使用NSPredicate将所有值相加

问题描述

我有一个使用Core Data的程序。当在循环中添加每个实体的数值时，我有点作弊。我读了关于使用NSPredicate过滤数据，但我不知道如何操纵数据或如何结果甚至存储。谢谢。

I have a program which uses Core Data. I was kinda cheating when adding the numeric values from each entity in a loop. I read about using NSPredicate to filter the data, but I don't know how to manipulate the data or how the results are even stored. Thanks.

推荐答案

如果符合您的要求，您可以分两步完成。

You can do it in two steps, if that fits your requirements.

1. 首先使用 NSPredicate 筛选您的数据，并将其全部保存在 NSArray 。


2. 然后使用带键值编码的复合运算符获取总和。

1. First filter your data with your NSPredicate and keep it all in an NSArray.
2. Then use a compound operator with Key-Value Coding to get the sum.

下面是一个如何做的例子。为了使其自包含，使用硬编码的数组而不是Core Data：

Below is an example of how it can be done. To make it self-contained, a hard-coded array is used instead of Core Data:

// In reality this array would be the result of a Core Data query:
NSArray *numbers = @[@{@"number":@3},
                     @{@"number":@2}];

NSNumber *sum = [numbers valueForKeyPath:@"@sum.number"];

这里的诀窍是 @sum 运算符。您可以阅读（以及另外几个类似的运算符）。

The trick here is the @sum compound operator. You can read about it (and another couple of similar operators) here.

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