How can I make a function that loops over a list and takes which data member to access as input parameter

Question

I have a datatype PlayerStats which contains a lot of different data members. I want to calculate a score which is different for each data member (the case below looks at statistics.nrOfGoals).

private double getScore()
{
    double strength = 0;
    foreach (PlayerStats statistics in this.statistics)
    {
        double dateDiff = Math.Abs(nowDate.Subtract(statistics.date).Days / (365.25 / 12));
        dateDiff = Math.Pow(dateDiff, Form1.historyFactor);

        strength += (statistics.nrOfGoals * ValueTable.PointsPerGoals   ) / dateDiff;
    }

    return strength;
}

How can I make this function general and accept which datamember to look at instead of creating a lot of similar looking functions?

Something like

private double getScore(Type type, Type type2)
{
    double strength = 0;
    foreach (PlayerStats statistics in this.statistics)
    {
        double dateDiff = Math.Abs(nowDate.Subtract(statistics.date).Days / (365.25 / 12));
        dateDiff = Math.Pow(dateDiff, Form1.historyFactor);

        strength += (statistics.type * ValueTable.type2) / dateDiff;
    }

    return strength;
}

Using a memberexpression. – CodeCaster Jan 21 '15 at 13:57 — CodeCaster, Jan 21 '15 at 13:57

score 4 · Accepted Answer · answered Jan 21 '15 at 13:58

4

You can give a function as a parameter with signature PlayerStats -> Double:

private double getScore(Func<PlayerStats,double> type, double type2)
{
    double strength = 0;
    foreach (PlayerStats statistics in this.statistics)
    {
        double dateDiff = Math.Abs(nowDate.Subtract(statistics.date).Days / (365.25 / 12));
        dateDiff = Math.Pow(dateDiff, Form1.historyFactor);

        strength += (type(statistics) * type2) / dateDiff;
    }

    return strength;
}

And then call it with:

getScore(x => x.nrOfGoals,ValueTable.PointsPerGoals);

x => x.nrOfGoals is a lambda-expression that defines some kind of function that (in this case) takes as input a PlayerStats instance and returns a double.

In the code, you can then see type as a "function"/"method" and call it with type(y) (with y a PlayerStats instance).

answered Jan 21 '15 at 13:58

Willem Van Onsem

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@Oday: is there a specific reason to make `type2` a function as well? – Willem Van Onsem Jan 21 '15 at 14:05
In the first function, the second operand is ValueTable.PointsPerGoals. In the second function, the second operand is ValueTable.type2. I can infer that both multiplication operands can be changed. – Oday Fraiwan Jan 21 '15 at 14:10
@Oday: no: the first function takes only one parameter: the `PlayerStats`, the `double` in the signature is the type of the result. A `Func` is probably only useful if the result can change (for instance a random number generator). Here it is reasonable to assume the weight will remain fixed... – Willem Van Onsem Jan 21 '15 at 14:16

score 1 · Answer 2 · answered Jan 21 '15 at 13:58

1

You could put the property names as string parameters and look up properties by name using reflection.

answered Jan 21 '15 at 13:58

Attila Szasz

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That will work, but it is more inefficient and less safe: using lambda-expressions, the compiler can check whether the aspect actually exists. Whereas reflection uses strings. If at one point you rename a property it will break. I think one better doesn't use reflection unless it is absolutely useful.. – Willem Van Onsem Jan 21 '15 at 14:06
I agree, but giving up type safety gives you maximum flexibility in naming any existing or future property to use in the statistics. – Attila Szasz Jan 21 '15 at 14:13

Andrey Korneyev · Answer 3 · 2015-01-21T14:05:02.130

1

You can pass Func<PlayerStats, double> to your function, like:

private double getScore(Func<PlayerStats, double> evaluator)
{
    double strength = 0;
    foreach (PlayerStats statistics in this.statistics)
    {
        double dateDiff = Math.Abs(nowDate.Subtract(statistics.date).Days / (365.25 / 12));
        dateDiff = Math.Pow(dateDiff, Form1.historyFactor);

        strength += evaluator(statistics) / dateDiff;
    }

    return strength;
}

And then call it like (in your shown case)

getScore(x => x.nrOfGoals * ValueTable.PointsPerGoals);

edited Jan 21 '15 at 14:05

answered Jan 21 '15 at 13:59

Andrey Korneyev

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The `ValueTable` probably gives, statistic-dependent weights? – Willem Van Onsem Jan 21 '15 at 13:59
1

@CommuSoft Maybe, but for me it is more likely looks like some constant (just based on its name). – Andrey Korneyev Jan 21 '15 at 14:04

How can I make a function that loops over a list and takes which data member to access as input parameter

3 Answers3