Is there a way of using bitwise operator to combine digit bits?

Question

For example, I have two digit bits:

0b0111111
0b0000110

I want to shift a state variable by 7 digits and combine them together.

0b00001100111111

Can I accomplish by shifting?

If you have to variables `x` and `y` with those values: `(y << 7) | x` — Pablo, Feb 22 '18 at 02:30
Are you really using C? Base-2 literals are only supported in C++14, not C11. — Dai, Feb 22 '18 at 02:30
@Dai Using xCode on Mac (not sure the exact compiler, I want to say GCC off the top of my head) allows for binary literals. — Matthew Kerian, Feb 22 '18 at 02:40
@user3386109 Why does that matter though? If binary liteterals work why would I want to make them not work? — Matthew Kerian, Feb 22 '18 at 04:07
@MatthewKerian Binary literals aren't technically part of the language. Which means that if you want your code to be portable, you can't use them. That's the purpose of `-pedantic`. It helps you write code that complies with the standard, and therefore has a better chance of porting correctly. — user3386109, Feb 22 '18 at 04:13

score 6 · Accepted Answer · answered Feb 22 '18 at 02:31

6

You do this by shifting the bottom number left 7 digits, then performing a bitwise OR of the result and the first number.

unsigned int a = 0x3f;
unsigned int b = 0x06;
unsigned int result = (b << 7) | a;

answered Feb 22 '18 at 02:31

dbush

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RyanArr · Answer 2 · 2018-02-22T17:07:20.493

0

unsigned char a = 0b00000110;
unsigned char b = 0b01111111;
unsigned short c = (b << 8); // shift everything left 8 bits to set the high bits
c &= 0xFF00; // clear out the lower bits - not necessary in C
c |= a; // set the lower 8 bits

edited Feb 22 '18 at 17:07

answered Feb 22 '18 at 02:33

RyanArr

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programmerj · Answer 3 · 2018-02-22T02:52:45.903

0

unsigned int X = 0b00111111;
unsigned int Y = 0b00000110;

unsigned int Z = ((X << 7) & 0xFF00) | Y;

edited Feb 22 '18 at 02:52

answered Feb 22 '18 at 02:36

programmerj

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This one fails, absolutely. – iBug Feb 22 '18 at 02:45
`0xFF00` is not the right mask for a 7-bit value that's been shifted by 7 bits. Not to mention that `Y` is the one that's supposed to get shifted. – user3386109 Feb 22 '18 at 03:28
Correct. Someone submitted that answer when I stepped away from my workstation for a few minutes. I think it was my evil twin brother. – programmerj Feb 22 '18 at 12:59

score -1 · Answer 4 · answered Feb 22 '18 at 02:31

-1

int a = 0b0111111;
int b = 0b0000110;

int combined = (a << 7) | b;

answered Feb 22 '18 at 02:31

Dai

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I would not use signed ints either – programmerj Feb 22 '18 at 02:33
1

When shifting left, there is no difference between arithmetic and logical shift. When shifting right, the type of shift depends on the type of the value being shifted. https://stackoverflow.com/questions/7622/are-the-shift-operators-arithmetic-or-logical-in-c#7632 – parvus Feb 22 '18 at 05:25

Is there a way of using bitwise operator to combine digit bits?

4 Answers4