Using functions to create and initialize an array from main in Visual Studio 2017

Question

The elements of the Array are not being initialized, because when I run the code I only see the values from the memory and not the values that I set in the vector. Can you explain to me why is this happening and what should I do in future to avoid this mistake? Please ignore the comments from my code

#include <iostream>
using namespace std;

//ne va crapa deoarece nu am initializat valorile vectorului, de asta cream o functie de initializare vector

void initialisingVector(int* vector, int n, int InitialValue)
{
    for (int i = 0; i < n; i++)
    {
        vector[i] = InitialValue;
    }

}
//mereu vom pune initializarea inainte de afisare

void displayingtheVector(int vector[], int n)
{
    cout << endl << "The vector is:";
    for (int i = 0; i < n; i++)
    {
        cout << vector[i] << ",";
    }
}

int* creatingAndInitialisingtheVector(int* n)//nu avem nevoie de nr de elemente deoarece le ia din main;si nici de vector;
{//nu avem nevoie de nimic la parametrii, totusi punem n deoarece ne da eroare-n main, ai comentat acolo

//vom folosi in* n ca parametru deoarece n e o valoare ce se va modifica , prin urmare e transmisa prin pointer

//vom avea eroare cu expected ; ,nu are legatura cu ; problema este de la int[] ,expected an identifier
//nu o sa putem nici o data pune int[] la tipul unei functii, mereu vom pune int*, int[]---sa incercam sa uitam forma asta;
//INTOTDEAUNA int*

    int vectorNou[30];
    int numberElements;
    int InitialValue;

    std::cout << std::endl << "Number of elements:";
    std::cin >> numberElements;
    *n = numberElements;

    std::cout << std::endl << "Initial value is:";
    std::cin >> InitialValue;


    initialisingVector(vectorNou, numberElements, 0);
    //afisareVector(vectorNou, numarElemente); nu are sens sa o facem in fct de creare

    return vectorNou;
}


int main()
{
    int listaPreturi[30];
    int numarProduse;

    /*std::cout << std::endl<<"Numarul de produse este:";
    std::cin >> numarProduse;

    initializareVector(listaPreturi, numarProduse, 0);
    afisareVector(listaPreturi,numarProduse);
    */

    //listaPreturi=crearesiInitializareVector();//ne va trebui o variabila care sa prinda vectorul creat de functie,de asta punem listaPreturi
    //vom obtine o alta eroare dupa ce egalam variabila cu functia; expression must be a modifiable lvalue==>listaPreturi e o variabila constanta, si trebuie sa fie modifiable
    //dc avem o fct care inhtoarce un vector nu vom putea prinde vectorul nou intr-un alt vector deja creat static.
    //il va prinde u8n vetor dinamic in schimb
    int nrElements = 0;
    int* vectorNew = creatingAndInitialisingtheVector(&nrElements);//functioneaza

    //afisareVector(vectorNou, sizeof(vectorNou)); e periculos folosind sizeof(vectorNou) deoarece vectorNou are 4 , utilizatoru poate vrea mai multe elemente

    displayingtheVector(vectorNew, nrElements);
}

I expect that the vector with the values set by the user to be displayed, and not the memory addresses.

With the code you show, what is the output you get, and what output did you expect? Please copy-paste the actual output into the question, and paste the actual output and modify it with the expected values. — Some programmer dude, Oct 21 '19 at 17:10
@Someprogrammerdude do you want me to write by hand the memory adresses from the compiler? — Corina, Oct 21 '19 at 17:14
[Enable warnings on your compiler](https://stackoverflow.com/questions/57842756/why-should-i-always-enable-compiler-warnings). It will tell you what you are doing wrong. — walnut, Oct 21 '19 at 17:15
@uneven_mark I don't have compiler errors, nor is the code crashing — Corina, Oct 21 '19 at 17:16
@Corina That may be the case, but your program still has [undefined behavior](https://stackoverflow.com/questions/2397984/undefined-unspecified-and-implementation-defined-behavior) and if you enable warnings your compiler will give you a warning about it (although warning messages may be hard to understand, in that case google for them and if necessary ask about them here). Also read the links I posted. — walnut, Oct 21 '19 at 17:17

Vlad from Moscow · Accepted Answer · 2019-10-21T17:19:01.500

The program has undefined behavior because the function creatingAndInitialisingtheVector returns pointer to the local array

int vectorNou[30];

that will not alive after exiting the function.

It seems you should declare the function with one more parameter the following way

size_t creatingAndInitialisingtheVector( int *a, size_t n );

and call the function in main like

size_t nrElements = 
    creatingAndInitialisingtheVecto( listaPreturi, sizeof( listaPreturi ) / sizeof( *listaPreturi ) );

The function definition can look like

size_t creatingAndInitialisingtheVector( int *a, size_t n)//nu avem nevoie de nr de elemente deoarece le ia din main;si nici de vector;
    {//nu avem nevoie de nimic la parametrii, totusi punem n deoarece ne da eroare-n main, ai comentat acolo

    //vom folosi in* n ca parametru deoarece n e o valoare ce se va modifica , prin urmare e transmisa prin pointer

    //vom avea eroare cu expected ; ,nu are legatura cu ; problema este de la int[] ,expected an identifier
    //nu o sa putem nici o data pune int[] la tipul unei functii, mereu vom pune int*, int[]---sa incercam sa uitam forma asta;
    //INTOTDEAUNA int*

        size_t numberElements;
        int InitialValue;

        std::cout << std::endl << "Number of elements:";
        std::cin >> numberElements;

        if ( n < numberElements ) numberElements = n;

        std::cout << std::endl << "Initial value is:";
        std::cin >> InitialValue;

        initialisingVector( a, numberElements, InitialValue);
        //afisareVector(vectorNou, numarElemente); nu are sens sa o facem in fct de creare

        return numberElements;
    }

score 0 · Answer 2 · answered Oct 21 '19 at 17:18

Inside of your function creatingAndInitialisingtheVector you are returning a pointer to memory that will go out of scope once the function returns. The memory is being allocated for the scope of the function and is freed after returning. I would recommend reading up on scope and how memory is allocated for things such as arrays in C++.

Using functions to create and initialize an array from main in Visual Studio 2017

2 Answers2