Programming

#include <iostream>
#include <string>
#include <ctype.h>

using namespace std;

// Making it global is important if you want unused elements
// to default to 0 and not garbage!
unsigned char m[256];

// Passing by reference avoids making a copy of a copy of a copy.
void revcomp(const string &input, string &output) {
        int n=input.length(), p=-1;

        output=input;
        while(n > 0) output[++p]=m[toupper(input[--n])];
}

int main(int argc, char *argv[])
{
        string str = argv[1], str2;
        // Do this once, and you can call revcmp as many times as you want without repeating it.
        m['A'] = 'T'; m['T'] = 'A'; m['C'] = 'G'; m['G'] = 'C';

        revcomp(str, str2);

        cout << str << " = " << str2 << endl;

        return 0;
}

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])];
}

char trans(char c) {
        if(c == 'C') return('G');
        else if(c == 'c') return('G');
        else if(c == 'G') return('C');
        else if(c == 'g') return('C');
        else if(c == 'A') return('T');
        else if(c == 'a') return('T');
        else if(c == 'T') return('A');
        else if(c == 't') return('A');
}

char m[256]={ ['A']='T' };