How to get a direction on the same train line?

Question

Can you help me with step by step logic that I need to get a direction on the same train line. with already having common train line with functions Next and Previous.

public IStation Next(IStation s)
{
    if (!_stations.Contains(s))
    {
        throw new ArgumentException();
    }
    var index = _stations.IndexOf(s);
    var isLast = index == _stations.Count -1;
    if (isLast)
    {
        return null;
    }
    return _stations[index + 1];
}

public IStation Previous(IStation s)
{
    if (!_stations.Contains(s))
    {
        throw new ArgumentException();
    }
    var index = _stations.IndexOf(s);
    var isFirst = index == 0;
    if (isFirst)
    {
        return null;
    }
    return _stations[index - 1];
}

And my function where I look for direction.

public string GetLineDirectiom(Station from, Station to, Line commonLine)
{
    bool fromNextTo = true;


    //to.Lines.Count();
    //to.ToString();
    var Final = commonLine.Next(from);
    while (Final != null)
    {

    }

    if (fromNextTo)
        return "next";
    else return "previous";
}

Answer 1

It looks like you are trying to "visit the stations along commonLine", starting at the from station.

The loop you have started is a valid start to that end; you need a variable to store the station you are currently visiting. Maybe the current variable name Final is a bit confusing to yourself here, because it is not the "final" station of the line, just the one you are currently visiting.

Therefore, let's name the variable currentStation. Then, you want to go to the next station until you have found to (and thereby know the direction), or until you have reached the end of the line:

var currentStation = from;
while (currentStation != null)
{
    if (currentStation == to)
    {
        return "next";
    }
    currentStation = commonLine.Next(currentStation);
}

Now, this checks whether to is "ahead". If it wasn't, you can proceed to checking whether it can be found in the other direction, again starting at from:

currentStation = from;
while (currentStation != null)
{
    if (currentStation == to)
    {
        return "previous";
    }
    currentStation = commonLine.Previous(currentStation);
}

If this loop doesn't find to either, apparently to is not on the line. Treat this case according to your preference.

Some remarks:

• Indicating the direction as "next" or "previous" may be a bit misleading. If it really is the direction of the line, think about something like "forward" and "backward", as "next" and "previous" indeed imply the direct next/previous elements in a list.
• While the above works, I do note that your Line object already has the stations in an indexed list. Therefore, a simpler way of achieving your goal might be to just determine the indices of the from and to stations on commonLine and compare which one is greater than the other.