# how to cut off  3 bits from a 32bit integer

**URL:** <https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202>\
**Category:** Programming\
**Created:** [December 8, 2020, 6:29pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202 "2020-12-08T18:29:37Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![BetaCarotin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/betacarotin/32/1098613_2.png) [@BetaCarotin](https://forum.arduino.cc/u/BetaCarotin)\
**Post date:** [December 8, 2020, 6:29pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/1 "2020-12-08T18:29:37Z")

</div>

Hi!  
I am reading a 32bit integer, but I only want 29bits out of it. the three first, most significant, are flags I dont want to use.

I tried & and | but I end up with weird results - maybe someone could give me an example how to cut of some bits?

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<div class="post-metadata">

**Author:** ![anon73444976](https://avatars.discourse-cdn.com/v4/letter/a/439d5e/32.png) [@anon73444976](https://forum.arduino.cc/u/anon73444976)\
**Post date:** [December 8, 2020, 6:34pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/2 "2020-12-08T18:34:53Z")

</div>

> I tried & and | but I end up with weird results

Post code, post results.

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<div class="post-metadata">

**Author:** ![hammy](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/hammy/32/238813_2.png) [@hammy](https://forum.arduino.cc/u/hammy)\
**Post date:** [December 8, 2020, 6:38pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/3 "2020-12-08T18:38:19Z")

</div>

[https://www.arduino.cc/en/pmwiki.php?n=Tutorial/BitMask](https://www.arduino.cc/en/pmwiki.php?n=Tutorial/BitMask)

---

<div class="post-metadata">

**Author:** ![BetaCarotin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/betacarotin/32/1098613_2.png) [@BetaCarotin](https://forum.arduino.cc/u/BetaCarotin)\
**Post date:** [December 8, 2020, 6:42pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/4 "2020-12-08T18:42:13Z")

</div>

```cpp
    CAN_config = canMsg.can_id;
    Serial.print("right from can: ");
    Serial.print(CAN_config);
    CAN_config << 3;
    Serial.print("  shifted 3 left: ");
    Serial.print(CAN_config);
    CAN_config >> 3;
    Serial.print("shifted three left: ");
    Serial.println(CAN_config);

    Serial.print("Message read: ");
    Serial.print(CAN_config);
    Serial.print(" = ");
    Serial.println(CAN_receive);

```

result:  
CAN\_config  
right from can: 2147483665 shifted 3 left: 2147483665 shifted three left: 2147483665  
Message read: 2147483665 = 17  
CAN\_config

---

<div class="post-metadata">

**Author:** ![ToddL1962](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/toddl1962/32/225060_2.png) [@ToddL1962](https://forum.arduino.cc/u/ToddL1962)\
**Post date:** [December 8, 2020, 6:47pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/5 "2020-12-08T18:47:43Z")

</div>

```cpp
void setup() {
  Serial.begin(9600);
  uint32_t y = 0xE1234567;
  Serial.println(y, HEX);
  y = y & 0x1fffffff;
  Serial.println(y, HEX);
}

void loop() {
}

```

---

<div class="post-metadata">

**Author:** ![BetaCarotin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/betacarotin/32/1098613_2.png) [@BetaCarotin](https://forum.arduino.cc/u/BetaCarotin)\
**Post date:** [December 8, 2020, 6:48pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/6 "2020-12-08T18:48:52Z")

</div>

As said, can\_id holds more bits than needed for adress decode (from the library):

```cpp
struct can_frame {
    uint32_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
    uint8_t  can_dlc;
    uint8_t  data[8];
};

```

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<div class="post-metadata">

**Author:** ![anon73444976](https://avatars.discourse-cdn.com/v4/letter/a/439d5e/32.png) [@anon73444976](https://forum.arduino.cc/u/anon73444976)\
**Post date:** [December 8, 2020, 6:49pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/7 "2020-12-08T18:49:17Z")

</div>

```cpp
CAN_config << 3;

```

Oops

`CAN_config <<= 3;` The rest of the solution is left as an exercise for the reader.

---

<div class="post-metadata">

**Author:** ![ToddL1962](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/toddl1962/32/225060_2.png) [@ToddL1962](https://forum.arduino.cc/u/ToddL1962)\
**Post date:** [December 8, 2020, 6:52pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/8 "2020-12-08T18:52:54Z")

</div>

```cpp
   CAN_config << 3;

```

This line shifts but doesn't assign it to anything.

Use:

```cpp
   uint32_t masked_config = CAN_config & 0x1fffffff;

```

Now masked\_config contains CAN\_config without the flags.

