactionDataBase : NSObject
{
NSMutableArray *tArray;
}
  - (id) objectInTArrayAtIndex: (NSUInteger) index;
- (void) insertObject: (Transaction *) tA
inTArrayAtIndex: (NSUInteger) index;
  - (NSUInteger) countOfTArray;
  @end

TransactionDataBase.m

     #import "TransactionDataBase.h"

   @implementation TransactionDataBase
  - (id) init
  {
self = [super init];
if (self) {
    tArray = [[NSMutableArray alloc] initWithCapacity:40];
   }
    return self;
  }

    - (id) objectInTArrayAtIndex: (NSUInteger) index
  {
  return [tArray objectAtIndex:index];
  }

  - (void) insertObject: (Transaction *) tA
    inTArrayAtIndex: (NSUInteger) index
 {
[tArray insertObject:tA atIndex:index];
   }

 - (NSUInteger) countOfTArray
  {
return [tArray count];
    }
   @end


  main.m:


  #import <Foundation/Foundation.h>
    #import "Transaction.h"
   #import "TransactionDataBase.h"

    int main(int argc, const char * argv[])
   {

      @autoreleasepool {

    Transaction *t1 = [[Transaction alloc] init];
    Transaction *t2 = [[Transaction alloc] init];
    Transaction *t3 = [[Transaction alloc] init];

    NSDate *nowDate = [[NSDate alloc]  initWithString:@"2.4.2012"];
    NSDate *nowDate2 = [[NSDate alloc] initWithString:@"2.4.2012"];
    [t1 setPayee:@"c" setAmount:[NSNumber numberWithInt:3000] setDate:nowDate];

    [t2 setPayee:@"c" setAmount:[NSNumber numberWithInt:5000] setDate:nowDate2];


    [t3 setPayee:@"y" setAmount:[NSNumber numberWithInt:1000] setDate:nowDate2];

    TransactionDataBase *db1 = [[TransactionDataBase alloc] init];
    [db1 insertObject:t1 inTArrayAtIndex:0];
    [db1 insertObject:t2 inTArrayAtIndex:1];
    [db1 insertObject:t3 inTArrayAtIndex:2];
    NSNumber *avgT = [db1 valueForKeyPath: "@avg.amount"];
    NSLog(@"%@", avgT);


}
return 0;
  }

Answer 1

I think you want NSNumber *avgT = [db1 valueForKeyPath:@”@avg.amount”]; You want to average over all the “amount” values in your array, not just in the t1 object — it doesn’t make sense to average over one value.