ios - 解析 IOS for loop sum total

Question

我正在尝试运行一个 for 循环并添加解析数据库中的所有对象以得出总数。这是我不断收到的错误:二进制表达式的无效操作数('NSNumber *' 和 'NSNumber *')

这是我的代码:

- (void)viewDidLoad
{
[super viewDidLoad];
PFQuery *query = [PFQuery queryWithClassName:@"Account"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    NSNumber *sum;
    for (PFObject *object in objects) {
        NSString *balanceString = object[@"startingBalance"];

        NSNumberFormatter *f = [[NSNumberFormatter alloc] init];
        [f setNumberStyle:NSNumberFormatterDecimalStyle];
        NSNumber *balanceNumber = [f numberFromString:balanceString];

        sum = sum + balanceNumber;
        NSLog(@"%f", sum);
    }


}];

Answer 1

您必须将 NSNumber 转换为基本类型(例如 float 或 int)才能执行算术运算

float sum = 0;
NSNumberFormatter *f = [[NSNumberFormatter alloc] init]; // and you should reuse formatter
[f setNumberStyle:NSNumberFormatterDecimalStyle];
for (PFObject *object in objects) {
    NSString *balanceString = object[@"startingBalance"];
    NSNumber *balanceNumber = [f numberFromString:balanceString];

    sum += [balanceNumber floatValue];
    NSLog(@"%f", sum);
}