Is it possible to dynamically allocate a vector? - c++

Question

I was told on here to use vectors instead of arrays for this particular solution ( My original solution using to dynamically allocate arrays) my problem is I don't really understand vectors, my book only covers a small section. So, maybe I am doing something wrong? I'm just trying to learn a different way in solving it with vectors. So, when I ask is it possible to dynamically allocate a vector. What I mean is it legal to do this std::vector<int*> vect= nullptr;, if not why? Also it would be really helpful to see an example how to modify my original solution with vectors. I'm a beginner just trying to learn from my mistakes.
#include <iostream>
#include <vector>
void sortAscendingOrder(int*, int );
void LowestTestScore(int*, int);
void calculatesAverage(int*,int);
int main()
{
std::vector<int*> vect= nullptr;
int input;
std::cout << "Enter the number of testscores you want to enter." <<std::endl;
std::cin >> input;
vect = new int[input];
for(int count =0; count < input; count++)
{
std::cout << "Enter the test score" << (count +1) <<":" <<std::endl;
std::cin >> vect[count] ;
while( vect[count] < 0)
{
std::cout <<"You enter a negative number. Please enter a postive number." <<std::endl;
std::cin >> vect[count];
}
}
sortAscendingOrder(vect,input);
for(int count =0; count < input;count++)
{
std::cout << "\n" <<vect[count];
std::cout << std::endl;
}
LowestTestScore(vect,input);
calculatesAverage(vect,input);
return 0;
}
void sortAscendingOrder(int* input,int size)
{
int startScan,minIndex,minValue;
for(startScan =0; startScan < (size-1);startScan++)
{
minIndex = startScan;
minValue = input[startScan];
for(int index = startScan+1;index<size;index++)
{
if(input[index] < minValue)
{
minValue = input[index];
minIndex = index;
}
}
input[minIndex]=input[startScan];
input[startScan]=minValue;
}
}
void LowestTestScore(int* input, int size)
{
int count =0;
int* lowest = nullptr;
lowest = input;
for(count =1; count <size;count++)
{
if(input[count] < lowest[0])
{
lowest[0] = input[count];
}
}
std::cout << "Lowest score" << *lowest;
}
void calculatesAverage(int* input, int size)
{
int total = 0;
int average =0;
for(int count = 0; count < size; count++)
{
total += input[count];
}
average =(total/size-1);
std::cout << "Your average is" << average;
}

Answer 1

Dynamic allocation of an array is required when you want to increase the size of an array at run-time. Just like arrays, vectors used contiguous storage locations for their elements. But, unlike arrays, their size can change dynamically, with their storage being handled automatically by the container.
Vectors do not reallocate each time an element is added to the container. It pre-allocates some extra storage to accommodate future growth. Libraries can implement different strategies for growth to balance between memory usage and reallocation, but in any case, reallocation should only happen at logarithmically growing intervals of size so that the insertion of individual elements at the end of the vector can be provided with amortized constant time complexity (see push_back)
To answer your question:
Yes, it is possible to dynamically allocate a vector and no, it requires no extra effort as std:vector already does that. To dynamically allocate vectors is therefore, a redundant effort.
You can write:
#include<vector>
typedef std::vector< std::vector<double> >vec_array;
vec_array name(size_x, std::vector<double>(size_y));
This means: size instances of a std::vector<double>, each containing size_y doubles (initialized to 0.0).
Or, you can simply use vector to increase the container size (knowing it handles storage requirements by itself) like:
#include <vector>
vector<int> vec_array;
int i;
for(i=0;i<10;i++)
vec_array.push_back(i);