C#

p1 = (n-1) % positions.Count;
p2 = (positions.Count + n - 2) % positions.Count;

public List<int> GetPositions(int n, int posCount)
{
  int pos1 = n % posCount;
  int pos2 = pos1 == 0 ? posCount - 1 : pos1 - 1;
  return new List<int>{pos1, pos2}; 
}
...
int n = 0;
List<int> positions = new List<int>{0,1,2,3,4};
while( n < 10)
{
  List<int> res = GetPositions(n, positions.Count)
  Console.WriteLine("Count{0}: Pos1 {1} - Pos2 {2}", n, res[0], res[1]);
  n++;
}

http://msdn.microsoft.com/en-us/library/system.string.format%28v=vs.110%29.aspx#FormatItem[^]

A format item has this syntax:

{index[,alignment][:formatString]}

...
alignment
Optional. A signed integer that indicates the total length of the field into which the argument is inserted and whether it is right-aligned (a positive integer) or left-aligned (a negative integer). If you omit alignment, the string representation of the corresponding argument is inserted in a field with no leading or trailing spaces.

Richard Deeming wrote:

he MSDN documentation is reasonably clear (for once!):

logicalweb wrote:

How it is different from below approach