Extremely slow random string generator - c++

Question

I came up with the below code to generate 100001 random strings.the strings should be unique. However, the below code takes hours to do the job. Can someone let me know how i can optimize it and why is it so slow?
string getRandomString(int length) {
static string charset = "abcdefghijklmnopqrstuvwxyz";
string result;
result.resize(length);
for (int i = 0; i < length; i++) {
result[i] = charset[rand() % charset.length()];
}
return result;
}
void main(){
srand(time(NULL));
vector<string> storeUnigrams;
int numUnigram = 100001;
string temp = "";
int minLen = 3;
int maxLen = 26;
int range = maxLen - minLen + 1;
int i =0;
while(i < numUnigram){
int lenOfRanString = rand()%range + minLen;
temp = getRandomString(lenOfRanString);
bool doesithave = false;
for(int j =0 ; j < storeUnigrams.size() ; j++){
if(temp.compare(storeUnigrams[j]) == 0){
doesithave = true;
break;
}
if(temp.compare(storeUnigrams[j]) < 0){
break;
}
}
if(!doesithave){
storeUnigrams.push_back(temp);
sort(storeUnigrams.begin(),storeUnigrams.end());
i++;
}
}

Answer 1

There are two factors that make your code slow:
Checking by linear search whether the string already exists – O(n)
Sorting the vector in each iteration – O(n log n)
Use e.g. a set for storing the strings – it's sorted automatically, and checking for existence is fast:
int main(){
srand(time(NULL));
set<string> storeUnigrams;
int numUnigram = 100001;
int minLen = 3;
int maxLen = 26;
int range = maxLen - minLen + 1;
while(storeUnigrams.size() < numUnigram){
int lenOfRanString = rand()%range + minLen;
storeUnigrams.insert(getRandomString(lenOfRanString));
}
}

Answer 2

This code generates a unique random number only once and stores it in random_once[i].
The first for loop generates ad stores the random number.
The second for loop is used to get the pre-rendered random numbers stored in the random_once[i] array.
Yes generating 100001 random numbers will take hours if not days.
#include <ctime>
#include <iostream>
using namespace std;
int main()
{
int numUnigram = 3001;
int size=numUnigram;
int random_once[100001];
cout<<"Please wait: Generatng "<<numUnigram<<" random numbers ";
std::cout << '-' << std::flush;
srand(time(0));
for (int i=0;i<size;i++)
{
//This code generates a unique random number only once
//and stores it in random_once[i]
random_once[i]=rand() % size;
for(int j=0;j<i;j++) if (random_once[j]==random_once[i]) i--;
//loading animation
std::cout << "\b\\" << std::flush;
std::cout << "\b|" << std::flush;
std::cout << "\b/" << std::flush;
std::cout << "\b-" << std::flush;
}
cout<<" \n";
// this code dispays unique random numbers stored in random_once[i]
for ( i=0;i<size;i++) cout<<" "<<random_once[i]<<"\t";
cout<<" \n";
return 0;
}

Answer 3

Philipp answer is fine. Another approach would be to use a Self-balancing Binary Search Tree like Red Black Tree instead of Vector. You can perform search and insets in log(n) time. If search is empty, insert the element.

Answer 4

Define your variables outside the while loop - because they are getting redefined at each iteration
int lenOfRanString = rand()%range + minLen; ;
bool doesithave = false;
Update
Thought it's advised in many books, in practice with all the new compilers, this will not significantly improve the performance

Answer 5

Use char arrays instead of strings (the string class does a lot of stuff behind the scenes)