ios - 使用 NSPredicate Core Data 中的数据库字段执行计算

Question

我创建了一个从我的数据库中恢复产品的方法，每个项目都有字段高度、宽度和 percErro(百分比误差)，

用户输入要搜索的宽高，eg:15 x 15cm

然后该方法应将兼容的产品与这些数字结合起来，同时考虑到其注册中允许的错误百分比。

假设数据库中的产品是 14 x 14cm，错误百分比是 10%(1.4cm)，那么您搜索的产品是 15 x 15cm

在我的查询中，我必须做这样的事情:

[query appendString: [NSString stringWithFormat: @ "(heigthInserted * percErro / 1000) <= (height * percErro / 100)"

[query appendString: [NSString stringWithFormat: @ "AND (widthInserted * percErro / 1000) <= (width * percErro / 100)"

 NSPredicate * predicate = [NSPredicate predicateWithFormat: query];
    [Request setPredicate: predicate];

这应该给我带来误差百分比不超过 10%(1.4 厘米)的所有产品

我知道 NSPredicate 不接受数学运算，我怎么能得到计算这个误差百分比的结果？

**

Solved

**

I accepted the answer of pbasdf and conclude that mathematical operators work in NSPredicate, but must be written according to the documentation of this link: Documentation

Below my 100% functional method with operators:

-(NSMutableArray*)findWithDimensoes:(NSMutableArray*)formatos :(float)largura :(float)altura{
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:[NSEntityDescription entityForName:TABLE_NAME inManagedObjectContext:managedObjectContext]];

    NSSortDescriptor *sortDescriptor1 = [[NSSortDescriptor alloc] initWithKey:@"altura" ascending:YES];
    NSSortDescriptor *sortDescriptor2 = [[NSSortDescriptor alloc] initWithKey:@"largura" ascending:YES];

    NSArray *arraySD = [NSArray arrayWithObjects:sortDescriptor1,sortDescriptor2,nil];
    [request setSortDescriptors:arraySD];
    [request setFetchLimit:20];

    NSMutableString *query = [[NSMutableString alloc] init];

    [query appendString:[NSString stringWithFormat:@" (habilitado == true) "]];

    if (formatos != nil) {
        for (ProdutoDimensao *d in formatos) {
            if ([d.id intValue] > 0) {
               if (([formatos indexOfObject:d] < formatos.count)) {
                    [query appendString:@" AND "];
                }

                [query appendString:[NSString stringWithFormat:@" (produtoDimensao.id == %d) ",[d.id intValue]]];
            }
        }
    }

    [query appendString:[NSString stringWithFormat:@" AND altura >=  from:subtract:(%lf,multiply:by:(%lf,percErro)) AND altura <= add:to:(%lf,multiply:by:(%lf,percErro)) ",altura,altura,altura,altura]];
[query appendString:[NSString stringWithFormat:@" AND largura >=  from:subtract:(%lf,multiply:by:(%lf,percErro)) AND largura <= add:to:(%lf,multiply:by:(%lf,percErro)) ",largura,largura,largura,largura]];


    NSPredicate *predicate = [NSPredicate predicateWithFormat:query];
    [request setPredicate:predicate];

    NSError *error = nil;
    NSArray *results = [managedObjectContext executeFetchRequest:request error:&error];


    return [results mutableCopy];

}

Answer 1

恐怕我不明白你的格式字符串(例如，percErro 可以从比较的双方中分解出来)。但是您的最后陈述是错误的:是可以在谓词中使用(基本)数学计算。正确的语法在 documentation 中对于 NSExpression。在您的情况下，您将需要这样的东西:

NSPredicate * predicate = [NSPredicate predicateWithFormat:@"divide:by:(multiply:by:(height,percErro),100) <= %@ AND divide:by:(multiply:by:(width,percErro),100) <= %@",heightInserted, widthInserted];