首先:新年快乐:-)
我正在尝试做什么
我试图划分核心数据中的两个属性,然后计算这些划分的平均值。这些属性由关键路径指定(例如 eur
、usd
、aud
)。
示例: 我有以下数据集:
date eur usd aud
------------------------------
2010-01-01 0.5 1.0 1.5
2010-01-02 0.6 1.1 1.6
2010-01-03 0.4 1.0 1.3
划分两个属性,例如欧元/美元,结果如下...
divide eur / usd:
------------------
2010-01-01 0.5
2010-01-02 0.54
2010-01-03 0.4
...然后计算这些数字的平均值(0.5 + 0.54 + 0.4)/3 = 0.48
我的代码
由于我希望由 Core Data 直接执行这些计算,因此我创建了以下表达式和获取请求:
NSExpression *fromCurrencyPathExpression = [NSExpression
expressionForKeyPath:fromCurrency.lowercaseString];
NSExpression *toCurrencyPathExpression = [NSExpression
expressionForKeyPath:toCurrency.lowercaseString];
NSExpression *divisionExpression = [NSExpression
expressionForFunction:@"divide:by:"
arguments:@[fromCurrencyPathExpression,
toCurrencyPathExpression]];
NSExpression *averageExpression = [NSExpression expressionForFunction:@"average:"
arguments:@[divisionExpression]];
NSString *expressionName = @"averageRate";
NSExpressionDescription *expressionDescription =
[[NSExpressionDescription alloc] init];
expressionDescription.name = expressionName;
expressionDescription.expression = averageExpression;
expressionDescription.expressionResultType= NSDoubleAttributeType;
NSFetchRequest *request = [NSFetchRequest
fetchRequestWithEntityName:NSStringFromClass([self class])];
NSPredicate *predicate =
[NSPredicate predicateWithFormat:@"date >= %@ AND date <= %@",startDate,fiscalPeriod.endDate];
request.predicate = predicate;
request.propertiesToFetch = @[expressionDescription];
request.resultType = NSDictionaryResultType;
NSError *error;
NSArray *results = [context
executeFetchRequest:request error:&error];
问题
但是,运行应用程序时,它崩溃并显示错误消息:
Unsupported argument to sum : (
"eur / usd"
我的代码有什么问题吗? 如何链接这两个计算并直接在 Core Data 中执行?
谢谢!
最佳答案
像@average
这样的“集合”函数表达式似乎只能与关键路径一起使用,但在用作propertiesToFetch
时不能与其他通用表达式组合使用。我没有这方面的引用,但其他人也注意到了这个问题:
- Fetch aggregate data from NSManagedObject using another expression as argument to sum: expression
- Performing multiplication (aggregation) with CoreData: how to?
因此,您可以分两步进行:首先执行一个获取请求,返回一个包含所有除法结果的数组:
NSExpression *fromCurrencyPathExpression = [NSExpression
expressionForKeyPath:@"eur"];
NSExpression *toCurrencyPathExpression = [NSExpression
expressionForKeyPath:@"usd"];
NSExpression *divisionExpression = [NSExpression
expressionForFunction:@"divide:by:"
arguments:@[fromCurrencyPathExpression,
toCurrencyPathExpression]];
NSString *expressionName = @"ratio";
NSExpressionDescription *expressionDescription =
[[NSExpressionDescription alloc] init];
expressionDescription.name = expressionName;
expressionDescription.expression = divisionExpression;
expressionDescription.expressionResultType= NSDoubleAttributeType;
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Entity"];
request.propertiesToFetch = @[expressionDescription];
request.resultType = NSDictionaryResultType;
NSError *error;
NSArray *ratios = [context executeFetchRequest:request error:&error];
结果是一个字典数组:
(lldb) po ratios
(NSArray *) $0 = 0x00000001001170f0 <_PFArray 0x1001170f0>(
{
ratio = "0.5454545454545454";
},
{
ratio = "0.4";
},
{
ratio = "0.5";
}
)
此外,通过“-com.apple.CoreData.SQLDebug 1”选项,我们可以看到划分已经在 SQLite 级别上执行:
sql: SELECT t0.ZEUR / t0.ZUSD FROM ZENTITY t0
然后您可以使用键值编码计算内存中所有比率的平均值:
NSNumber *average = [ratios valueForKeyPath:@"@avg.ratio"];
结果是
(lldb) po average
(NSNumber *) $0 = 0x000000010012fd60 0.4818181818181818
关于objective-c - 用于在 Core Data 中执行计算的链式表达式,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14122086/