Question

The following codes try to generate random strings over K runs. But we want the newly generated strings to be totally different with its reference string.

For that I tried to use ‘continue’ to restart the random string generation process. However it doesn’t seem to work. What’s wrong with my approach below?

#include <iostream> #include <vector> #include <fstream> #include <sstream> #include <time.h> using namespace std;   // In this code we want to print new string that is entirely different with   // with those in initVector    template <typename T> void  prn_vec(std::vector < T >&arg, string sep='') {   // simple function for printing vector     for (int n = 0; n < arg.size(); n++) {         cout << arg[n] << sep;      } }   int main  ( int arg_count, char *arg_vec[] ) {      // This is reference string     vector <string> initVec;     initVec.push_back('A');     initVec.push_back('A');     initVec.push_back('A');     initVec.push_back('A');      vector <string> DNA;       DNA.push_back('A');       DNA.push_back('C');       DNA.push_back('G');       DNA.push_back('T');      for (unsigned i =0; i< 10000; i++) {         vector <string> newString;        for(unsigned j=0; j<initVec.size(); j++) {           int dnaNo = rand() % 4;          string newBase = DNA[dnaNo];          string oldBase = initVec[j];           int sameCount = 0;          if (newBase == oldBase) {             sameCount++;          }           if (sameCount == initVec.size()) {               continue;          }           newString.push_back(newBase);         }         cout << 'Run ' << i << ' : ';        prn_vec<string>(newString);        cout << endl;      }      return 0; } 

Answer 1

Your code looks fine on first glance, unless I am missing a big part of your requirements. Read this before you use rand(). Except of course, the continue part. What you are trying to do is see if this is the same as the initVector or not, right? A simple comparison would do before you push it in or print to the console.

int sameCount = 0; if (newBase == oldBase) {  sameCount++; } // sameCount can be 1 at most, 0 otherwise // this check never return true if (sameCount == initVec.size()) { continue; } 

The sameCount variable is initialized each time you create a new entry to the newString and goes out of scope at the closing } of the for loop. So, it will not be incremented to function as a proper check against duplicate generation. You should ideally, use a std::set and keep inserting in it. Duplicates are not allowed and you are saved from a lot of trouble.

More on using rand() srand() and random number generation:

From the comp.lang.c FAQ:

[…]the low-order bits of many random number generators are distressingly non-random

If you want to keep your randome numbers in the range

[0, 1, ... N - 1] 

a better method compared to the simple rand() % N (as advised in the link) is to use the following:

(int)((double)rand() / ((double)RAND_MAX + 1) * N) 

Now, if you were to run your program, every time you will get the same set of 10000 odd random DNA strands. Turns out this is because:

It’s a characteristic of most pseudo-random number generators (and a defined property of the C library rand) that they always start with the same number and go through the same sequence.

from another FAQ of comp.lang.c.

To get different strands across runs try the following:

#include <iostream> #include <vector> #include <fstream> #include <sstream> #include <ctime> #include <cstdlib> using namespace std;     int main  ( int arg_count, char *arg_vec[] ) {      // most pseudo-random number generators      // always start with the same number and      // go through the same sequence.      // coax it to do something different!     srand((unsigned int)time((time_t *)NULL));      // This is reference string     string initVec('AAAA');         // the family     string DNA('ACGT');      for (unsigned i =0; i< 5; i++) {        string newString;        for(unsigned j=0; j<initVec.size(); j++) {          int dnaNo = (int)((double)rand() / ((double)RAND_MAX + 1) * 4);          char newBase = DNA[dnaNo];                   newString += newBase;        }                // ideally push in a std::set                 // for now keep displaying everything          if (newString != initVec) {                cout << 'Run ' << i << ' : ' << newString << endl;              }          }      return 0; }