Stuck with spiral triangle - c++

Question

I am looking for an algorithm that numbers and outputs a triangle with sides n (or half of a square) where n is an input of the program. But the numbering starts at the top of the triangle, goes down the diagonal, back along the bottom row and up the left edge. If there is an interior remaining it goes diagonally down from the highest number and continues.
Here is an example:
1
9 2
8 10 3
7 6 5 4
Here is my code and it results in:
1
10 2
9 8 3
7 6 5 4
Is there any algorithm for this of program if there any please explain to me.
The above program works well with row size less than 3 but doesn't for size above 3.
#include<iostream.h>
#include<conio.h>
void main()
{
int n,i,j,v=0;
static int k;
clrscr();
cout<<"Enter the number of rows : ";
cin>>n;
for(i=0;i<n;i++)
{
for(j=0;j<=i;j++)
{
v++;
}
}
for(i=0;i<n;i++)
{
for(j=0;j<i;j++)
{
cout<<v;
cout<<"\t";
v--;
}
while(k==i)
{
k++;
cout<<k;
cout<<"\t";
}
cout<<"\n";
}
getch();
}

Answer 1

Here's a solution that doesn't use any array storage. The spiral can be thought of as a set of right angled triangles inside each other. The function iterates over all the rows and columns and for each position, it calculates what triangle the element is on by finding the closest distance to the edge of the outer triangle, then computes its adjusted position (x, y) relative to the top-left corner of that inner triangle, the number of rows of the inner triangle (r), and the start number of the inner triangle (start+1). Then it outputs a number based on whether it lies on the diagonal, horizontal or vertical side.
#include <iostream>
#include <iomanip>
using namespace std;
int main(void) {
int rows;
cout << "Enter the number of rows : ";
cin >> rows;
int i, j;
for(i = 0; i < rows; i++)
{
for(j = 0; j <= i; j++)
{
// find the closest side:
int distance = j; // distance to vertical side
if(i-j < distance)
distance = i-j; // distance to diagonal side
if((rows-1)-i < distance)
distance = (rows-1)-i; // distance to horizontal side
int r = rows - distance * 3;
// compute position on inner triangle:
int x = j - distance;
int y = i - distance * 2;
// compute start number for inner triangle:
int start = (((rows+1)*rows)/2) - (((r+1)*r)/2);
// output number based on side:
if(x==y) // diagonal side
cout << setw(2) << (start+y+1) << " ";
else if(y==(r-1)) // horizontal side
cout << setw(2) << (start+(r*2)-(x+1)) << " ";
else // vertical side
cout << setw(2) << (start+(r*3)-(y+2)) << " ";
}
cout << endl;
}
return 0;
}
Demo
Take the example where rows equals 7. In that case the distance value for each element will be:
(0)
0 0
0 (1) 0
0 1 1 0
0 1 (2) 1 0
0 1 1 1 1 0
0 0 0 0 0 0 0
All elements with the same distance value form a triangle. The number of rows of the outer triangle is 7, the next smaller one has 4, then 1. So r = rows - (distance * 3).
The top-left corner of the outer triangle is at row 0, column 0. The first inner triangle is row 2, column 1, the next one is at row 4, column 2. So the position of a given row/column position on the inner triangle that it lies on is found by subtracting distance * 2 from the row and distance from the column, so y = i - (distance *2) and x = j - distance.
The inner triangle column is stored in x. The inner triangle row is stored in y. In the example above, the values in brackets are the top-left corners of each triangle, where x = 0 and y = 0. For example for the top-left corner of the triangle with distance = 1, i = 2 and j = 1, so x = 1 - 1 = 0, and y = 2 - (1 * 2) = 0.
The start value is found by calculating the number of elements in the entire large triangle ((row+1)*row)/2 and then subtracting the number of elements remaining, which is the number of elements in the inner triangle.
For a triangle with n rows, the total number of elements is ((n+1)*n)/2, as shown below for rows = 5:
1 X 0 0 0 0 0
2 X X 0 0 0 0
3 X X X 0 0 0
4 X X X X 0 0
5 X X X X X 0
1 2 3 4 5 6
To count the number of X's, we can see that it's half the number of elements in a rectangle of (5+1)*5, so half of 30, which is 15.
If there are 2 triangles one inside the other, like this:
X
X X
X O X
X O O X
X X X X X
and we want to count the number of X's, then we can calculate the size of the entire triangle using the above formula to get 15, and then calculate the size of the inner triangle which has 2 rows as ((2+1)*2)/2 = 3, and subtracting the smaller from the larger gives 15 - 3 = 12. So if there are 12 X's, then the first O must be number 13. That's how we can calculate the number to output for the top-left corner of the inner triangle.
Once you've calculated all that it's just a matter of working out which side of the inner triangle the element is on and printing it out.

Answer 2

Here is a general idea as to how to solve this with recursion. There is probably a way to solve this in a space-efficient way, but I leave that for someone else. So let's say we store this as an array of arrays accessed by x[i][j] and let's say the size of sides of the triangle are n. You can google for how to dynamically create an array of arrays.
Now what we want is an equation for the outside of the triangle. Numbers along the diagonal are (i(i+1))/2 for 1 <= i <= n. Numbers along the left edge are 1+(i(i-1))/2 for 1 <= i <= n. And numbers along the bottom are 1+(n(n-1))/2 .. (n(n+1))/2.
So now to the recursion. Let j be the size of the remaining triangle yet to be numbered, k the highest number you've encountered before and (l,m) the index of the top of the triangle. Use the equations above and the preceding information to calculate the number and store that into the remaining triangle array. If there is yet another interior, recurse with the highest number and index of the top. And so on.
Example for side size 4. First number the outside. Highest prior number is 0. Index of the first position is (0,0)
1
9 2
8 x 3
7 6 5 4
We are not done because we still have an interior of size one. So when you recurse you, the position of the top of the triangle is (2,2), the highest number so far is 9 and the size of the triangle remaining is 1.
Now try with side size 5. After the first numbering we'd get:
1
12 2
11 x 3
10 x x 4
9 8 7 6 5
And the triangle remaining also starts at (2,2) as for side 4. But the size of the triangle remaining is now 2 and the highest number seen so far is 12.
After all the recursion is done, then print out of the table.

Answer 3

Easy way is to use an array.
#include <iostream>
using namespace std;
int main(){
int n;
cout <<"Enter the number of rows : ";
cin >> n;
int **tri = new int *[n];
for(int i=0; i<n; i++){
tri[i] = new int[i+1];
}
int v = 0, r = 0, c = 0;
for(int k = n-1; 0 <= k; k -=3){//k is total side size / 3, next k -2 -1
if(k==0){
tri[r][c] = ++v;
break;
}
for(int i = 0; i < k; ++i)//↘
tri[r++][c++] = ++v;
for(int i = 0; i < k; ++i)//←
tri[r][c--] = ++v;
for(int i = 0; i < k; ++i)//↑
tri[r--][c] = ++v;
r += 2; ++c;//next start position
}
//print
for(r = 0; r < n; ++r){
for(c = 0; c <= r; ++c)
cout << tri[r][c] << '\t';
cout << endl;
}
for(int i=0; i<n; i++){
delete[] tri[i];
}
delete[] tri;
}