Partition of Array by Element given X - c++

Question

I Am Trying To Find Partition Of Array ,On Condition By Checking Variable x ,when less then x they will be on one side or else on another. but my code need some correction.
HERE am not able to find the error , i will be thankful to you if you help me.
Code is:-
#include<iostream>
using namespace std;
int partition(int arr[],int n,int x){
for(int i=0;i<n;){
if(arr[i]<x){
i++;
}
else if(arr[i]==x){
int temp=arr[i];
arr[i]=arr[n];
arr[n]=temp;
i--;
}
else if(arr[i]>x){
int temp=arr[i];
for(int j=i;j<n;j++){
arr[j]=arr[j+1];
}
arr[n]=temp;
i--;
}
}
return 0;
}
int main(){
int n;
cin>>n;
int arr[n];
for(int i=0;i<n;i++){
cin>>arr[i];
}
int x;
cin>>x;
partition(arr,n,x);
for(int i=0;i<n;i++){
cout<<arr[i]<<"\t";
}
return 0;
}
Input >> array={2,10,15,1,3,15} ,x=10
Expected << {2,1,3,10,15,15}
Output I get << nothing .

Answer 1

The code isn't giving any output because, first, the "cin" and "cout" are in upper case which is syntactically incorrect, secondly, the variable j is in different case in loop statement and body inside the second else-if clause in the partition function, same goes for the "I" in the first for loop in the main() function. Sort this out and you should be good to go.

Answer 2

First in C++ the size of an array must be a compile-time constant. So for example, consider the following examples:
int n = 10;
int arr[n]; //INCORRECT
The correct way to write the above would be:
const int n = 10;
int arr[n]; //CORRECT
Similarly, in your code,
int n;
cin>>n;
int arr[n]; //INCORRECT because n is not a constant expression
Second, in your code, when you wrote:
arr[n] = temp; Undefined behavior
you're going out of bounds and so you have undefined behavior.
Solution
You can use std::stable_partition and std::vector to solve your problem as shown below:
#include <iostream>
#include <vector>
#include <algorithm>
int main()
{
int n;
std::cout <<"Enter n:"<<std::endl;
std::cin >> n;
std::vector<int> arr(n); //create a vector of size n instead of an array
std::cout<<"Enter elements: "<<std::endl;
//iterate and take input from user
for(int &elem: arr){
std::cin >> elem ;
}
int x;
std::cout << "Enter x: "<<std::endl;
std::cin>>x;
//use std::partition
std::stable_partition(arr.begin(), arr.end(), [x](int i){return (i < x);});
std::cout<<"This is the partitioned vector: "<<std::endl;
for(int i=0;i<n;i++)
{
std::cout<<arr[i]<<"\t";
}
return 0;
}
Output
The output of the above program is as follows:
Enter n:
6
Enter elements:
2
10
15
1
3
15
Enter x:
10
This is the partitioned vector:
2 1 3 10 15 15
which can be seen here.