Calculate lessthan value using NSExpression?

问题

For the following block of code:

NSString *formul=@"5 < 9";

NSExpression *e = [NSExpression expressionWithFormat:formul];

int result = [[e expressionValueWithObject:nil context:nil] intValue];

NSLog(@"formule:%d", result);

I got error:

due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "5 < 9 == 1"'

回答1:

You can use NSPredicate instead:

NSString *formul = @"15 < 9";
NSPredicate *predicate = [NSPredicate predicateWithFormat:formul];
BOOL b = [predicate evaluateWithObject:nil];

In Swift:

let formula = "15 < 9"
let predicate = NSPredicate(format: formula)
let b = predicate.evaluate(with: nil)
print(b) // false

来源：https://stackoverflow.com/questions/18871071/calculate-lessthan-value-using-nsexpression