计算大数乘法出错

[kingstarwmx]

777777*9999999结果是7777769222223.000576

- (NSString *)createResultString:(NSString *)calcuStr{ NSDecimalNumber *decimalResult = [MSParser parserComputeNumberExpression:calcuStr error:nil]; NSInteger decimalNum = 9; NSDecimal decimal = decimalResult.decimalValue; NSDecimal desDecimal; NSDecimalRound(&desDecimal, &decimal ,decimalNum, NSRoundPlain); NSDecimalNumber *result = [NSDecimalNumber decimalNumberWithDecimal:desDecimal]; NSString *res = [result stringValue]; return res; }