C语言中的地图编号

Question

How can i map numbers like this:

1 => 0x01;
2 => 0x03;
3 => 0x07;
4 => 0x0F;
....
8 => 0xFF;

I have only 8 numbers to map and i need this for my RT embedded system so the solution must be efficient. Is there a way to implement this using ENUM or DEFINE? I don't want to use switch statement. Should i use an array:

BYTE bMap[8] = 
{
    0x01,
    0x03,
    0x07,
    0x0F,
    ....
    0xFF,
}

or is there another way? Thank you! Max.

Answer 1

The two most obvious solutions would be:

1. Use an array. const uint8_t masks[] = { 1, 3, ... } .
2. Your mask seems to be "the i + 1 rightmost bits should be set", so you can trivally compute that at runtime using (1 << (i + 1)) - 1 which is easier to implement and less error-prone.

Answer 2

If you only have 8 values and that's not going to change, use the array. But note that the mapping would be 0=>0x01, 1=>0x03, ... etc., because C indexing is zero-based.

Also, look for a pattern in your numbers: you could find a logic or arithmetic operation that will set the least significant N bits in a byte. Ie N => (2 * N) -1

Answer 3

There's nothing wrong with using the lookup table to get your numbers but you could consider another approach.

If you're simply looping through those values in order, each one can be obtained by left-shifting the previous and setting the low order bit to 1:

      0000 0001 (x01)

<< 1: 0000 0010
 | 1: 0000 0011 (x03)

<< 1: 0000 0110
 | 1: 0000 0111 (x07)

<< 1: 0000 1110
 | 1: 0000 1111 (x0f)

<< 1: 0001 1110
 | 1: 0001 1111 (x1f)

So, something like:

for (unsigned int i = 1; i < 0x100; i = (i << 1) | 1) ...

should do the trick.

The only possible advantage I can see that may have would be not having to go out to memory for a lookup table. Depending on your hardware architecture, that may or may not be a problem.

The following complete program:

#include <stdio.h>

int main (void) {
    unsigned int i;
    for (i = 1; i < 0x100; i = (i << 1) | 1)
        printf ("%02x ", i);
    putchar('\n');
    return 0;
}

shows it in action:

01 03 07 0f 1f 3f 7f ff