cant find error in my applied logic - c++

Question

question:
Given an array of elements of length N, ranging from 0 to N-1, your task is to write a program that rearranges the elements of the array. All elements may not be present in the array, if element is not present then there will be -1 present in the array. Rearrange the array such that A[i] = i and if i is not present, display -1 at that place.
my code:
#include <iostream>
using namespace std;
int main()
{
long int n;
cin>>n;
long int a[n];
for(int i=1;i<=n;i++)
{
cin>>a[i];
}
for (int i=1;i<=n;i++)
{
while(a[i]==i&&a[i]==-1)
{
int temp=a[i];
a[i]=a[temp];
a[temp]=temp;
}
}
for(int i=1;i<=n;i++)
cout<<a[i]<<" ";
return 1;
}
output:
6
-1 4 2 3 -1 5
-1 4 2 3 -1 5
can anyone please help me in finding out my error in the logic apllied?
Thanks in advance.

Answer 1

for(int i=1;i<=n;i++)
Wrong, elements go from 0 to N-1, not 1 to N
while(a[i]==i&&a[i]==-1)
This will never happen, you are asking for a[i] to be equal to both i and -1, which means asking i to be equal to -1, which won´t happen in your loop.
For a simple answer, you need to sort the list and then process that. For an efficient answer, you will want to make a boolean array of size N and then iterate the array and check which values are present. Then you iterate the boolean array to write the number when it is present or -1 when its not.

Answer 2

The easiest way to solve this kind of problem is to get a pack of playing cards and lay them out on your desk, and then solve the problem by hand. Write down every step you take, and then write code that performs those steps.
Because cards start at 1, rather than 0, I use the 10 as a 0. Use the joker to indicate an empty space (-1 in your problem description).
Take five cards and lay them out in six spaces
2 10 4 J 3 1
Starting with position 0. Pull the 2 out and replace it with -1, so your cards look like this:
J 10 4 J 3 1
And you're holding the 2 in your hand.
Then, go to position 2, pull out the 4 and put the 2 there. Now you have
J 10 2 J 3 1
And you're holding 4 in your hand. Go to position 4 (where the 3 is). Replace the 3 with 4, you have:
J 10 2 J 4 1
And 3 in the hand. Position 3 contains a joker. So you put the 3 in that position and put the joker aside. You now have:
J 10 2 3 4 1
So you move to the next position, 1. Pick up the 10 and put a joker in that spot. The 10 goes to position 0, so take the joker from position 0, place the 10 there, and you have:
10 J 2 3 4 1
You don't have anything in your hand now, so you move forward, checking positions 2, 3, and 4, which already have a[i] == i. But position 5 contains a 1. So you pick it up, place a joker in that position, and then replacing the joker in position 1 with the value 1 that you just pulled from position 5. Your array now looks like this:
10 1 2 3 4 J
And you're done.
Do that a few times with different arrangements of cards, writing down the steps you took. After a few practice runs, you should be able to write down the general algorithm for solving the problem. Then you write a program to implement that solution.
The idea of this kind of problem is to help you develop these problem solving steps. Over time, you'll be able to go straight to code with the simpler problems, but you'll find that building a physical model is very useful with more complex problems. If you step away from the computer, you're not tempted to start programming before you've solved the problem. You'll find that doing things way will save you a lot of frustration.