Replacing if else statement with any design pattern or better approach

Question

This code doesn't look clean and this if condition can grow

public int VisitMonth(int months)
    {
        int visit = 0;

        if (months <= 1)
        {
            visit = 1;
        }
        else if (months <= 2)
        {
            visit = 2;
        }
        else if (months <= 4)
        {
            visit = 3;
        }
        else if (months <= 6)
        {
            visit = 4;
        }
        else if (months <= 9)
        {
            visit = 5;
        }
        else if (months <= 12)
        {
            visit = 6;
        }
        else if (months <= 15)
        {
            visit = 7;
        }
        else if (months <= 18)
        {
            visit = 8;
        }
        else if (months <= 24)
        {
            visit = 9;
        }
        else if (months <= 30)
        {
            visit = 10;
        }
        else if (months <= 36)
        {
            visit = 11;
        }
        else if (months <= 48)
        {
            visit = 12;
        }
        else if (months <= 60)
        {
            visit = 13;
        }
        else
        {
            visit = 14;
        }
        return visit;
    }

Is there any better solution to this problem? Sadly that function isn't linear so it's not easy to code that in a mathematical way.

Answer 1

Should be more suitable to be reused :You can write a "Interval" Class with "inRange" methode like this :

 public struct Interval<T>
       where T : IComparable
{
    public T Start { get; set; }
    public T End { get; set; }
    public T Visit { get; set; }

    public Interval(T visit, T start, T end)
    {
        Visit = visit;
        Start = start;
        End = end;
    }

    public bool InRange(T value)
    {
      return ((!Start.HasValue || value.CompareTo(Start.Value) > 0) &&
          (!End.HasValue || End.Value.CompareTo(value) >= 0));
    }
}

And then use like this :

public static readonly List<Interval<int>> range = new List<Interval<int>>
        {
                new Interval<int>(1, 0, 1),
                new Interval<int>(2, 1, 2),
                new Interval<int>(3, 2, 4),
                new Interval<int>(4, 4, 6),
                new Interval<int>(5, 6, 9),
                new Interval<int>(6, 9, 12),
                new Interval<int>(7, 12, 15),
                new Interval<int>(8, 15, 18),
                new Interval<int>(9, 18, 24),
                new Interval<int>(10, 24, 30),
                new Interval<int>(11, 30, 36),
                new Interval<int>(12, 36, 48),
                new Interval<int>(13, 48, 60),
                new Interval<int>(14, 60, int.MaxValue)
        };

var months = 5;
var visit = range.Where(x => x.InRange(months)).Select(x => x.Visit).FirstOrDefault();

Answer 2

Possibly in C# 8 (this feature is not official yet, but works in recent IDEs if you turn it on):

int months = ...;
int visit = months switch
{
    int j when j <= 1 => 1,
    int j when j <= 2 => 2,
    int j when j <= 4 => 3,
    int j when j <= 6 => 4,
    int j when j <= 9 => 5,
    // ...
    _ => 42 // default
};

You can do similar in earlier C#, since this is a method:

public int VisitMonth(int months)
{
    switch (months)
    {
        case int j when j <= 1: return 1;
        case int j when j <= 2: return 2;
        case int j when j <= 4: return 3;
        // etc
        default: return 14;
    }
}