Read from array two bytes at a time? - c

Question

I declare an array
uint8_t data_buffer[64];
I write into it, and then I need to iterate over it looking at the data it stores. The data is written in groups of two.
Right now I am doing something messy like this
for(int i = 1; i < BUFFER_LEN + 1; i += 2)
{
if(data_buffer[i] == 0xff && data_buffer[i+1] == 0xff)
{
write_led_states(i/2 + 1, OFF);
}
else
{
write_led_states(i/2 + 1, ON);
}
}
But I'd much prefer to be able to read two bytes of data at a time from the buffer so I could do something more like this:
for(int i = 1; i < BUFFER_LEN + 1; i++)
{
if(data_buffer[i] == 0xffff) // where data_buffer is being read two bytes at a time
{
write_led_states(i, OFF);
}
else
{
write_led_states(i, ON);
}
}
Another way of saying what I want to do is to remap the indices so they go from looking like this:
[1][2][3][4][5][6][7][8]
to like this:
[1...][2...][3...][4...]
Is this possible?

Answer 1

Change this:
uint8_t data_buffer[64];
to this:
uint16_t data_buffer[32];
uint_16_t uses 2 bytes (16 bits) per element (and adjust the size to half of 64).
PS: Your code seems to access an array out of bounds, which invokes Undefined Behavior, here:
for(int i = 1; i < BUFFER_LEN + 1; i += 2)
Array indexing starts from 0, so this should start from 0 and the condition should be i < BUFFER_LEN.
However, with my appraoch, you don't need to increment by two, so you simply do a clean for loop, like this:
for(int i = 0; i < BUFFER_LEN; ++i)

Answer 2

Simply use a uint16_t instead, which is 2 bytes per element,
uint16_t data_buffer[32];
be careful with endianness if it can be a problem.
Note: it appears that you don't know that arrays are 0 indexed, so
for (int i = 1; i < BUFFER_LEN + 1; i += 2)
shoul probably be (most likely),
for (int i = 0; i < BUFFER_LEN; i += 2)
And in case you do what I suggest, then each element is 2 bytes so you'd need half the number of elements and of course you would increment i by 1
for (int i = 0; i < BUFFER_LEN; ++i)

Answer 3

You can combine the two uint8_t elements into a single uint16_t by bhtshifting:
for (int i = 1; i < BUFFER_LEN + 1; i += 2) {
uint16_t pair = data_buffer[i] << 8 | data_buffer[i+1];
LEDState newLEDState = (pair == 0xffff) ? OFF : ON; // replace "LEDState" with the proper type
write_led_states(i/2 + 1, newLEDState);
}