Not Understanding Script for Moving Character

Unity Engine
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1

I’m a beginner on Unity and C#. I’m trying to make my character move. I found a Youtube video where the script goes like this:

var vel = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical")) * _speed;

vel.y = _rb.velocity.y;

_rb.velocity = vel;

I can’t understand the last 2 ones.

I realized that if I take vel.y = _rb.velocity.y; out, the character can still jump, but it gets blocked immediately after.

If I remove _rb.velocity = vel;, the character can jump, but it can’t move.

My question is: Why?

I appreciate any help :slight_smile:

2 
// this first line constructs a Vector3 that looks like this: [X, 0, Z]
// (X and Z are our inputs multiplied by movement speed)
Vector3 vel = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical")) * _speed;

// here we replace the middle zero with the Rigidbody's original velocity
// (this means we are only changing the X and Z components of the velocity,
// and the Y (up/down) component is preserved)
vel.y = _rb.velocity.y;

// assign velocity to Rigidbody
_rb.velocity = vel;

I would personally express this code more like this:

var velocity = _rb.velocity;

_rb.velocity = new Vector3(
    x: Input.GetAxis("Horizontal") * _speed,
    y: velocity.y, // y is unchanged
    z: Input.GetAxis("Vertical") * _speed
);

And soon-ish, when Unity updates their supported C# lang version🙏, you’ll be able to do it like this:

_rb.velocity = _rb.velocity with
{
    x = Input.GetAxis("Horizontal") * _speed,
    z = Input.GetAxis("Vertical") * _speed,
};
1 Like
3

It may be helpful to you to start with some basic C# tutorials so you understand what these simple statements are doing.

In the case of the first line it is doing THREE things:

  • reading X and Y input
  • making a Vector3 named vel out of the input
  • scaling it by speed.

Tutorials that cram all that in one line are doing you a disservice by confusing what is going on.

The second line is injecting the y component of the rigidbody int your vel vector.

The third line is assigning the vel vector back to the rigidbody’s .velocity field

When you understand the parts involved that I just listed above you will understand the answers to your questions.

Meanwhile, I suggest trying this decompositional approach:

Imphenzia: How Did I Learn To Make Games:

IMNSHO, the absolute LAST thing we need is new syntactic sugar that can only be used on extremely-recent versions of Unity confusing new users to the system. That’s not a helpful thing at all to anybody on this entire planet.

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4

I’m completely tired of people stuck writing C# 3.0 code their whole lives for some vague reason that allegedly has to do something with compatibility or not scaring the newbies. It’s a lazy excuse. Unity devs should absolutely be getting exposed to perfectly readable, years old features of the language they’re using, instead of treating each other like children incapable of handling a little with { }.

5

And yet every day we have several posts right in this forum from people who have used these crazy constructs to pack 27 different statements into a single statement 300+ characters long with 47 dots in it.

“I have this weird NullReferenceException error that I just can’t figure out!”

Looking up I see this post flagged as “Beginner” so I’m thinking simple-simple-simple. I stand by that.

1 Like
6

Thank you guys for your feedback!

I can comprehend the first line of code. My problem were those following 2.
I kind of understand their purpose, but not how they work.

I’ll try a different way.

Maybe I’ll crack their meanings in the future!

Thank you once again :slight_smile:

7

Draw pictures of what is going on if it helps.

A Vector3 is a box with three cells labeled x, y and z

Note what goes into them at each step.

It will actually make sense when you follow the data.

1 Like