---

<div class="post-metadata">

**Author:** ![BetaCarotin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/betacarotin/32/1098613_2.png) [@BetaCarotin](https://forum.arduino.cc/u/BetaCarotin)\
**Post date:** [December 8, 2020, 7:00pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/9 "2020-12-08T19:00:38Z")

</div>

> ToddL1962:
> 
> ```arduino
> void setup() {
> 
> ```

Serial.begin(9600);  
&nbsp;uint32\_t y = 0xE1234567;  
&nbsp;Serial.println(y, HEX);  
&nbsp;y = y & 0x1fffffff;  
&nbsp;Serial.println(y, HEX);  
}

void loop() {  
}

> ```arduino
> 
> ```

That one does not work, when It gives me FFFFFFF  
FFFFFFF, when I enter with:

```arduino
  uint32_t y = 0xfffffff;
    Serial.println(y, HEX);
    y = y & 0x1fffffff;
    Serial.println(y, HEX);

```

> TheMemberFormerlyKnownAsAWOL:
> 
> ```arduino
> CAN_config << 3;
> 
> ```
> 
> Oops
> 
> ```arduino
> CAN_config <<= 3;
> 
> ```
> 
> The rest of the solution is left as an exercise for the reader.

Ohh noooooo. yes, that one solved it .... so stupid I am 🙂

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<div class="post-metadata">

**Author:** ![anon73444976](https://avatars.discourse-cdn.com/v4/letter/a/439d5e/32.png) [@anon73444976](https://forum.arduino.cc/u/anon73444976)\
**Post date:** [December 8, 2020, 7:15pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/10 "2020-12-08T19:15:33Z")

</div>

> That one does not work,

Count the 'f's

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<div class="post-metadata">

**Author:** ![noiasca](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/noiasca/32/1126910_2.png) [@noiasca](https://forum.arduino.cc/u/noiasca)\
**Post date:** [December 8, 2020, 7:33pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/11 "2020-12-08T19:33:12Z")

</div>

when you mask

0xfffffff; // 7 f!

with

0x1fffffff;

it's absolutely correct, that you end up with nothing more than

0xfffffff;

If I go back to your entry post, you wanted to cut off the MOST significant 3 bits. Than the left shift will give you a wrong result, because it will shift out 3 bits and add zeros on the LSB

```cpp
void setup() {
  Serial.begin(115200);
  uint32_t in = 0xffffffff; //all 32 (!!!) bits set
  Serial.println(in, HEX);
  Serial.println(in, BIN);
  uint32_t outA = in & 0x1fffffff;
  Serial.println(outA, HEX);
  Serial.println(outA, BIN);
  in <<= 3;
  Serial.println(in, HEX);
  Serial.println(in, BIN);
}

void loop() {
}

```

FFFFFFFF  
11111111111111111111111111111111  
1FFFFFFF  
11111111111111111111111111111  
FFFFFFF8  
11111111111111111111111111111000

imho masking is the correct way, if you want what you have asked for in your entry post.

AWOL was faster, but I'll keep my post...

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<div class="post-metadata">

**Author:** ![ToddL1962](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/toddl1962/32/225060_2.png) [@ToddL1962](https://forum.arduino.cc/u/ToddL1962)\
**Post date:** [December 8, 2020, 7:50pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/12 "2020-12-08T19:50:46Z")

</div>

It's amazing how a simple masking problem generated so much discussion.

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<div class="post-metadata">

**Author:** ![BetaCarotin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/betacarotin/32/1098613_2.png) [@BetaCarotin](https://forum.arduino.cc/u/BetaCarotin)\
**Post date:** [December 8, 2020, 9:57pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/13 "2020-12-08T21:57:50Z")

</div>

Well, no- the masking is wrong!  
And the number of f is correct.

When I move 32 bits 3 positions to the left AND after that I move them 3 position s to the right I will erase the 3 most significant bits - meaning I will make them zero.  
By this I ensure that the number left is encoded as a 29bit value- not as a 32bit value.

All the obscure bit mask magic everyone tries to apply will alter the bits in a unpredictable way- the first 3 bits can be zeros- they can be ones- I dont know- I just dont wont them.

My example with the F shows that when all bits are set to 1 (32 x 1) the result is not 29 x1 (and that it is when you just cut of the first 3msb) - so bit masking does not work to cut of 3 bits.

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<div class="post-metadata">

**Author:** ![anon73444976](https://avatars.discourse-cdn.com/v4/letter/a/439d5e/32.png) [@anon73444976](https://forum.arduino.cc/u/anon73444976)\
**Post date:** [December 8, 2020, 10:03pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/14 "2020-12-08T22:03:38Z")

</div>

Please show us the code that you say doesn't work.  
Note that if you use signed types, the result may not be what you hope for.

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<div class="post-metadata">

**Author:** ![anon57585045](https://avatars.discourse-cdn.com/v4/letter/a/ed8c4c/32.png) [@anon57585045](https://forum.arduino.cc/u/anon57585045)\
**Post date:** [December 8, 2020, 10:46pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/15 "2020-12-08T22:46:57Z")

</div>

> All the obscure bit mask magic everyone tries to apply will alter the bits in a unpredictable way

There is nothing obscure about bit masking. "Everyone" is suggesting it because it's extremely common and well understood. Perhaps you should take a breather and try to understand it. Then you would realize, it can't possibly do what you say.

