How to implement this using the bitwise & (and)?

Question

For a program I am writing to connect to a scanner with three output trays (pockets) I need to make use of an SDK. After a call to the SDK I receive an int that represents the status of the pockets. To determine this "pocket" status the following is in the documentation.

Get status of the output pockets. To determine whether a pocket is full or empty, check the returned value using the bitwise AND (&) operator. Valid values are:

Csd.POCKET.P1_EMPTY Pocket 1 is empty.
Csd.POCKET.P2_EMPTY Pocket 2 is empty.
Csd.POCKET.P1_FULL Pocket 1 is full.
Csd.POCKET.P2_FULL Pocket 2 is full.
Csd.POCKET.P3_EMPTY Pocket 3 is empty.
Csd.POCKET.P3_FULL Pocket 3 is full.

I have never used bitwise operators so I am quite at a loss. The values of the above "Pocket" struct are as follows:

public struct POCKET
{
  public const int P1_EMPTY = 1;
  public const int P1_FULL = 16;
  public const int P2_EMPTY = 2;
  public const int P2_FULL = 32;
  public const int P3_EMPTY = 4;
  public const int P3_FULL = 64;
}

I have read up on bitwise operators and I know what they do, but I am at a loss implementing it for this specific case.

Thank you all in advance.

Take a look at this http://stackoverflow.com/questions/8447/what-does-the-flags-enum-attribute-mean-in-c — DavidG, Jul 14 '16 at 14:06
E.g. `if (returned_value & POCKET.P2_FULL == POCKET.P2_FULL) {...}` checks if pocket 2 is full — Dmitry Bychenko, Jul 14 '16 at 14:17
Also it may be worth mentioning that you are missing an enum value for 8. — DavidG, Jul 14 '16 at 14:23
@DavidG, I thought as much. But the SDK isn't really well written imo. — Gecko, Jul 14 '16 at 14:28
I'd assume it's because there used to be a P4 (or they planned on adding it later) — DavidG, Jul 14 '16 at 14:29

Dmitry Bychenko · Accepted Answer · 2016-07-14T14:40:11.057

1

Typical patterns for testing bit flags are

// Entire key match  
if (returned_value & value_to_test == value_to_test) {
  ...
}

// Partial key match  
if (returned_value & value_to_test != 0) {
  ...
}

E.g. if you want to test if pocket #3 is full:

if (returned_value & POCKET.P3_FULL == POCKET.P3_FULL) {
  ...
}

You can combine flags via | and test for partial match of such combined flag:

const int ALL_ARE_FULL = POCKET.P1_FULL | POCKET.P2_FULL | POCKET.P3_FULL;

...

// Test if any part of the flag is match (i.e. any pocket - at least one - is full)
// Please, notice != 0 comparison
if (returned_value & ALL_ARE_FULL != 0) {
   ...
}

// Compare with this: all three pockets are full
if (returned_value & ALL_ARE_FULL == ALL_ARE_FULL) {
   ...
}

edited Jul 14 '16 at 14:40

answered Jul 14 '16 at 14:21

Dmitry Bychenko

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Could this also work? (pocketStatus & Csd.POCKET.P1_FULL) > 0. I just found it in someones old code using this library. – Gecko Jul 14 '16 at 14:30
1

@stefchri: in your *particular case* `> 0` will do, but it's a *bad practice*, e.g. if const had happen to be `POCKET.P1_FULL == 1 << 31` the compasion would have *failed*. Please, stick to `... & key == key` (*entire* `key` match) and `... & key != 0` (*partial* `key` match) – Dmitry Bychenko Jul 14 '16 at 14:32

How to implement this using the bitwise & (and)?

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