C++ passing dynamic arrays to functions

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How do I write a function that takes a dynamic array from the main and creates 2 arrays, 1 for positive values and other for negative and return the two arrays to the main?

this what i've tried:

What I have tried:

C++

#include <iostream>
using namespace std;
void PosNegArr(int* arr, int s, int* p, int* n, int cp, int cn) {

    for (int i = 0; i < s; i++) {
        if (arr[i] < 0) {
            arr[i] = n[i];
        }
        else {
            arr[i] = p[i];
        }
    }
}

int main() {
    int s, c1, c2;

    cout << "Enter number of elements:";
    cin >> s;
    int* A = new int[s];
    cout << "Enter the elements:";
    for (int i = 0; i < s; i++) {
        cin >> A[i];
    }
    for (int i = 0; i < s; i++) {
        if (A[i] < 0) {
            c2++;
        }
        else {
            c1++;
        }
    }
   int* p = new int[c1];
    int* n = new int[c2];
   
    PosNegArr(A, s, p, n, c1, c2);
    cout << "positive elements are :" << endl;
    for (int i = 0; i < c1; i++) {
        cout << p[i] << ' ' << endl;
    }
    cout << "Negative elements are:" << endl;
    for (int i = 0; i < c2; i++) {
        cout << n[i] << ' ' << endl;
    }
}

Posted 12-Mar-20 5:34am

Member 14771149

Updated 12-Mar-20 9:22am

KarstenK

v2

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4 solutions

Solution 4

Richard already gave you the solution. Anyway a bit of modern C++ could be worth to see.

C++

#include <iostream>
#include <vector>
#include <tuple>
#include <algorithm>

using namespace std;

tuple < vector <int>, vector <int> > pos_neg(const vector<int> & v );

int main()
{
  size_t count;
  cout << "Enter number of elements: ";
  cin >> count;
  vector <int > v(count);
  for (size_t n=0; n<count; ++n)
  {
    cin >> v[n];
  }

  auto  [vp, vn] = pos_neg(v);

  cout << "non-negative: ";
  for (const auto & x : vp)
    cout << x << " ";
  cout << "\n";

  cout << "negative: ";
  for (const auto & x : vn)
    cout << x << " ";
  cout << "\n";
}


tuple < vector <int>, vector <int> >  pos_neg(const vector<int> & v )
{
  vector <int> vpos, vneg;
  for_each( v.begin(), v.end(), [&vpos, &vneg](int i){ i < 0 ? vneg.push_back(i) : vpos.push_back(i); } );
  return make_tuple(vpos, vneg);
}

Posted 12-Mar-20 9:23am

CPallini

Updated 14-Mar-20 13:49pm

v2

Comments

Richard MacCutchan 13-Mar-20 5:13am

Cool!

Solution 3

Trial and error is a learning method where you learn from your errors.
The debugger is a tool that allow you to see step by step what your code is doing.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.
[Update]

Quote:
How do I write a function that takes a dynamic array from the main and creates 2 arrays, 1 for positive values and other for negative and return the two arrays to the main?

Just a little writing should have gave you clues to solution:

a[0]=1  => p[0]=1
a[1]=0  => p[1]=0
a[2]=-1 => n[0]=-1
a[3]=3  => p[2]=3
a[4]=-3 => n[1]=-3
a[5]=1  => p[3]=1

it look obvious that you need 3 indexes for a, p and n.

Posted 12-Mar-20 8:35am

Patrice T

Updated 14-Mar-20 14:03pm

v2

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Richard MacCutchan · Accepted Answer · 2020-03-12T08:08:00

The instructions say that the function should create the new arrays and resturn them to the caller. So you need something like:

C++

void PosNegArr(int* source, int size, int** newpos, int** newneg)
{
    int* positives = new int[size];
    int* negatives = new int[size];
    memset(positives, 0, size);
    memset(negatives, 0, size);

    for (int i = 0; i < size; i++)
    {
        if (source[i] < 0)
        {
            negatives[i] = source[i];
        }
        else
        {
            positives[i] = source[i];
        }
    }
    *newpos = positives;
    *newneg = negatives;
}

You can then call it like:

C++

int* positiveList;
int* negativeList;

// collect input values ...

PosNegArr(array, countize, &positiveList, &negativeList)

// on return the two arrays will be populated with the numbers

John R. Shaw · Accepted Answer · 2020-03-12T06:07:00

Given that he/she does provide enough information to solve the problem I'll leave it there.

Stop and think about the problem. The arguments to "PosNetarr" are not arbitrary, they are met to be used. What are you doing with the argument/parameters? Nothing as far as I see. You are suppose to be copying them to 'p' and 'n', not to the original 'Arr'. The limits on 'p' is 'cp' (count p length) and 'cn' (count n length). You are supposed to be copying elements from 'Arr' to 'p' and 'n', not the other way around.

I was tempted to provide a solution, but I will leave it up to you.

Hint:

C++

if (arr[i] < 0 && n_index < cn)
{
    n[n_index++] = arr[i];
}

C++ passing dynamic arrays to functions

4 solutions

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