Convert members of struct in a list to array

Question

To process data from a log file, I read the data into a list.

When I tried to convert from the list to an array for the graphing routine, I ran into trouble.

For the sake of this discussion, let's say the log file contains three values* - x, y and theta. In the routine that does file I/O, I read the three values, assign them to a struct and add the struct to PostureList.

The plotting routine, wants the x, y and theta to be in individual arrays. My thought was to use the ToArray() method to do the conversion but when I tried the syntax below, I got an error - see error in comment below. I have an alternate approach to do the conversion but wanted to get advice on better approaches.

I'm very new to C#. Thanks in advance for your help.

NOTE: * In reality, the log file contains many different pieces of information that have varying payload sizes.

struct PostureStruct
{
    public double x;
    public double y;
    public double theta;
};

List<PostureStruct> PostureList = new List<PostureStruct>();

private void PlotPostureList()
{
    double[] xValue = new double[PostureList.Count()];
    double[] yValue = new double[PostureList.Count()];
    double[] thetaValue = new double[PostureList.Count()];

    // This syntax gives an error:
    //   Error 1 'System.Collections.Generic.List<TestNameSpace.Test.PostureStruct>'
    //   does not contain a definition for 'x' and no extension method 'x' accepting a first
    //   argument of type 'System.Collections.Generic.List<TestNameSpace.Test.PostureStruct>'
    //   could be found (are you missing a using directive or an assembly reference?)
    xValue = PostureList.x.ToArray();
    yValue = PostureList.y.ToArray();
    thetaValue = PostureList.theta.ToArray();

    // I could replace the statements above with something like this but I was wondering if
    // if there was a better way or if I had some basic mistake in the ToArray() syntax.
    for (int i = 0; i < PostureList.Count(); i++)
    {
        xValue[i] = PostureList[i].x;
        yValue[i] = PostureList[i].y;
        thetaValue[i] = PostureList[i].theta;
    }

    return;
}

Answer 1

The ToArray extension method can only be used on IEnumerables. To transform an IEnumerable, for example from your struct to a single value, you can use the Select extension method.

var xValues = PostureList.Select(item => item.x).ToArray();
var yValues = PostureList.Select(item => item.y).ToArray();
var thetaValues = PostureList.Select(item => item.theta).ToArray();

You don't need to define the size of the arrays or create them with new, the extension method will take care of that.

Answer 2

you are trying to reference x directly on list.

PostureList.y

you need to do it on specific member like

PostureList[0].y

i guess you need to select all the x from your list. For that you can do this

xValue = PostureList.Select(x => x.x).ToArray();

Answer 3

You can use this way to convert your List<PostureStruct> to individual arrays:

double[] xValue = PostureList.Select(a => a.x).ToArray();
double[] yValue = PostureList.Select(a => a.y).ToArray();
double[] thetaValue = PostureList.Select(a => a.theta).ToArray();

This is all you have to do and the arrays will have the right size (same as the list's lenght).

Answer 4

You can either loop through the list:

  double[] xValue = new double[PostureList.Count()];
  double[] yValue = new double[PostureList.Count()];
  double[] thetaValue = new double[PostureList.Count()];

  foreach (int i = 0; i < PostureList.Count; ++i) {
    xValue[i] = PostureList[i].x;
    yValue[i] = PostureList[i].y;
    thetaValue[i] = PostureList[i].theta;
  }

  ...

Or use Linq, but in different manner:

  double[] xValue = PostureList.Select(item => item.x).ToArray();
  double[] yValue = PostureList.Select(item => item.y).ToArray();
  double[] thetaValue = PostureList.Select(item => item.theta).ToArray();
  ...