问将公式字符串转换为数学公式的Xcode

EN

@interface MyExprTest : NSObject
-(void)run;
@end

@implementation MyExprTest

-(void)run {
    NSExpression *e = [(NSComparisonPredicate *)[NSPredicate
        predicateWithFormat:@"2*FUNCTION(2*PI*$t,'sin')==0"]
        leftExpression];
    NSMutableDictionary *vars = [NSMutableDictionary
                             dictionaryWithObjectsAndKeys:
                             [NSNumber numberWithDouble:0.04], @"t",
                             nil];
    NSLog(@"%@", [e expressionValueWithObject:self context:vars]);
}

-(double)PI {
    return M_PI;
}

@end

@interface NSNumber (Trigonometry)
- (NSNumber*)sin;
@end

@implementation NSNumber (Trigonometry)

- (NSNumber*) sin {
    double result = sin([self doubleValue]);
    return [NSNumber numberWithDouble:result];
}

@end

int main (int argc, const char * argv[]) {
    MyExprTest *t = [[MyExprTest alloc] init];
    [t run];
    return 0;
}