128個ASCII字符（不可打印（控制）或可打印字符）的分類可用以下函數（在類庫<cctype>中）區分：

isalnum() Check if character is alphanumeric (function )

isalpha() Check if character is alphabetic (function )

isblank() Check if character is blank (function )

iscntrl() Check if character is a control character (function )

isdigit() Check if character is decimal digit (function )

isgraph() Check if character has graphical representation (function )

islower() Check if character is lowercase letter (function )

isprint() Check if character is printable (function )

ispunct() Check if character is a punctuation character (function )

isspace() Check if character is a white-space (function )

isupper() Check if character is uppercase letter (function )

isxdigit() Check if character is hexadecimal digit (function )

128個ASCII字符的編碼并非毫無規律，按十六進制編碼去看，非打印控制字符編排在前面，數字、大寫字母、小寫字母都是一個取整的十六進制數 1開始排在一起。

可打印标點符号（punctuation，上圖紅色字體）被數字、大寫字母、小寫字母分成了四塊。

如果你去考慮字母大小寫的轉換，你會看到其編碼的精心安排之處：

printf("%d\n",'a'-'A'); // 32, 100000b printf("%c\n",'a'^32); // A, 1000001^100000，相當于加32 printf("%c\n",'A'^32); // a, 1100001^100000，相當于減32

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