Bitshift Char Array

Question

If I want to bit shift the integer 5 by 3, so int a = 5; int b = a << 3;, the result would be 40 in decimal as 5 is 101 and 40 is 101000.

What if however, I have the following char array: 00000 00101 and by bit shifting three to the left, I want the result to be 00001 01000. So I want to accommodate for the 0's padding. What do you suggest?

Mysticial · Accepted Answer · 2011-11-10 20:13:00Z

3

If you meant an actual char array, you can use memmove() and memset():

char str[] = "0000000101";

int shift = 3;
int length = strlen(str);

memmove(str, str + shift,length - shift);
memset(str + length - shift,'0',shift);

//  Result:
//  "0000101000"

edited Nov 10, 2011 at 20:13

answered Nov 10, 2011 at 20:02

Mysticial

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8 Comments

Dave Over a year ago

You're shifting right, rather than left, and overflowing the array, because you memmove() length bytes, not length - shift.

Mysticial Over a year ago

Thanks, I saw your first comment and fixed it a few min. ago. EDIT: I'm shifting left. I've tested it.

Dave Over a year ago

Yes you are, I mentally flipped the order of src and dst in memmove. Your current code is correct.

Daniel Fischer Over a year ago

I think OP wanted to shift by three bits. Can't do that with memmove.

Mysticial Over a year ago

@DanielFischer: It's not entirely clear what the OP wants. The way I'm reading it, the bits are stored as a char array where each char is either '0' or '1'. So it's unclear until the OP clarifies it.

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TJD · Accepted Answer · 2011-11-10 20:08:39Z

1

Access the buffer with a 16-bit pointer, use htons to take care of endian issues

char c[2] = {0, 5};

uint16_t* p16 = (uint16_t*)c;

*p16 = htons((ntohs(*p16) << 3));

answered Nov 10, 2011 at 20:08

TJD

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