Input2points x1, y1, x2, y2
OutputLinearEquation

'input 2 points' x and y coordinates
Function Input2points(x1, y1, x2, y2)
	x1=Inputbox("Please enter the X coordinate for pointA:", "X coordinate for pointA")
	y1=Inputbox("Please enter the Y coordinate for pointA:", "Y coordinate for pointA")
	x2=Inputbox("Please enter the X coordinate for pointB:", "X coordinate for pointB")
	y2=Inputbox("Please enter the Y coordinate for pointB:", "Y coordinate for pointB")
	'wrong input: non-numeric character or no character was entered		
	If IsNumeric(x1) = false or IsNumeric(y1) = false or IsNumeric(x2) = false or IsNumeric(y2) = false then
		Msgbox "Non-numeric character or no character was entered." & vbCrLf & "Click [OK] to enter the coordinates for the 2 points again."
		Input2points x1, y1, x2, y2
	'wrong input: the 2 points should not be the same		
	ElseIf x1=x2 and y1=y2 then
		Msgbox "PointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") are the same." & vbCrLf & "Click [OK] to enter the coordinates for the 2 points again."
		Input2points x1, y1, x2, y2
	End if
End Function

'caculate the linear equation y=mx+b which joins the 2 points
Function OutputLinearEquation
	Dim orim, orib, m, b
	If x1<>x2 then
		orim=(y2-y1)/(x2-x1)
		orib=y1-orim*x1
		'use 2 decimals places in the output linear equationn
		m=FormatNumber(orim,2)
		b=FormatNumber(orib,2)
	
		If y1<>y2 then
		
			'normal case: y=mx+b
			If m<>1 and m<>-1 and b>0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=" & m & "x+" & b
			
			'special case: y=mx+0 should be displayed as y=mx
			Elseif m<>1 and m<>-1 and b=0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=" & m & "x"

		   'special case: y=mx+-b should be displayed as y=mx-b
			Elseif m<>1 and m<>-1 and b<0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=" & m & "x"  & b
			
			'special case: y=1x+b should be displayed as y=x+b
			Elseif m=1 and b>0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=x+" & b
			
			'special case: y=1x+0 should be displayed as y=x
			Elseif m=1 and b=0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=x"

			'special case: y=1x+-b should be displayed as y=x-b
			Elseif m=1 and b<0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=x" & b

			'special case: y=-1x+b should be displayed as y=-x+b
			Elseif m=-1 and b>0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=-x+" & b
			
			'special case: y=-1x+0 should be displayed as y=-x
			Elseif m=-1 and b=0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=-x"

			'special case: y=-1x+-b should be displayed as y=-x-b
			Elseif m=-1 and b<0 then
				Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=-x" & b
			
			End If
		
		'special case: horizontal line
		Else
			Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "y=" & y1
		
		End If
	
	Else 'special case: vertical line
		Msgbox "The linear equation joins " & "pointA(" & x1 & "," & y1 & ") and pointB(" & x2 & "," & y2 & ") is" & vbCrLf & "x=" & x1
	
	End if
End Function