Shift characters in a string like rot-13

Blockland Rot-N Function.cs

// made on 12/24/2017
// rotNString 'string', shift # (postive or negative work), %m for whether to allow nonletters to pass through

function shiftCharS(%c, %offset, %mode)
{
	%letters = "abcdefghijklmnopqrstuvwxyz";
	if (%offset !$= (%offset | 0)) // first %offse
		return "offset isn't a number";
	if (strLen(%c) != 1)
		return "one character only";
	if (%c $= (%c | 0))
		switch(%mode)
		{
			case (0):
				return "letters only - no numbers";
			case (1): 
				return %c;
		}
	if (strPos(%letters, %c) == -1 && strPos(strUpr(%letters), %c) == -1) // not 0
		switch(%mode)
		{
			case (0): 
				return "no symbols";
			case (1):
				return %c;
		}
	if (strCmp(%c, strUpr(%c)))
		return getSubStr(%letters, ((strPos(%letters, %c) + %offset) % 26), 1); 
	else
		return getSubStr(strUpr(%letters), ((strPos(strUpr(%letters), %c) + %offset) % 26), 1); 
}

function rotNString(%string, %n, %m)
{
	if (%string $= "")
	{
		error("No blank strings.");
		return;
	}
	
	for (%i = 0; %i < %strLen = strLen(%string); %i++)
	{
		%rotNString = %rotNString @ shiftCharS(getSubStr(%string, %i, 1), %n, %m); // %m was 1
	}
	return %rotNString;
}