copying elements from one array to another

Question

I am getting a crash error at run time and not sure what exactly to do with the function or how to get the data for it.

FUNCTION DETAILS

Write a function that accepts an int array and size as arguments, then create a new array that is one element bigger than the given. Setting the first element to 0, then copying over what is in the argument array to the new array.

MAIN DETAILS

Use in a program reading int n from input, then read int n from file data name data passing it to element shifter, then printing it to output (one per line).

#include <cstdlib>
#include <iostream>
#include <fstream>
using namespace std;

int element_shift(int elmts[], int size) {
  int new_size = size + 1;
  int shifter[new_size];
  int *elmt_sft;
  shifter[0] = 0;
  for (int i = 1; i >= new_size; i++) {
    shifter[i + 1] = elmts[i];
  }
  return *elmt_sft;
}

int main() {
  fstream infile;
  infile.open("D:\\data.txt");
  int n, x;
  infile >> x;
  cout << "size of array: ";
  cin >> n;
  const int ARRAY_SIZE = n + x;
  int elements[ARRAY_SIZE];
  element_shift(elements, ARRAY_SIZE);

  system("PAUSE");
  return EXIT_SUCCESS;
}

I'd recommend reading an introductory book on C++, [look here](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — Begemoth, Apr 18 '12 at 04:56
Couple things to think about: 1. `for(int i=1;i>=new_size;i++)` think about the comparisons (also keep in mind arrays start at zero). 2. `return *elmt_sft;` When you dereference this pointer, where is it pointing to? — Jesse Good, Apr 18 '12 at 04:57

score 1 · Answer 1 · answered Apr 18 '12 at 05:10

First of all ARRAY_SIZE declared in the main function is not a constant variable but defined at run-time depending on user inputs. This means that the array elements should be created dynamically. On the other hand you read some x variable which is only used to define the size of the array and didn't initialized the array at all. I guess that the problem statement is to read the size of the array from the input, then the data of the array from the file.

There are also lot of mistakes in element_shift function.

Your code should look like something similar to this:

#include <cstdlib>
#include <iostream>
#include <fstream>
using namespace std;

void element_shift(int* elmts, int size)
{
    int new_size = size + 1;
    int* shifter = new int[new_size];
    shifter[0] = 0;
    for(int i = 0; i < size; ++i)
    {
        shifter[i + 1] = elmts[i];
    }
    delete [] elmts;
    elmts = shifter;
}


int main()
{
    fstream infile;
    infile.open("D:\\data.txt");

    int n;
    cout << "size of array: ";
    cin >> n;

    int* elements = new int[n];

    for (int i = 0; i < n; ++i) {
        infile >> elements[i];
    }

    element_shift(elements, n);

    for (int i = 0; i < n; ++i) {
        std::cout << elements[i] << std::endl;
    }

    return EXIT_SUCCESS;
}

score 0 · Answer 2 · edited Oct 19 '13 at 09:30

First off, you spend alot of time creating the shifted array but don't return it back.

int element_shift(int elmts[], int size) {
  int new_size = size + 1;
  int shifter[new_size];
  int *elmt_sft;
  shifter[0] = 0;
  for (int i = 1; i >= new_size; i++) {
    shifter[i + 1] = elmts[i];
  }
  return *elmt_sft;
}

The elmt_sft pointer is never assigned. You are trying to access memory that is not there by using *elmt_sft. This may be causing your error. Also this function has no way of returning the new array shifter because that variable is locally declared and will disappear once the function exits. If you want to create something new in the function and still have it in memory once the function exits, I recommend creating the array dynamically and returning a pointer to it.

This is untested but should start you in the right direction. It will return a separate dynamically allocated array that will not override your other one.

int* element_shift(int elmts[], int size) {
  int *result_array = new int[size + 1]; //dynamically create new array MAKE SURE TO DELETE
  result_array[0] = 0; //set 0 index to 0
  for (int i = 1; i < size + 1; i++)//start at 1 of the result and put value in
  {
    result_array[i] = elmts[i - 1];
  }
  return result_array; //returning pointer
}

copying elements from one array to another

FUNCTION DETAILS

MAIN DETAILS

2 Answers2