是否有字符串形式的數學表達式的現成解析器？

Question

給出字符串：

"10 + 20 - 5 * 7 + (20/5)"

是否有任何可用的解析器，可以評估這個，或者我需要寫我自己？

Answer 1

看看this發表於newacct。

NSString *equation = @"10 + 20 - 5 * 7 + (20/5)"; 

// dummy predicate that contains our expression 
NSPredicate *pred = [NSPredicate predicateWithFormat: 
         [equation stringByAppendingString:@" == 42"]];//42 is just a rightExpression to make it an equation 
NSExpression *exp = [pred leftExpression]; 
NSNumber *result = [(NSComparisonPredicate *)exp expressionValueWithObject:nil context:nil]; 
NSLog(@"%@", result); // logs "-1"