Thread: another dynamic memory allocation problem

the question is;
Write a program that lets users keep track of the last time they talked to each of their friends. Users should be able to add new friends (as many as they want!) and store the number of days ago that they last talked to each friend. Let users update this value (but don't let them put in bogus numbers like negative values). Make it possible to display the list sorted by the names of the friends of by how recently it was since they talked to each friend.

Below is what I have done. The problem I am having is that the array will not grow in size. (Another problem which I have not tackled yet is how to remember if the same user updates their noDays figure, but will give that a go after the array grows);

Code:

#include <iostream>
#include <string>


using namespace std;


int *growArray (int * friends, int * size)
{
    *size *= 2;
    int *new_friends = new int[*size];
    for (int i=0; i < *size; i++) {
        new_friends[i] = friends[i];
    }
    delete [] friends;
    return new_friends;
}


string *growArray2 (string * name1, int * size)
{
    *size *= 2;
    string *new_name2 = new string[*size];
    for (int i=0; i < *size; i++) {
        new_name2[i] = name1[i];
    }
    delete [] name1;
    return new_name2;
}


void printArray (string *name1, int *friends,int size, int element_set)
{
    cout << "the total size of the array is: " << size << endl;
    cout << "Values in the array: " << endl;
    for (int i = 0; i < element_set; ++i)
    {
        cout << name1[i] << "[" << i << "] = " << friends[i] << " days since you last spoke to them" << endl;
    }
}


int main()
{
    string exit = "X";
    string name;
    int noDays;
    int next_element = 0;
    int size = 4;
    int *friends = new int[size];
    string *name1 = new string [size];
        cout << "Please enter your friends name" << endl;
        cin >> name;
        cout << "Please enter no of days you last spoke to them" << endl;
        cin >> noDays;
    while(true) {
        if (size == next_element+1) {
            friends = growArray(friends, &size);
            name1 = growArray2(name1, &size);
        }


        friends[next_element] = noDays;
        name1[next_element] = name;
        next_element++;
        printArray(name1, friends, size , next_element);
        cout << "Please enter your friends name or X to exit" << endl;
        cin >> name;
        if (name == exit) {
            break;
        }
        cout << "Please enter no of days you last spoke to them" << endl;
        cin >> noDays;


        }
        delete name1;
        return 0;
}

Also I am not supposed to use vectors this is why it is written like this. It is a chapter on dynamic memory allocation and pointers

Have you considered using a structure or class to hold the friend information instead of the parallel arrays?

In your growArray() functions you are trying to access your array name1[] out of bounds. You have incremented the size of the array but remember name1[] only has the original size number of elements.

Jim

Thanks for the help initially I wrote a different code with a structure but I must have been coding it wrong because it didn't work. I have rectified my code now and change the pointer of "size" to an address. The arrays are growing however it is not printing out the value of the grown array as it would if it were a pointer. Is this correct so far (I still need to sort out if a user puts in multiple values and update them and print them in ascending order) ? I am struggling with this one. Everyone I speak to says use vectors so I may skip ahead to that chapter but want to try at least (also classes have not been covered yet this is why I haven't used any).

Code:

#include <iostream>
#include <string>


using namespace std;


int *growArray (int *friends, int &cur_size)
{


    int *new_friends = new int[(cur_size==0 ? 1 : cur_size*2)];
    for (int i=0; i < cur_size; ++i) {
        new_friends[i] = friends[i];
    }
    //*cur_size = (*cur_size==0 ? 1 : *cur_size*2); //added here
    delete []friends;
    return new_friends;
}


string *growArray2 (string *name1, int &cur_size2)
{


    string *new_name2 = new string[(cur_size2==0 ? 1 : cur_size2*2)];
    for (int i=0; i < cur_size2; ++i) {
        new_name2[i] = name1[i];
    }
    //*cur_size2 = (*cur_size2==0 ? 1 : *cur_size2*2);


    delete []name1;
    return new_name2;
}


void printArray (string *name1, int *friends, int &cur_size3, int element_set)
{


    cout << "the total size of the array is: " << cur_size3 << endl;
    cout << "Values in the array: " << endl;
    for (int i = 0; i < element_set; ++i)
    {
        cout << name1[i] << "[" << i << "] = " << friends[i] << " days since you last spoke to them" << endl;
    }
}


int main()
{
    string exit = "X";
    string name;
    int size = 10;
    int noDays;
    int next_element = 0;
    int *friends = new int[size];
    string *name1 = new string [size];
        cout << "Please enter your friends name" << endl;
        cin >> name;
        cout << "Please enter no of days you last spoke to them" << endl;
        cin >> noDays;
    while(true) {


        if (size==next_element+1) {


            friends = growArray(friends, size); //changed here
            name1 = growArray2(name1, size); //changed here


        }
        friends[next_element] = noDays;
        name1[next_element] = name;
        next_element++;
        printArray(name1, friends, size , next_element);
        cout << "Please enter your friends name or X to exit" << endl;
        cin >> name;
        if (name == exit) {
            break;
        }
        cout << "Please enter no of days you last spoke to them" << endl;
        cin >> noDays;
    }
        //printArray(name1, friends, size , next_element);
        return 0;
}

Have you set a breakpoint and run this program through your debugger, watching the variables as you single step through the program?

Jim

No I have not, I have not even been taught debugging yet it is a later chapter in the e-book. I am beginning to think maybe it is time to skip ahead and read through, as it would make some previous problems easier.

Learn to use the debugger, the sooner the better.

Jim

I have managed to sort it, but it is incrementing the "size" of the array by 1 each time, which I thought wastes memory? There were no errors when I ran the debugger

Code:

 
#include <iostream>
#include <string>


using namespace std;


int *growArray (int *friends, int &cur_size)
{


    int *new_friends = new int[(cur_size==0 ? 1 : cur_size*2)];
    for (int i=0; i < cur_size; ++i) {
        new_friends[i] = friends[i];
    }
    //*cur_size = (*cur_size==0 ? 1 : *cur_size*2); //added here
    delete []friends;
    return new_friends;
}


string *growArray2 (string *name1, int &cur_size2)
{


    string *new_name2 = new string[(cur_size2==0 ? 1 : cur_size2*2)];
    for (int i=0; i < cur_size2; ++i) {
        new_name2[i] = name1[i];
    }
    //*cur_size2 = (*cur_size2==0 ? 1 : *cur_size2*2);
    delete []name1;
    return new_name2;
}


void printArray (string *name1, int *friends, int &cur_size3, int element_set)
{


    cout << "the total size of the array is: " << cur_size3 << endl;
    cout << "Values in the array: " << endl;
    for (int i = 0; i < element_set; ++i)
    {
        cout << name1[i] << "[" << i << "] = " << friends[i] << " days since you last spoke to them" << endl;
    }
}


int main()
{
    string exit = "X";
    string name;
    int size = 0;
    int noDays;
    int next_element = 0;
    int *friends = new int[size];
    string *name1 = new string [size];
        cout << "Please enter your friends name" << endl;
        cin >> name;
        cout << "Please enter no of days you last spoke to them" << endl;
        cin >> noDays;
    while(true) {


        size=next_element+1;
        friends = growArray(friends, size); //changed here
        name1 = growArray2(name1, size); //changed here
        friends[next_element] = noDays;
        name1[next_element] = name;
        next_element++;
        printArray(name1, friends, size , next_element);
        cout << "Please enter your friends name or X to exit" << endl;
        cin >> name;
        if (name == exit) {
            break;
        }
        cout << "Please enter no of days you last spoke to them" << endl;
        cin >> noDays;
    }
        //printArray(name1, friends, size , next_element);
        return 0;
}