如何使用谓词从核心数据中获取偶数和奇数值 [英] How to fetch even and odd values from core data using Predicate

问题描述

我想从我的核心数据实体中获取偶数和奇数，请查看下面的代码并进行更正，因为它使应用程序崩溃。

I want to fetch even and odd values from my core data entity, please look into below code and correct, because its crashing the app.

        NSPredicate *predicate;
        if ([leadFilter.rank isEqualToString:@"Even"]) {
            predicate = [NSPredicate predicateWithFormat:@"(street%2) = 0"];
        }
        else
            predicate = [NSPredicate predicateWithFormat:@"street%2 != 0)"];
        [predicateArray addObject:predicate];

是无法解析街道％2 = 0。

推荐答案

您可以直接在Core Data中执行此操作，但是语法有点麻烦。要仅获取 street 的值，请按以下步骤设置获取：

You can do this directly in Core Data, but the syntax is a little hairy. To fetch only values of street that are even, set up your fetch like this:

NSExpression *evenOddExpression = [NSExpression expressionWithFormat:@"modulus:by:(%K,2)", @"street"];

NSFetchRequest *evenOddFetch = [NSFetchRequest fetchRequestWithEntityName:@"ENTITY_NAME"];

NSExpression *zeroExpression = [NSExpression expressionForConstantValue:@0];
NSPredicate *evenOnly = [NSComparisonPredicate predicateWithLeftExpression:evenOddExpression rightExpression:zeroExpression modifier:NSDirectPredicateModifier type:NSEqualToPredicateOperatorType options:0];
evenOddFetch.predicate = evenOnly;

结果将是具有 street均等值的任何托管对象。

更新：在Marcus指出 NSDictionaryResultType 实际上并不需要。

Update: Made simpler after Marcus pointed out that NSDictionaryResultType wasn't actually needed.

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