Question

const char chars[] = "abcdef ...."; 
char result[...];
memcpy(result, chars, sizeof(chars)); 
for (unsigned i = 0; i < (sizeof(chars)-1); ++i) {
    unsigned j = rand() % sizeof(chars);
    char tmp = result[j];
    result[j] = result[i];
    result[i] = tmp;
} 

problem writing the result to a text file.

Answer 1

Your problem is that the character value 0 isn’t mapped to A. Rather, a character which will will be printed as A is equal to 65. You can see the full table here, if you ever need it.

Anyway, the code solution is simple.
Instead of

cout<<letter[i]<<" ";

You can do:

cout<<letter[i]+'A'-1<<" ";

(the -1 is because you numbered the letters from 1 to 26 instead o 0 to 25).