How to combine 2 4-bit unsigned numbers into 1 8-bit number in C

Question

I have 2 4-bit numbers (X0X1X2X3 and Y0Y1Y2Y3) and I want to combine them so that I would create an 8-bit number like that:

X0X1X2X3 
Y0Y1Y2Y3  => X0Y0X1Y1X2Y2X3Y3

I know how to concatenate them in order to create the X0X1X1X3Y0Y1Y2Y3 but I am stuck on how to compose them in the pattern explained above.

Any hints?

How about a loop taking one bit each from the two 4-bit numbers each iteration? — Some programmer dude, Jul 12 '16 at 11:56
See http://stackoverflow.com/questions/30539347/2d-morton-code-encode-decode-64bits — Bathsheba, Jul 12 '16 at 12:03
Start_Number = (Start_Number_High << 4) | (Start_Number_Low & 0xF); — Gaurav Chandarana, May 16 '19 at 14:34

score 3 · Answer 1 · answered Jul 12 '16 at 12:00

3

A very quick way of interleaving four-bit numbers is with a look-up table:

unsigned int spread[] = {
    0x00, 0x01, 0x04, 0x05, 0x10, 0x11, 0x14, 0x15, 
    0x40, 0x41, 0x44, 0x45, 0x50, 0x51, 0x54, 0x55
};

You can use spread[] array, which "spreads" bits of the original, to construct your output as follows:

unsigned int res = (spread[x] << 1) | spread[y];

The trick is in the construction of the look-up table. Its values are selected in such a way that the bits of the index are interleaved with zeros. For example, 0x07, or 0111₂, becomes 0x15, or 00010101₂.

answered Jul 12 '16 at 12:00

Sergey Kalinichenko

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Of course, this is feasible only if the number is small. If you have too many bits this `spread` array may be too big to be worthwhile. – Nathan Fellman Jul 12 '16 at 12:32
1

@NathanFellman A table with 256 entries would be sufficient to deal with most numbers one byte at a time, rather than one bit at a time. Since OP specifically asks for four-bit numbers, it shouldn't be an issue for him. – Sergey Kalinichenko Jul 12 '16 at 12:39

score 3 · Accepted Answer · answered Jul 12 '16 at 12:29

Here's a fairly direct way of performing this transformation:

uint8_t result;
result |= (x & 8) << 4;
result |= (y & 8) << 3;
result |= (x & 4) << 3;
result |= (y & 4) << 2;
result |= (x & 2) << 2;
result |= (y & 2) << 1;
result |= (x & 1) << 1;
result |= (y & 1) << 0;

score 0 · Answer 3 · answered May 22 '21 at 16:36

0

You can also do this in one line.

 uint8_t result =  (varX << 4) | (varY);

answered May 22 '21 at 16:36

alkman

1
1

How to combine 2 4-bit unsigned numbers into 1 8-bit number in C

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