Invalid Conversion Problem in C++

Question

I have the following snippet:

string base= tag1[j];

That gives the invalid conversion error.

What's wrong with my code below? How can I overcome it.

Full code is here:

#include <iostream>
#include <vector>
#include <fstream>
#include <sstream>
#include <time.h>
using namespace std;


int main  ( int arg_count, char *arg_vec[] ) {
    if (arg_count < 3 ) {
        cerr << "expected one argument" << endl;
        return EXIT_FAILURE;
    }

    // Initialize Random Seed
    srand (time(NULL));

    string line;
    string tag1     = arg_vec[1];
    string tag2     = arg_vec[2];

    double SubsRate = 0.003;
    double nofTag   = static_cast<double>(atoi(arg_vec[3])); 

    vector <string> DNA;
      DNA.push_back("A");
      DNA.push_back("C");
      DNA.push_back("G");
      DNA.push_back("T");


      for (unsigned i=0; i < nofTag ; i++) {

          int toSub = rand() % 1000 + 1;

          if (toSub <= (SubsRate * 1000)) {
              // Mutate
              cout << toSub << " Sub" << endl;

              int mutateNo = 0;
              for (int j=0; j < tag1.size(); j++) {

                  mutateNo++;


                  string base = tag1[j]; // This fail

                  int dnaNo = rand() % 4;

                  if (mutateNo <= 3) {
                     // Mutation happen at most at 3 position
                        base = DNA[dnaNo];
                  }

                  cout << tag1[j] << " " << dnaNo << " " << base  <<  endl;
                  //cout << base;

              }
               cout << endl;

          }
          else {
              // Don't mutate
              //cout << tag1 << endl;
          }

      }
    return 0;
}

Why do I get an Invalid conversion from char to const char* when looping over a string?

Answer 1

string tag1 = arg_vec[1];

tag1 is a string literal.

string base = tag1[j]; is initialized with a char instead of char *.

Try, char base = tag1[j];

Answer 2

Change it to

char base = tag1[j];

Answer 3

One problem is that the error message says the program expects one argument when it actually requires two. You should probably follow the Unix conventions and show the required usage too (or instead):

if (arg_count != 3) {
    cerr << "Usage: " << arg_vec[0] << " tag1 tag2";
    return EXIT_FAILURE;
}

The names 'argc' and 'argv' are very conventional (and the only major alternative I've seen is 'ac' and 'av'). It might be worth sticking with that.

Answer 4

The std::string operator [] returns a single char. string cannot be instantiated with a single char.

Use:

string base = string( 1, tag1[j] ) instead

Answer 5

There is no constructor for string that takes just a char (which is what tag1[j] is). You have a couple options:

string base;  // construct a default string
base  = tag1[j]; // set it to a char (there is an 
                 //   assignment from char to string, 
                 //   even if there's no constructor

or

string base( 1, tag1[j]);  // create a string with a single char

Or as Josh mentioned, you can define base as a char since you're not performing any string operations on it anyway. If you decide to do this you'll need to change DNA to be a vector<char> (and change the initialization of DNA to using chars instead of strings).