C ++动态数据–如何获取数据以及如何摆脱数据

Question

The code below – it's a skeleton of a program operating on the dynamic collection of data. The idea is to use a structure containing two fields: the first stores the number of elements in collections, and the second is the actual collection (a dynamically allocated vector of ints). As you can see, the collection is filled with the required amount of pseudo-random data. Unfortunately, the program requires completion, as the most important function.

Here's what i expect from the function:

1. if the collection is empty, it should allocate a one-element vector and store a new value in it.

2. if the collection is not empty, it should allocate a new vector with a length greater by one than the current vector, then copy all elements from the old vector to the new one, append a new value to the new vector and finally free up the old vector.

    #include <iostream> #include <cstdlib> #include <ctime> using namespace std; struct Collection { int elno; int *elements; }; void AddToCollection(Collection &col, int element) { //the first part of the funtion if (col.elno==0){ col.elements= new int[1]; col.elements[0]= element; } //this is the second part but i do not know how to do it. //Please help me to complete*************** else { int *temp; temp = new[]; } } void PrintCollection(Collection col) { cout << "[ "; for(int i = 0; i < col.elno; i++) cout << col.elements[i] << " "; cout << "]" << endl; } int main(void) { Collection collection = { 0, NULL }; int elems; cout << "How many elements? "; cin >> elems; srand(time(NULL)); for(int i = 0; i < elems; i++) AddToCollection(collection, rand() % 100 + 1); PrintCollection(collection); delete[] collection.elements; return 0; } 

Answer 1

vector container is originally dynamic container. so u can use vector.

Just declare vector variable in structure and use it in AddToCollection function.

struct Collection {
    int elno;
    std::vector<int> elements;
};
void AddToCollection(Collection &col, int element) {
    col.elements.push_back(element);
    col.elno++;
}

like this.

Answer 2

Here is what you are looking for:

void AddToCollection(Collection &col, int element)
{
    if(col.elements == NULL)
    {
        col.elements = new int[1];
        col.elements[0] = element;
        col.elno = 1;
    }
    else
    {
        int *newArr = new int[col.elno+1];
        for(int i = 0; i < col.elno; i++)
        {
            newArr[i] = col.elements[i];
        }
        newArr[col.elno] = element;

        delete[] col.elements;
        col.elements = new int[col.elno+1];
        for(int i = 0; i < col.elno+1; i++)
        {
            col.elements[i] = newArr[i];
        }

        delete[] newArr;
        newArr = NULL; // avoid dangling pointer
        col.elno++;
    }
}

For sure using vector container is a great ideea but the exercise require no modification to the main function. The objective of this exercise is to help the student to understand dynamically allocated memory .