Diference between 0 and (1 << 0)?

[weeix]

From the Collision Tutorial

// I use this ISceneNode ID to indicate a scene node that is
// not pickable by getSceneNodeAndCollisionPointFromRay()
ID_IsNotPickable = 0,

// I use this flag in ISceneNode IDs to indicate that the
// scene node can be picked by ray selection.
IDFlag_IsPickable = 1 << 0,

What's the difference between 0 and (1 << 0)?

(I read from MSDN Document that the value of a left-shift expression x << y is x * 2y.
So 1 << 0 is 1 * 2(0) = 1 * 0 = 0. Did I misunderstand?)

[weeix]

I didn't understand this one either,

node = smgr->addAnimatedMeshSceneNode(smgr->getMesh("../../media/dwarf.x"),
0, IDFlag_IsPickable | IDFlag_IsHighlightable);

IDFlag_IsPickable | IDFlag_IsHighlightable is (1 << 0) | (1 << 1) = 0 | 2 = 2?

0 0 0 0 OR
0 0 1 0
--------
0 0 1 0

[greenya]

What's the difference between 0 and (1 << 0)?

The difference is that 0 == 0 , and 1 << 0 == 1. In tutorial it written in this way to show that enum can be extended and next values will be "1<<2", "1<<3", ...

(I read from MSDN Document that the value of a left-shift expression x << y is x * 2y.
So 1 << 0 is 1 * 2(0) = 1 * 0 = 0. Did I misunderstand?)

The links show a power of y, not a miltiplication.
"x << y is x * 2y" <--is not the same as--> "x << y is x * 2^y" as we know 2^0 == 1.

[greenya]

I didn't understand this one either,

node = smgr->addAnimatedMeshSceneNode(smgr->getMesh("../../media/dwarf.x"),
0, IDFlag_IsPickable | IDFlag_IsHighlightable);

IDFlag_IsPickable | IDFlag_IsHighlightable is (1 << 0) | (1 << 1) = 0 | 2 = 2?

0 0 0 0 OR
0 0 1 0
--------
0 0 1 0

The result of "IDFlag_IsPickable | IDFlag_IsHighlightable" is:
0 0 0 1 // 1 == 1 << 0
0 0 1 0 // 2 == 1 << 1
--------
0 0 1 1 // 3

[weeix]

I don't understand why these shift operation are invented. Are they necessary?

Could I use:

Code: Select all

        ID_IsNotPickable = 0,
        IDFlag_IsPickable = 1,
        IDFlag_IsHighlightable = 2

instead?

[greenya]

Yes.

For first 3 members you can use just:
ID_IsNotPickable = 0,
IDFlag_IsPickable = 1,
IDFlag_IsHighlightable = 2

but if you need to add more, you have to use 4 (not 3), than 8 (not 4), than 16 (not 5) and so on.

[weeix]

1 2 4 8 16 32 64 ..

I see. It's easier to use x << y in this case.

But I still don't understand why can't I just use 1 2 3 4 5 6 .. happily? What is the purpose of this complication? O_o

[greenya]

We use 1,2,4,8,16,... (power of 2 values) when we want to pack into single integer value couple of independent triggers (true/false value or 1/0). When we do so, we simply use bit-logic operators to check values.

[slavik262]

What is the purpose of this complication? O_o

It's a carry-over from C. Since binary is base-2, the bit shift operator lets you access each individual bit easily. 1 << 0 is the first bit (in an 8-bit char, 0000 0001), and 1 << 5 (32) is the 5th bit (0001 0000). Using bit shift and binary logic commands such as &, |, and ^ (not && and ||), you can do things like use an int as a field of 32 booleans by controlling each bit.

[weeix]

BTW, thanks everyone for answering my question!

[DtD]

I wouldn't necessarily call it a carry over from C, it is computers in general. It is basically for accessing the bits inside the byte, so one byte can act like 8 bools. It is generally used for flexibility stuff.

[slavik262]

I was just referencing the fact that C had it first, but yeah, it is a pretty generic programming feature.

[terier]

Well then here's another one for you to figure out:
Why did they invent the so-called bit fields?

[slavik262]

It makes things more convenient when you're doing storage, but there's still places where manual bit-shifting is needed. For example, in networking it's required to convert from host to network byte order and vice-versa (for little-endian machines). Now, this does usually come in a pre-made function (htons and ntohs, respectively), but I'm just trying to point out that it still has plenty of practical uses.

[Mikhail9]

Well then here's another one for you to figure out:
Why did they invent the so-called bit fields?

Fundamentally, they invented them because the hardware itself exposes a set of flip-flops (a hardware boolean) to the firmware, which are grouped together into sets of 4, 8, 16 and so on. These make the most sense to the software as nybbles, bytes, words, and so on, so that's how you work with them.

It's really a holdover from writing machine language to access hardware directly. You still do a lot of it in embedded programming, where setting the bits in a memory location directly affects the microcrontroller you're working with. I program my microcontrollers in C, so those bit-shift operators are my best friend.