Dynamic array from an input file - c++

Question

I'm a beginner, so I'm sorry if this is really dumb question/problem.
The assignment that I have is printing out a dynamic array from an input file. I tried googling it and I found some similar problems... but the answers were all like "use vectors" etc but we haven't learned those yet. It's also said that a function must be used. This is what I came up with:
#include <iostream>
#include <fstream> //file input
using namespace std;
int *out(int *arr, int siz){
arr = new int[siz];
for (int i = 0; i < siz; i++) {
cout << arr [i] << " ";
}
return arr; //this should print out the array later???
}
int main(){
int siz;
int *arr;
ifstream inf ("input.txt");
inf >> siz; //
for (int i = 0; i < siz; i++) {
inf >> arr[i];
}
inf.close();
cout << "This array contains following elements: ";
*arr = *out(arr, siz) ;
delete[] arr;
return 0;}
So, it doesn't give any errors with Dev-C++ but when I try to run it, it crashes. I tried debugging it and then it gave me "segmentation error" or something like that. Then of course, I googled it and there must be something wrong with the pointers, right? Could you help me out? Thanks.

Answer 1

You are trying to access arr, when arr has not been allocated or initialized to a valid array. Your main needs to allocate arr before using arr to populate elements:
So, here's the changed version:
#include <iostream>
#include <fstream> //file input
using namespace std;
void out(int *arr, int siz){
for (int i = 0; i < siz; i++) {
cout << arr [i] << " ";
}
}
int main(){
int siz;
int *arr;
ifstream inf ("input.txt");
inf >> siz;
arr = new int[siz]; // added
for (int i = 0; i < siz; i++) {
inf >> arr[i];
}
inf.close();
cout << "This array contains following elements: ";
out(arr, siz);
delete[] arr;
return 0;
}

Answer 2

arr is an uninitialized pointer.
Do arr = new int[size]; before you read data into arr.

Answer 3

You haven't allocated memory to the array, which you'd likely need to do with malloc. Once you've read in the size of the array, allocate the memory.
inf >> siz;
arr = malloc(siz * sizeof(*int));
//Rest of program
//delete[] arr; <- you use this with the new keyword
free(arr); //Use 'free' with malloc
return 0;

Answer 4

I think what you want might be sth like this
#include <iostream>
#include <fstream>
int main(){
int siz(0);
std::ifstream inf ("input.txt");//Assume that the input file and this file are in the same folder
inf >> siz; //Assume that the first number in input file is the size of array
int *arr=new int[siz];
for (int i = 0; (siz-i)&&inf ; ++i) {
inf >> arr[i];
}
inf.close();
std::cout << "This array contains following elements: ";
for (int i = 0; siz -i ; ++i ) {
std::cout << arr [i] << " ";
}
delete[] arr;
return 0;
}