Dynamic array from an input file

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.

Use a more recent and C++11 standard conforming compiler (e.g. GCC), and compile with all warnings and debug info (g++ -Wall -g). Then use std::vector. Learn how to use the debugger (gdb) — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 20, 2014 at 10:57
Other than that, your parts of your code make no sense (The thing with the return value...) — deviantfan
– deviantfan, Commented Oct 20, 2014 at 10:58
You are reading your file to an unalloacated memory 'arr'. First allocate your memory and then read — 999k
– 999k, Commented Oct 20, 2014 at 10:58

Nard · Accepted Answer · 2014-10-20 10:58:07Z

1

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

answered Oct 20, 2014 at 10:58

Nard

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3 Comments

Yann Over a year ago

Yeah, I have no idea why I thought that. I'm having a look around to see what could have lead me to believe that.

Nard Over a year ago

It's a fair concern, just wasn't a valid argument in this case. C does not allow non-const integrals for array size during declaration.

Yann Over a year ago

THAT'S IT. I was trying to remember which language it was.

Dr. Debasish Jana · Accepted Answer · 2014-10-20 11:01:24Z

0

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;
}

answered Oct 20, 2014 at 11:01

Dr. Debasish Jana

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Comments

Yann · Accepted Answer · 2014-10-20 10:58:59Z

0

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;

answered Oct 20, 2014 at 10:58

Yann

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Comments

Lewis · Accepted Answer · 2014-10-20 11:28:42Z

0

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;
}

answered Oct 20, 2014 at 11:28

Lewis

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