iOS计算器: 表达式的精准计算（计算字符串数学表达式）MathStringExpression

 NSDecimalNumber* computeResult = [MSParser parserComputeNumberExpression:floatString error:nil];
    
    NSDecimal decimal = computeResult.decimalValue;
    NSDecimal desDecimal;
    NSDecimalRound(&desDecimal, &decimal , 2, NSRoundPlain);
    result = [[NSDecimalNumber decimalNumberWithDecimal:desDecimal] stringValue];

@interface QCTQCTCalculatorViewTableViewCell ()



/**
 负责计算表达式的值
 */
@property (strong, nonatomic) CQTMoneyCalculator *moneyCalculator;

/**
 计算器视图（读者自己实现，后续考虑开源）
 */
@property (weak, nonatomic) QCTCalculatorView *cellView;

/**
 负责计算表达式的值
 */
@property (strong, nonatomic) CQTMoneyCalculator *moneyCalculator;

result = [ self.moneyCalculator kn_evaluateExpression:strArr];

- (NSString *)kn_evaluateExpression:(NSString *)expressionString {
    if (expressionString.length<=0) {
        return nil;
    }
    
    // 处理非法的表达式：最后一个字符是 -/*.+ 其中的一种就return 去掉；目的是去除运算符
    
    
            if ([@".+-÷/*×x" containsString:[expressionString substringFromIndex:expressionString.length-1]]) {//÷
                
                
                // 写一个递归计算
                
                NSString *tmp = [expressionString substringToIndex:expressionString.length-1];
                
//                NSString *tmp = [expressionString substr

                 return [self kn_evaluateExpression: tmp];
                
            }
    
    
    NSString *sanitizedString = [self sanitizedString:expressionString];
    NSString *floatString = [NSString stringWithFormat:@"1.0*%@", sanitizedString];//1.0*1+1+9/8
    
    NSString * result = nil;
    
    NSDecimalNumber* computeResult = [MSParser parserComputeNumberExpression:floatString error:nil];
    
    NSDecimal decimal = computeResult.decimalValue;
    NSDecimal desDecimal;
    NSDecimalRound(&desDecimal, &decimal , 2, NSRoundPlain);
    result = [[NSDecimalNumber decimalNumberWithDecimal:desDecimal] stringValue];
    
    NSLog(@"保留2位小数计算结果为：%@",result);
    
    
    return result;
}