Question on generating weird elements

[shamieh]

I need to generate a vector such that [6, 5, 4, 3, 2, 1, 7, 8, 9, . . . , n − 4, n − 3, n, n − 1, n − 2] for n = 100, 200, 400, 800, 1600, 3200, 6400.

For example: for n = 16,
[6,5,4,3,2,1,7,8,9,10,11,12,13,16,15,14]

I can't understand the pattern here. I know I can code this if someone can explain to me the pattern.

[dhayden]

The first 6 elements are 6, 5, 4, 3, 2, 1. The next elements are 7, 8, 9, ... n. So to populate the vector, use 2 loops. The first loop does the first 6 elements. The next loop does elements 7 through n

[shamieh]

That would never work, also - note: the last 3 elements are weirdly placed such that n, n-1, n-2

[shamieh]

Here is what I ending up using as my solution dhayden. Were you thinking of something faster? I know you are alot more versed in c++ then me, I've seen you help others before. Thanks in advance. This is what I'm currently using.

#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;

void printvector(vector<int> v)
{
	if (v.size() == 0)
	{
		cout << "Empty" << endl;
		return;
	}
	for (int i = 0; i < v.size(); i++)
	{
		cout << v[i] << " ";
	}
}
int main()
{
	vector<int>test;
	int N = 16;
	int t = N;
	t = t / 2;
	int tempSize;
	bool firstCase = true;
	int counter = 3;

	for (int i = 0; i < (t - 1); i++)
	{
		test.push_back(N - counter);
		counter++;
	}

	tempSize = counter - 4;
	cout << "counter = " << counter << endl;
	reverse(test.begin(), test.end());

	//Add extra 3 cases
	//n, n-1, n-2
	test.push_back(N);
	test.push_back(N - 1);
	test.push_back(N - 2);

	counter = 1;
	for (int i = 0; i < tempSize; i++)
	{
		test.insert(test.begin(), counter);
		counter++;
	}
	cout << endl;
	printvector(test);
	
	system("pause");
}

[JLBorges]

#include <iostream>
#include <vector>

int main()
{
    for( int n : { 10, 16, 22, 27 } ) if( n > 9 )
    {
        std::vector<int> seq { 6, 5, 4, 3, 2, 1 } ; // initialise with [1,6] in reverse

        for( int v = 7 ; v < n-2 ; ++v ) seq.push_back(v) ; // append [7,n-3]

        seq.insert( seq.end(), { n, n-1, n-2 } ) ; // append [n-2,n] in reverse

        std::cout << "n == " << n << "  =>  [ " ;
        for( int v : seq ) std::cout << v << ' ' ;
        std::cout << "]\n" ;
    }
}

http://coliru.stacked-crooked.com/a/0fcc6db20a9613a3

[shamieh]

Yeah but isn't your way slower since your algorithm is O(n^2) (quadratic) and mine is only O(n)?

[JLBorges]

#include <iostream>
#include <vector>

int main()
{
    const int N = 16 ;

    std::vector<int> seq { 6, 5, 4, 3, 2, 1 } ; // initialise with [1,6] in reverse

    for( int v = 7 ; v < N-2 ; ++v ) seq.push_back(v) ; // append [7,N-3]

    seq.insert( seq.end(), { N, N-1, N-2 } ) ; // append [N-2,N] in reverse

    for( int v : seq ) std::cout << v << ' ' ;
    std::cout << '\n' ;
}

http://coliru.stacked-crooked.com/a/fcb6a47ed4d49189