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<div class="post-metadata">

**Author:** ![ToddL1962](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/toddl1962/32/225060_2.png) [@ToddL1962](https://forum.arduino.cc/u/ToddL1962)\
**Post date:** [December 8, 2020, 10:54pm UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/16 "2020-12-08T22:54:32Z")

</div>

> BetaCarotin:  
> Well, no- the masking is wrong!  
> And the number of f is correct.
> 
> My example with the F shows that when all bits are set to 1 (32 x 1) the result is not 29 x1 (and that it is when you just cut of the first 3msb) - so bit masking does not work to cut of 3 bits.

I have been doing bit masking for 35 years. I assure you it works. Here is the code you claim doesn't work:

```arduino
 uint32_t y = 0xfffffff;
    Serial.println(y, HEX);
    y = y & 0x1fffffff;
    Serial.println(y, HEX);

```

You have 7 f's. If you mask with 0x1fffffff you get the same thing...0xfffffff.

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<div class="post-metadata">

**Author:** ![BetaCarotin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/betacarotin/32/1098613_2.png) [@BetaCarotin](https://forum.arduino.cc/u/BetaCarotin)\
**Post date:** [December 9, 2020, 6:46am UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/17 "2020-12-09T06:46:40Z")

</div>

And the desired result is 1fffffff !  
So it does not work.  
Why is that so hard to understand?

CAN identifiers are 29bit long, out of the CAN frame drops a 32bit value, containing flags at the beginning. These flags need to be cut off before processing the adress. Otherwise I will decode a flag as a part of an adress.

So how would you do bit masking= logical operations , that will set bits to zero- no matter what the other variable is- without altering all the other bits (29) you dont want to change? You dont! You just shift 3 left and three right, done.

So please try understand what I asked for.

---

<div class="post-metadata">

**Author:** ![sherzaad](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/sherzaad/32/226173_2.png) [@sherzaad](https://forum.arduino.cc/u/sherzaad)\
**Post date:** [December 9, 2020, 7:18am UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/18 "2020-12-09T07:18:15Z")

</div>

> BetaCarotin:  
> And the desired result is 1fffffff !  
> So it does not work.  
> Why is that so hard to understand?

It's YOU who FAILS TO UNDERSTAND!!!!  
The masking suggested WORKS!!!  
Here's another example code to demostrate it:

```arduino
const uint32_t BIT_MASK_29Bits = 0x1fffffff;

void setup() {
  uint32_t val1 = 0x1abcdef2;
  uint32_t val2 = 0xff456789;

  Serial.begin(115200);

  Serial.print("val1 = 0x");
  Serial.print(val1, HEX);
  Serial.print(", applying mask 0x");
  Serial.println(val1 & BIT_MASK_29Bits, HEX);

  Serial.print("val2 = 0x");
  Serial.print(val2, HEX);
  Serial.print(", applying mask 0x");
  Serial.print(val2 & BIT_MASK_29Bits, HEX);

}

void loop() {
  // put your main code here, to run repeatedly:

}

```

and the result:

```arduino
val1 = 0x1ABCDEF2, applying mask 0x1ABCDEF2
val2 = 0xFF456789, applying mask 0x1F456789

```

As you can see, in val2, the 3 most significant bits WERE MASKED

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<div class="post-metadata">

**Author:** ![jimLee](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jimlee/32/214865_2.png) [@jimLee](https://forum.arduino.cc/u/jimLee)\
**Post date:** [December 9, 2020, 7:24am UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/19 "2020-12-09T07:24:33Z")

</div>

> ToddL1962:  
> It's amazing how a simple masking problem generated so much discussion.

Naw, its just Covid.

-jimlee

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<div class="post-metadata">

**Author:** ![noiasca](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/noiasca/32/1126910_2.png) [@noiasca](https://forum.arduino.cc/u/noiasca)\
**Post date:** [December 9, 2020, 7:31am UTC](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202/20 "2020-12-09T07:31:38Z")

</div>

> Why is that so hard to understand?

we don't know why you struggle at that topic.  
But you could put the posted codes in your Arduino and try to follow what we are saying.

> CAN identifiers are 29bit long, out of the CAN frame drops a 32bit value, containing flags at the beginning. These flags need to be cut off before processing the adress. Otherwise I will decode a flag as a part of an adress.

so, mask the 32bit variable with

0b00011111111111111111111111111111

and you will see, that the first 3 bits are gone.

[Next page](https://forum.arduino.cc/t/how-to-cut-off-3-bits-from-a-32bit-integer/686202.md?page=2)