Swift switch判断rating空值未进入默认分支问题求解

• 空字符串匹配异常：直接用字符串做大小比较时，遵循字典序比较规则（逐位对比字符ASCII值），空字符串""的优先级低于所有非空字符串，必然满足"" <= "1.9"的判断条件，因此不会进入default分支。这种写法还存在隐藏逻辑bug：评分值为"10.0"时，字符串比较会判定"10.0" < "1.9"（第一位字符同为"1"，第二位字符"0"的ASCII值小于"9"），判断结果完全错误。
• 强制解包崩溃：当user_details为nil，或其rating属性为nil时，user_details?.rating的返回值是nil，对nil值做强制解包!会直接触发运行时崩溃。

核心修正两点：

1. 前置拦截所有异常场景：把nil、空字符串、非数字格式的评分值统一归类到全空星级的展示逻辑
2. 把评分从字符串转为Double类型做数值比较，彻底规避字符串字典序比较的逻辑错误

修复后的代码如下：

// 前置校验：过滤空值、非法格式值
guard let ratingStr = user_details?.rating, 
      let rating = Double(ratingStr) else {
    // 所有异常场景统一展示全空星级
    star1.image = UIImage(named: "staremp")
    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
    return
}

// 基于数值做区间判断，逻辑准确无歧义
switch rating {
case 1.0..<1.5:
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
case 1.5..<2.0:
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star-half")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
case 2.0..<2.5:
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
case 2.5..<3.0:
    star1.image = UIImage(named: "star")
    star2.image = UIImage(named: "star")
    star3.image = UIImage(named: "star-half")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
// 可按相同区间规则补全3分、4分、5分的对应分支
default:
    star1.image = UIImage(named: "staremp")
    star2.image = UIImage(named: "staremp")
    star3.image = UIImage(named: "staremp")
    star4.image = UIImage(named: "staremp")
    star5.image = UIImage(named: "staremp")
}

内容的提问来源于stack exchange，提问作者Swift