Array without a specified size without using vector [duplicate] - c++

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How do I use arrays in C++?
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Why aren't variable-length arrays part of the C++ standard?
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Closed 1 year ago.
I am trying to make a program that reads files with numbers and sort the numbers with different algorithms and there is multiple files that are going to be read and each file has a different amount of integers. so what i need to do is read those integers and cast them into a array.
but for some reason in c++ you cant have a array with an undefined size so what is a solution that i can use? And i can't use vectors (school project)
Here is my program
#ifndef SORT_H
#define SORT_H
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
// =============================================================================
// FILE READER
// =============================================================================
void fileReader(string fileName, int arr[]){
ifstream myfile(fileName);
int pos = 0;
string number;
if(myfile.is_open()){
while (getline(myfile, number))
{
arr[pos] = stoi(number);
pos++;
}
}
}
// =============================================================================
// SWAPER
// =============================================================================
void swap(int* a, int* b)
{
int temp = *a;
*a = *b;
*b = temp;
}
// =============================================================================
// QUICKSORT
// =============================================================================
int partition(int arr[], int beg, int end){
int pivot = arr[end];
int index = beg - 1;
for(int j = beg; j < end; j++){
if(arr[j] <= pivot){
index++;
swap(&arr[index], &arr[j]);
}
}
swap(&arr[index+1], &arr[end]);
return (index+1);
}
void quicksortAlgo(int arr[], int beg, int end){
if(beg < end){
int pi = partition(arr, beg, end);
quicksortAlgo(arr, beg, pi-1);
quicksortAlgo(arr, pi+1, end);
}
}
template <typename T>
void quicksort(T arr[], int n)
{
quicksortAlgo(arr, 0, n-1);
for(int x = 0; x < n; x++)
cout << arr[x] << endl;
}
// =============================================================================
// HEAPSORT
// =============================================================================
void heapify(int arr[], int n, int i){
int max = i;
int left = 2*i + 1; // Left side
int right = 2*i + 2; // Right side
cout << "max: " << arr[max] << endl;
// if there is a left child for root and if is
// bigger then root
if(left < n && arr[max] < arr[left]){
max = left;
}
// If there is a right child of root and if is
// bigger then root
if(right < n && arr[max] < arr[right]){
max = right;
}
if(max != i){
swap(&arr[i], &arr[max]);
heapify(arr, n, max);
}
}
void heapsortAlgo(int arr[], int n){
for (int i = n/2 - 1; i >= 0; i--){
heapify(arr, n, i);
}
for (int i = n-1; i >= 0; i--){
swap(&arr[0], &arr[i]);
heapify(arr, i, 0);
}
}
template <typename T>
void heapsort(T arr[], int n)
{
heapsortAlgo(arr, n);
for (int i = 0; i < n; i++){
cout << arr[i] << " ";
}
}
// =============================================================================
// INTSERTIONSORT
// =============================================================================
template <typename T>
void insertionsort(T arr[], int n)
{
int holePostion;
int valueToInsert;
for (int i = 1; i < n; i++){
valueToInsert = arr[i];
holePostion = i;
while(holePostion > 0 && arr[holePostion-1] > valueToInsert){
arr[holePostion] = arr[holePostion - 1];
holePostion = holePostion - 1;
}
arr[holePostion] = valueToInsert;
}
}
int main(){
string filedest;
string arrSize;
cout << "enter file destenation: ";
cin >> filedest;
int arr[];
int n = sizeof(arr) / sizeof(arr[0]);
fileReader(filedest, arr);
quicksort(arr, n);
return 0;
}
#endif

Answer 1

As #MikeCAT mentioned, you can use std::vector<T>.
You can add push elements at the back of the vector by std::vector<T>::push_back().
Alternatively, you can also resize the vector array by using std::vector<T>::resize() and then add elements at a specific location similar to what you are doing in your fileReader() function. You can also insert elements at a specific location by calling std::vector<T>::insert()
.
Do have a look at the time complexities if you have a time constraint in your programming task.