Question

I wrote a C++ program with the goal of quickly inserting an element into a sorted vector. It works sometimes but not all of the time and I have not been able to figure out why. When I follow the algorithm with a paper and pencil it works out but something is wrong. Please help?

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

vector<int> sortedVec;

int main() {
    // Random seed
    srand(time(NULL));

    // Put in n random elements
    for (int i = 0; i < 10; i++) sortedVec.push_back(rand()%10);

    // Sort the vector
    bool swapped = true;
    int endDecrement = 0;
    while (swapped) {
        swapped = false;
        endDecrement++;
        for (int i = 0; i < sortedVec.size()-endDecrement; i++) {
            if (sortedVec.at(i) > sortedVec.at(i+1)) {
                int swap = sortedVec.at(i);
                sortedVec.at(i) = sortedVec.at(i+1);
                sortedVec.at(i+1) = swap;
                swapped = true;
            }
        }
    }

    cout<<"Sorted random list:"<<endl;
    for (int i = 0; i < sortedVec.size(); i++) cout<<sortedVec.at(i)<<endl;

    int toInsert = rand()%10;
    cout<<"Random element to insert = "<<toInsert<<endl;

    // Insert a random int to the sorted vector
    int minIndex = 0;
    int maxIndex = sortedVec.size()-1;
    while (true) {
        int mid = (maxIndex-minIndex)>>1;
        if (toInsert == sortedVec.at(mid) || maxIndex-minIndex < 2) {
            sortedVec.insert(sortedVec.begin()+mid, toInsert);
            break;
        }
        else if (toInsert < sortedVec.at(mid)) maxIndex = mid;
        else if (toInsert > sortedVec.at(mid)) minIndex = mid;
    }

    cout<<"Random list with inserted element:"<<endl;
    for (int i = 0; i < sortedVec.size(); i++) cout<<sortedVec.at(i)<<endl;

    return 0;
}

Answer 1

I changed it and am pretty sure it works in all cases now:

if (toInsert < sortedVec.at(0)) sortedVec.insert(sortedVec.begin(), toInsert);
else if (toInsert > sortedVec.at(sortedVec.size()-1)) sortedVec.push_back(toInsert);
else {  
    int minIndex = 0;
    int maxIndex = sortedVec.size();
    while (true) {
        int mid = (maxIndex+minIndex)>>1;
        if (toInsert == sortedVec.at(mid) || maxIndex-minIndex < 2) {
            sortedVec.insert(sortedVec.begin()+mid+1, toInsert);
            break;
        }
        else if (toInsert < sortedVec.at(mid)) maxIndex = mid;
        else if (toInsert > sortedVec.at(mid)) minIndex = mid;
    }
}

@LeGEC and @chac, thank you for looking into the algorithm I posted, it helped a lot. @Jerry Coffin, I did get std::upper_bound to work for this but not for a variable of my Node class, thank you as well though.