为什么在for循环中不能将i作为index方法的参数使用

错误根因 #

待解密的字符串包含空格字符，你定义的alphabet数组中未收录空格，调用alphabet.index(i)查找空格索引时会返回nil，nil无法参与数值运算，因此抛出异常。

修复方案 #

方案1：调整字母表适配所有字符 #

在字母表中增加空格元素，同步修改模运算的除数即可：

message = "rvtufkmbrdpr fmasmegppreemvemeuaiv tmgb"
messageArray = message.chars
decrypted = ""
# 末尾新增空格元素，总长度变为28
alphabet = ["","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"," "]
for i in(messageArray)
    letter = alphabet[((alphabet.index(i)) + 14) % 28]
    decrypted += letter
end
puts decrypted

方案2：单独处理空格字符 #

无需修改原有字母表逻辑，碰到空格直接跳过位移步骤，原样保留即可：

message = "rvtufkmbrdpr fmasmegppreemvemeuaiv tmgb"
messageArray = message.chars
decrypted = ""
alphabet = ["","a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
for i in(messageArray)
    if i == " "
        decrypted += " "
        next
    end
    letter = alphabet[((alphabet.index(i)) + 14) % 27]
    decrypted += letter
end
puts decrypted

两种方案都可以解决index方法调用报错的问题，运行后可正常输出解密后的字符串。

内容的提问来源于stack exchange，提问作者Hugo Salcedo