逻辑NSExpression评价

Question

此代码：

NSString* equation = @"2.5*3"; 
NSExpression* expresion = [NSExpression expressionWithFormat:equation, nil]; 
NSNumber* result = [expresion expressionValueWithObject:nil context:nil]; 
NSLog(@"%@", result); // 7.5 

但是这一次不，它在一个NSInvalidArgumentException

NSString* equation = @"2.5<=3"; 
NSExpression* expresion = [NSExpression expressionWithFormat:equation, nil]; 
NSNumber* result = [expresion expressionValueWithObject:nil context:nil]; 
NSLog(@"%@", result); //I wanted result to be 1, as the expression is true 

结束

有谁知道是否有使用NSExpression评价逻辑的方式像那样的表情？

谢谢。

Answer 1

使用NSPredicate代替NSExpression：

NSString* equation = @"2.5<=3"; 
NSPredicate* pre = [NSPredicate predicateWithFormat:equation]; 
BOOL result = [pre evaluateWithObject:nil]; 
NSLog(@"%@", (result ? @"YES" : @"NO"));