将选择的文字从大写字母转换为小写字母（反之亦然）；
Convert a selection of text from uppercase to lowercase (or vice versa).
返回一个字符串，该字符串中的所有字母都被转换为小写字母。
Returns a string where all alphabetic characters have been converted to lowercase.
这个例程检查字符是否在A到Z之间，然后将其转换为小写字母并将它直接和其他字符比较。
This routine checks to see if a character is between a and Z, then converts it to lowercase and compares it; it compares other characters directly.
它把这个传递给toupper函数，如果你们使用过，它表示的是把小写字母,转换为大写字母。
It's passing to this function called toupper, which if you've never used it it actually does what it says it makes it touppercase.
它把这个传递给toupper函数，如果你们使用过，它表示的是把小写字母,转换为大写字母。
It's passing to this function called toupper, which if you've never used it it actually does what it says it makes it touppercase.
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