Cycle through an array in either direction based on a bool - c

Question

I'm looking for a method of looping through some array in either direction based on some passed bool value, with the same functionality as:
void Transfer(bool *_payload, int _size, bool _isLSB)
{
if (_isLSB)
{
for (int i = _size - 1; i >= 0; i--)
{
digitalWrite(dataPin, _payload[i]);
}
}
else
{
for (int i = 0; i < _size; i++)
{
digitalWrite(dataPin, _payload[i]);
}
}
}
or
void Transfer(bool *_payload, int _size, bool _isLSB)
{
int _index = 0;
if (_isLSB) _index = _size - 1;
for (;;)
{
printf("%d",_payload[_index]);
if (_isLSB) _index--;
else _index++;
if (_isLSB && _index < 0) break;
if (!_isLSB && _index >= _size) break;
}
}
Other than creating a method that reverses the array, is there a nice simplification of this?

Answer 1

You can define the starting and ending point and the increment conditionally:
void Transfer(bool *_payload, int _size, bool _isLSB)
{
int increment = _isLSB ? -1 : 1;
int i = _isLSB ? _size : -1; // one before the area to scan
int end = _isLSB ? -1 : _size; // one past the area
while ((i += increment) != end) // incr/decr before testing
{
digitalWrite(dataPin, _payload[i]);
}
}
We do not know in advance which way the index will be changing (incrementing or decrementing), so we can't use less-than or greater-than in the loop condition. And after processing the last item the index will be modified once more, hence the stopping point is one past the area being processed.
Similarly we need the starting point one position before the scanned area, so that after incrementing (or decrementing) the index we process the valid, first item.

Answer 2

You can calculate the direction and the start/end position for the for loop depending on _isLSB
void Transfer(bool* _payload, int _size, bool _isLSB) {
int dir;
int start;
int end;
if(_isLSB) {
dir = -1;
start = _size-1;
end = -1;
}else {
dir = 1;
start = 0;
end = _size;
}
for(int i = start; i != end; i+=dir) {
digitalWrite(dataPin, _payload[i]);
}
}

Answer 3

What you could do for example, since in C true and false are expanded to integer values in reality, is to use said integer value for calculations.
In the following example I will extract the main problem from your question which is: Looping over a size in a direction depending on a boolean value
#include <math.h>
#include <stdbool.h>
#include <stdio.h>
int main()
{
int size = 5;
bool condition = false;
// Option #1
printf("Option #1\n");
for (int i = (size - 1) * condition; (i >= 0 && condition) || (i < size && !condition); i += 1 * pow(-1, condition))
{
printf("%d", i);
}
// Option #2
printf("\nOption #2\n");
int i = (size - 1) * condition;
for (;;)
{
printf("%d", i);
i += 1 * pow(-1, condition);
if ((i < 0 && condition) || (i >= size && !condition))
break;
}
return 0;
}
With the main function giving the following output for condition = true
Option #1
01234
Option #2
01234
And the following output for condition = false
Option #1
43210
Option #2
43210