JayByte JayByte - 1 year ago 74
C# Question

How to iterate through a list so that the last iteration step goes back to the first object?

I have a list of points that form a plane. I want to create edges between consecutive points and add them to another list.

Here is the code I currently have:

// Get points forming the plate
ArrayList points = part.points;

// Number of points forming the plate
int pointCount = points.Count;

// Create edges
List<LineSegment> edges = new List<LineSegment>();
for (int i = 0; i < pointCount - 1; i++)
// Get start and end points
Point start = points[i];
Point end = points[i+1];

// Create edge
LineSegment edge = new LineSegment(start, end);

// Add edge to the list

It doesn't quite work because it doesn't create the last edge between the last and the first points on the list. What would be the way to correct it? I could make it work with an if statement like this:

for (int i = 0; i < pointCount; i++)
// Get start and end points
Point start = points[i] as Point;
Point end;
if (i == pointCount-1) end = points[0] as Point;
else end = points[i+1] as Point;

// Rest of the code here

But I'm sure there is a more elegant way to do it. In Python I would start the loop from -1 so that the first edge would actually be connecting the last point to the first point, but that is not possible in C#.

EDIT: The points list is given as an ArrayList by the API.

Answer Source

An 'elegant' solution is using a modulo:

for (int i = 0; i < pointCount; i++)
    // for i+1 == pointCount this will yield points[0]
    Point end = points[(i+1) % pointCount] as Point;

However, I believe the if statement you used is more readable.

Note: Use a List<T> instead of ArrayList, too.

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