Finds the non-repeated number in an array

array_pop.c

#include <stdio.h>
#include <stdlib.h>
_Bool in_array();
int *pop_array();

void main() {
   int numbers[] = { 1, 2, 2, 4, 4, 1, 5, 6, 5 };
   int length = sizeof( numbers ) / sizeof( numbers[ 0 ] );
   int lucky = 0;
    
   for( int i = 0; i < length; i++ ) {
       int *n_a = pop_array( i, numbers, length );
       
       if( !in_array( numbers[ i ], n_a, (length-1) ) ) {
            lucky = numbers[ i ];
        }
        free( n_a );
    }
    
    printf( "Lucky number is %d", lucky );
}

_Bool in_array( int value, int *a, int length ) {
   
    for( int i = 0; i < length; i++ ) {
        
        if( value == *( a+i ) )
            return 1;
    }
    
    return 0;
}

int *pop_array( int index, int a[], int length ) {
    int *new_array = malloc( sizeof( int ) * (length ) );
    int i = 0;
    int p_i = 0;
    
    while( i < length ) {
        if( i == index ) {
            *(new_array+i) = a[i+1];
            i++;
            continue;
        }
        *(new_array+p_i) = a[i];
        i++;
        p_i++;
    }
    
    return new_array;
}