What is a character but a smaller integer? Single-character constants count as integers.

4 is not enough: printf("%d %d %d %d\n", 'A', 'T', 'C', 'G'); prints 65 84 67 71 You will note these correspond exactly to the ASCII character table.

The array is a look-up table, like a Caesar cipher, big enough to cover the entire ASCII range. This is very simple to do and lets you translate with a single instruction.

It's also one reason it should be a global variable. Global variables are guaranteed to have any contents you don't initialize, set to binary 0. Not that we're using any other contents, but if garbage gets in, it's reassuring to know garbage won't come back out.

The same reason I used strdup -- to make a new string of correct length. I don't really care what's in it as long as it's long enough. I think this is more efficient than calling string.push_back() 570 times.

But strings have a lot of properties of vectors, so I could also have done this:

Code:

void revcomp(const string &input, string &output) {
        int n=input.length(), p=-1;
        output.resize(n);
        while(n > 0) output[++p]=m[toupper(input[--n])];
}

...which is probably better, especially if input happens to be large. No point copying it then just overwriting it.