C++ - Randomly select strings without selecting it more than once - c++

Question

I have a hand full of objects each containing several strings each. Right now its set up as structures each containing a map with keys 1...n, one for each string (map<int,string> strs), this can be changed if a better way exists. I need to randomly access all of these str ings without overlapping and know that i'm done. How can I do this, either with the maps or another data structure? Thanks.

Answer 1

Here's some code for a Fisher-Yates shuffle:
template <class T>
std::vector<T> shuffle(std::vector<T> &vect)
{
std::vector<T> shuffled = vect;
for(int i = shuffled.size()-1; i >= 1; i--) {
int idx = rand() % (i+1);
T tmp = shuffled[idx];
shuffled[idx] = shuffled[i];
shuffled[i] = tmp;
}
return shuffled;
}
This will take in a vector, and return a copy of it in a random order. If you have a vector of strings, you can use it like this (I'm using c++11 here):
int main()
{
srand(time(NULL));
std::vector<std::string> strs = {"foo", "bar", "baz", "stack", "overflow"};
for(auto &str : shuffle(strs)) {
std::cout << str << std::endl;
}
return 0;
}
Of course, if you're lazy, like me, there's always the random_shuffle() function in <algorithm>:
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
int main()
{
std::vector<std::string> strs = {"foo", "bar", "baz", "stack", "overflow"};
std::random_device rd;
std::mt19937 g(rd()); // Use a good random number generaor
std::random_shuffle(strs.begin(), strs.end(), g); // this does the shuffle
for(auto &str : strs) {
std::cout << str << std::endl;
}
return 0;
}
Hope this helps!

Answer 2

A horrible solution, don't do this. Very slow for large candidate vectors, this has n-squared complexity. Shuffling is better, it has linear complexity.
std::vector<int> RandomThing(int number, int min, int max)
{
assert(!"RandomThing" && min < max);
std::vector<int> candidates;
for(int i=min; i<max; i++)
candidates.push_back(i);
std::vector<int> result;
for(int i=0; i<number;)
{
int candidate_index = rand() % candidates.size();
result.push_back(candidates[candidate_index]);
std::vector<int>::iterator it = candidates.begin();
std::advance(it, candidate_index);
candidates.erase(it);
}
return result;
}