C++ Proper way of creating a dynamic array from a file [duplicate]

Question

I'm a beginner and maybe the solution here is easier that I think it is, but I have spend a lot of time trying to fix this problem. The main goal is to create a dynamic array from a file.

I have this code:

#include <iostream>
#include <fstream> 

using namespace std;

void out(int *arr, int siz){
    for (int i = 0; i < siz; i++) {
        cout << arr [i] << " ";
        if((i + 1) % 5 == 0){
            cout << endl;
        }        
    }
}

int main(){
    int *arr;
    int i = 1;
    int len = 0;

    ifstream fin;
    fin.open("input.txt");
    int s; 
    fin >> s;     
    arr = new int[s];

    while(!fin.eof()){ 
        fin >> arr[i]; 
        len++;       
        i++;
    }
    fin.close(); 

    out(arr, len); 
    delete[] arr;

    return 0;
}

And file looks like this:

But the output looks like this:

0 84 65 80 62 
98 2 4 75 69

The problem is that it counts first element as 0, instead of 11.

Expected output:
    11 84 65 80 62 
    98 2 4 75 69

I have tried changing parts of the code, but I still didn't manage to find the proper solution. Do you have any ideas what I'm doing wrong?

The first element in your array is 0 because i is initialized to 1 (arrays in C++ starts at 0) — Frodyne
– Frodyne, Commented May 30, 2022 at 10:36

Frodyne · Accepted Answer · 2022-05-30 11:16:16Z

1

You can do something like this:

#include <iostream>
#include <fstream> 
#include <vector>

void out(const std::vector<int>& arr) {
    for (size_t i = 0; i < arr.size(); i++) {
        std::cout << arr[i] << " ";
        if ((i + 1) % 5 == 0) {
            std::cout << std::endl;
        }
    }
}

int main() {
    std::vector<int> arr;

    std::ifstream fin("input.txt");

    for (int data; fin >> data;)
        arr.push_back(data);

    fin.close();

    out(arr);
}

I changed the C-style array to an std::vector and changed the loop condition (both Remy Lebeau and I posted comments detailing why this is correct). This in turn made your "off by one" indexing error go away (the cause of the initial 0).

answered May 30, 2022 at 11:16

Frodyne

4,02310 silver badges16 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Remy Lebeau · Accepted Answer · 2022-05-30 10:38:28Z

0

use a vector;

#include <vector>

int main(){

    vector<int> arr;

    ifstream fin;
    fin.open("input.txt");
    int s; 
    

    while(!fin.eof()){
        int temp; 
        fin >> temp;
        arr.push_back(temp);
    }
    fin.close(); 

    for(auto v:arr) cout<<v<<"  ";
    cout<<endl;
    arr.clear();

    return 0;
}

edited May 30, 2022 at 10:38

Remy Lebeau

609k36 gold badges516 silver badges875 bronze badges

answered May 30, 2022 at 10:31

satyam

214 bronze badges

1 Comment

Remy Lebeau Over a year ago

Why is iostream::eof inside a loop condition (i.e. while (!stream.eof())) considered wrong?

Collectives™ on Stack Overflow

C++ Proper way of creating a dynamic array from a file [duplicate]

2 Answers 2

Comments

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Linked

Related