how to replace each element (of struct type) with another element in an array using Linq?

Question

I have a Point struct:

public struct Point
{
    public Point(double x, double y, double z)
    {
        X = x;
        Y = y;
        Z = z;
    }

    public double X;
    public double Y;
    public double Z;
}

then in the following code, I am creating an array of points and then dividing each points by a value 2

//Create a test array of Points
Point[] testPointArray = new Point[2];
testPointArray[0] = new Point(2, 4, 8);
testPointArray[1] = new Point(6, 12, 24);

//Divide by 2
testPointArray = testPointArray.Select(point => point = new Point(point.X / 2, point.Y / 2, point.Z / 2)).ToArray();

Question: In the above code, I am using the new operator to replace each point with a newly created point. How can I directly make changes in the existing point (i.e. avoid creating new points)? I tried to do the following code, but the syntax is wrong:

testPointArray = testPointArray.Select(point =>
{
    point.X = point.X / 2;
    point.Y = point.Y / 2;
    point.Z = point.Z / 2;
}).ToArray();

You should create new instances, structs are value types and they should be immutable. — jalsh
– jalsh, Commented May 20, 2022 at 10:01
if you used class instead of struct and its inside a list you can try ForEach, see example. — Bagus Tesa
– Bagus Tesa, Commented May 20, 2022 at 10:03
value types are copied when passed as arguments, hence any modification happening on them will happen within the scope of that function. This can cause a lot of confusion\errors. Thus it's recommended that you keep your structs immutable. See this answer — jalsh
– jalsh, Commented May 20, 2022 at 10:08
IMO (bring pitchforks and torches!) LINQ was meant as a functional extension (see this and this). I would use a plain old for loop or, given you have 2 points, just inline the calls and skip looping altogether. — orhtej2
– orhtej2, Commented May 20, 2022 at 10:18

TVOHM · Accepted Answer · 2022-05-20 10:04:05Z

3

testPointArray = testPointArray.Select(point =>
{
    point.X /= 2;
    point.Y /= 2;
    point.Z /= 2;
    return point;
}).ToArray();

In your example your lambda returns nothing - adding return point returns the mutated point for your select.

I agree with jalsh's comment that you should create new points, something like:

testPointArray = testPointArray.Select(point => new Point(point.X / 2, point.Y / 2, point.Z / 2))
            .ToArray();

answered May 20, 2022 at 10:04

TVOHM

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6 Comments

jalsh Over a year ago

good answer, fyi for OP, this is still creating new points, you're just not initializing it yourself

skm Over a year ago

In both cases, you are still creating a new point. As, Jalsh mentioned, the Structs are immutable so, I should either use class to avoid creating new points OR else I must create a new point.

jalsh Over a year ago

One way to see this happening is by running ForEach instead of Select try: Array.ForEach(testPointArray, point =>{point.X = point.X / 2; point.Y = point.Y / 2; point.Z = point.Z / 2;}); this will return the same (old) values although you have modified your fields

orhtej2 Over a year ago

@jalsh will that work if you're not pasing point as ref point? Doesn't seem to: ideone.com/FCSuPT

jalsh Over a year ago

@orhtej2 yes, it won't... that was my point (pun not intended). this shows that the function is operating on a COPY of Point

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