c语言ascii码转换字符串（通过ctype库的字符分类函数来认识128个ASCII字符及其编码规律）

128个ASCII字符（不可打印（控制）或可打印字符）的分类可用以下函数（在类库<cctype>中）区分：

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个ASCII字符的编码并非毫无规律，按十六进制编码去看，非打印控制字符编排在前面，数字、大写字母、小写字母都是一个取整的十六进制数 1开始排在一起。

可打印标点符号（punctuation，上图红色字体）被数字、大写字母、小写字母分成了四块。

如果你去考虑字母大小写的转换，你会看到其编码的精心安排之处：

printf("%d\n",'a'-'A'); // 32, 100000b printf("%c\n",'a'^32); // A, 1000001^100000，相当于加32 printf("%c\n",'A'^32); // a, 1100001^100000，相当于减32

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