你好，有人介意解释一下这段代码是什么意思吗？

Question

Essentially, my friend wrote this code and did what i had done in these three lines whereas mine was nearly a page, i don't understand what it means could someone help please.

msg[i] = toupper(msg[i]);
msg[i] = ((msg[i] - 'A')+rotation)%26;
printf("%c", calpha[msg[i]]);

Answer 1

// convert a - z character to upper case A - Z
msg[i] = toupper(msg[i]);

// A - Z values are 65 - 90. Subtract from uppercase character 65 (which is A)
// which moves it to 0 - 25 range. Add some shift value to it (rotation)
// %26 brings it back to 0 - 25 range.
msg[i] = ((msg[i] - 'A')+rotation)%26;

// print calpha value for given index (our 0 - 25 result)
printf("%c", calpha[msg[i]]);