CS50 Pset2 Substitution check50报output not valid ASCII text错误

问题现象 #

程序本地运行输出看似符合预期，但check50检测存在2个失败用例，核心异常特征：

• 使用全大写密钥时，所有加密测试正常通过
• 使用全小写密钥、大小写混合密钥加密时，报错output not valid ASCII text

完整check50检测结果：

:) substitution.c exists
:) substitution.c compiles
:) encrypts "A" as "Z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
:) encrypts "a" as "z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
:) encrypts "ABC" as "NJQ" using NJQSUYBRXMOPFTHZVAWCGILKED as key
:) encrypts "XyZ" as "KeD" using NJQSUYBRXMOPFTHZVAWCGILKED as key
:) encrypts "This is CS50" as "Cbah ah KH50" using YUKFRNLBAVMWZTEOGXHCIPJSQD as key
:( encrypts "This is CS50" as "Cbah ah KH50" using yukfrnlbavmwzteogxhcipjsqd as key
    output not valid ASCII text
:( encrypts "This is CS50" as "Cbah ah KH50" using YUKFRNLBAVMWZteogxhcipjsqd as key
    output not valid ASCII text
:) encrypts all alphabetic characters using DWUSXNPQKEGCZFJBTLYROHIAVM as key
:) does not encrypt non-alphabetical characters using DWUSXNPQKEGCZFJBTLYROHIAVM as key
:) handles lack of key
:) handles too many arguments
:) handles invalid key length
:) handles invalid characters in key
:) handles duplicate characters in key
:) handles multiple duplicate characters in key

根因分析 #

核心bug出在subs函数生成小写密钥映射数组的逻辑中，括号位置书写错误：

// 错误代码：括号位置错误，将key[i3] > 0的布尔结果传给了islower
if (islower(key[i3] > 0)) {
    LOWER[i3] = key[i3];
}
else {
    LOWER[i3] = key[i3] + 32;
}

错误逻辑导致：

1. islower接收到的参数永远是key[i3] > 0的布尔值（0或1），这两个值都不属于小写字母ASCII范围，因此判断永远为假，固定走else分支
2. 当密钥为全大写时，大写字母ASCII值+32刚好对应小写字母ASCII值，结果巧合正确，因此全大写密钥的测试全部通过
3. 当密钥包含小写字母时，小写字母ASCII值（97_{122）+32得到129}154，超出标准可打印ASCII范围（0~127），因此触发output not valid ASCII text报错

另外代码中还存在一个潜在逻辑漏洞：key_check函数仅判断了密钥长度小于26的场景，未判断长度大于26的场景，虽然当前测试用例未触发该问题，但不符合题目要求。

修复方案 #

1. 修正小写映射数组的判断条件括号位置，确保islower正确接收密钥字符作为参数：

// 修正后代码
if (islower(key[i3]) > 0) {
    LOWER[i3] = key[i3];
}
else {
    LOWER[i3] = key[i3] + 32;
}

2. 补全密钥长度校验逻辑，将长度判断从「小于26报错」改为「不等于26报错」：

// 修正后长度校验
if (leng != 26) {
    printf("Key must contain 26 characters.\n");
    return 1;
}

修复完成后重新编译运行，所有check50用例即可正常通过。

内容的提问来源于stack exchange，提问作者Gabriel Accetta