Printing My Vector Doesn't Work (No Compiler Errors)

Question

What I must make:

A program that makes a double vector with at least 30 negative and positive elements. I also must have functions to calculate/print the standard deviation, the mean value, the highest value, and the lowest value of the vector. Next, I will need a split() function to split the values of the double vector into 2 new double vectors, positive and negative (the former will have all values from 0 - max_positive_value, the latter will have all nonzero negative values). Finally, I need one last function to display both of the modified vectors.

My Issue:

Nothing whatsoever is being printed from my split() function! For debugging purposes, I made it so that it prints all the values of the positive and negative vectors, but it prints nothing. The compiler makes no mention of any error of any kind, which doesn't really help!

Code Block in Question:

double split(const vector<double> &data)
{
    vector <double> positive;
    vector <double> negative;
    int j = 0; // iterate inner

    // add to pos/neg vector depending on value of data[i]
    for (int i = 0; i < data.size(); ++i)
    {

        if (data[i] >= 0)
        {
            positive[j] = data[i];
            j++;

        } else if (data[i] < 0)
        {
            negative[j] = data[i];
            j++;
        }
    }

    // debugging, print vectors
    for (int vC = 0; vC < positive.size(); ++vC)
    {
        cout << positive[vC] << endl;
    }

    for (int vC = 0; vC < negative.size(); ++vC)
    {
        cout << positive[vC] << endl;
    }

    return 0;

}

Full code:

#include <iostream>
#include <vector>
#include <ctime> // both this and the next header are needed for
#include <cstdlib> // rand() to randomly generate numbers 
#include <cmath> // sqrt

using namespace std;

// passing by reference uses less resources
double split(const vector<double> &data)
{
    vector <double> positive;
    vector <double> negative;
    int j = 0; // iterate inner

    // add to pos/neg vector depending on value of data[i]
    for (int i = 0; i < data.size(); ++i)
    {

        if (data[i] >= 0)
        {
            positive[j] = data[i];
            j++;

        } else if (data[i] < 0)
        {
            negative[j] = data[i];
            j++;
        }
    }

    // debugging
    for (int vC = 0; vC < positive.size(); ++vC)
    {
        cout << positive[vC] << endl;
    }

    for (int vC = 0; vC < negative.size(); ++vC)
    {
        cout << positive[vC] << endl;
    }

    return 0;

}

// calculate std deviation
double calculateSD(const vector<double> &data)
{
    // calculate sum of data
    double sum = 0.0;
    for (size_t i = 0; i < data.size(); ++i)
        sum += data[i]; 

    // calculate average
    double avg = sum / data.size();
 
    // calculate and return the standard deviation
    double stdDev = 0.0;
    for (size_t i = 0; i < data.size(); ++i)
        stdDev += (data[i] - avg) * (data[i] - avg);
    return sqrt(stdDev / data.size());
}



int main()
{
    // vector with minimum 30 values
    vector <double> data (30);

    // seed rand() with time
    srand (time(NULL));
    
    // fill vector with neg/pos num
    for (int i = 0; i < data.size(); ++i)
    {
        data[i] = rand() - (RAND_MAX / 2);
        // cout << data[i] << endl;
    }

    split(data);

}

NOTE: I have not implemented the needed highest/lowest/mean functions, yet.

Answer 1

Your vectors

vector <double> positive;
vector <double> negative;

are empty, so accessing their elements is undefined behavior. You need

if (data[i] >= 0)
{
    positive.push_back(data[i]);
} else if (data[i] < 0)
{
    negative.push_back( data[i]);          
}