iOS - 当字符串以算术运算符开始时，NSPredicate string.length总是计算为0 - * / [英] iOS - NSPredicate string.length always evaluates to 0 when string starts with arithmetic operators + - * /

问题描述

我有一个简单的方法，使用NSPredicate返回的行数，其中comments.length> 0。

I have a simple method that uses NSPredicate to return the number of rows where comments.length > 0.

问题是，用+ - *或/，length属性总是计算为0，因此该行从计数中排除。

Problem is, I found that when the Comment column starts with + - * or /, the length property always evaluates to 0, and thus the row is excluded from the count.

我在SQLite浏览器中打开表，列是一个VARCHAR。使用SQLite查询来检查字符串长度是否正常（LENGTH（ZComments）> 0），因此这必须是一个CoreData问题。

I opened the table in SQLite browser and verified the column is a VARCHAR. Using a SQLite query to check string length works just fine (LENGTH(ZComments) > 0), so this must be a CoreData issue.

-(int)getCommentsCount{
    NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
    [request setIncludesSubentities:YES];
    [request setEntity:[NSEntityDescription entityForName:@"InspectionRecord" inManagedObjectContext:managedObjectContext]];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(comments.length > 0)"];
    [request setPredicate:predicate];

    NSError *err;
    NSUInteger count = [managedObjectContext countForFetchRequest:request error:&err];

    //count is ALWAYS 0 if 'comments' starts with + - * or /     WHYYY???      
    return count;
}

我可以通过检查空/空字符串而不是使用.length，但是我真的想知道为什么当字符串以某些字符开头时.length失败。

I was able to work around this by checking for empty/null string instead of using .length, but I'd really like to know why .length fails when the string starts with certain characters.

这是发生在任何人吗？

推荐答案

您不能在Core Data fetch请求中使用 length $ b（当Core Data将获取请求
转换为SQLite查询时，将忽略.length部分）。但是你可以简单地比较一个空字符串：

You cannot use Objective-C functions like length in a Core Data fetch request (and the ".length" part is simply ignored when Core Data translates the fetch request to a SQLite query). But you can simply compare with an empty string instead:

 [NSPredicate predicateWithFormat:@"comment != ''"]

对于涉及长度的其他查询，可以使用MATCHES运算符与
a正则表达式： CoreData谓词：string属性长度？。

For other queries involving the length, you can use the MATCHES operator with a regular expression as shown here: CoreData predicate: string property length?.

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