Traversing INT array in two ways - c

Question

Traversing INT array in two ways is a robotic funny code (in C).
I have an array of positions like this: int pos[] = {0, 45, 90, 135, 180, 135, 90, 45};
These positions are used to move a servo motor.
45 90 135
\ | /
\ | /
\ | /
0 ----------- 180
In main loop() I check distance from an obstacle, and if it's < xx Cm my servo must rotate to next step (next array position) until it finds a free way ( > xx Cm ).
My main is easy:
int main (int argc, const char * argv[]) { for (;;) find(); }
and my core function (find) is this:
void find() {
for ( i=0; i<sizeof(pos); i++ ) // Traversing position array
{
distance = rand() % 7; // Simulate obstacle distance
move( pos[i] ); // Simulate movements
if (i==sizeof(pos)) { i=1; } // Try to reset the "i" counter. PROBLEM!
if ( distance<=5 ) continue; // Is there an obstacle?
sleep(2); // Debug sleep
find(); // Similar recursion
}
}
I don't know what is wrong in this code, but I need to move servo until is there not an obstacle.
Example:
At position 90 I find an obstacle. I want to loop array from left to right and viceversa controlling distance every step. If I don't find a freeway, print("ko") else print("ok").
How do I fix this code to work correctly?

Answer 1

You really want i < sizeof(pos) / sizeof(*pos) rather than i < sizeof(pos). The size of an array is not the number of its elements but rather the total byte count it occupies in memory.
sizeof(pos) yields 8 * sizeof(int). If an int is 4 bytes, you are looping 32 times instead of 8.
Also, i == sizeof(pos) will never be true in the body of the loop because the condition of the for statement limits i to sizeof(pos) - 1.

Answer 2

If I understand your question correctly, you want the servo to make a sweeping movement from left to right and then back from right to left. Measuring the distance to an object that can be in front of the robot at each angle. If there is a free way ahead of the robot, the find method returns.
int pos[] = {0, 45, 90, 135, 180, -1};
void find()
{
int i = 0;
int direction = 1;
do {
move(pos[i]);
i += direction;
if (pos[i+direction] == -1) direction = -1;
if (i==0) direction = 1;
} while(measure_distance() <= 5);
}
Instead of recursion, there is a while loop that only exits when there is a distance greater then 5.
The 'pos' array has a sentinel at the end (-1). This is an invalid angle and can be used to find the end of the array. There is no need to calculate the number of elements.
The left-right, right-left movement comes from using the 'direction' variable. It is rather easy to detect either the beginning (i==0) or the end of the 'pos' array (pos[i+1] == -1), at which point we reverse the direction.
There is also no need to repeat the angles after 180 degrees. The sequence we get is:
0 45 90 135 180 135 90 45 0 45 90 ...
We can even reduce the code with one line...
...
if (pos[i+direction] == -1 || i == 0) direction *= -1;
...
Cheers,
Johan

Answer 3

Don't forget to initialize your rand function using
/* initialize random seed: */
srand ( time(NULL) );
distance = rand() % 7;

Answer 4

It's probably good practise to say:
#define POSLENGTH 8
and then iterate using i<POSLENGTH: as others have pointed out using sizeof(pos) is probably not going to work.
Also, arrays in C are 0 based: elements go 0,1,2,3,...n-1.
So, you need to say:
if (i==POSLENGTH-1) i=0;

Answer 5

Try using a while loop instead of a for loop. Increment the value when there is no obstacle and break when you find an obstruction:
{
......
......
i = rand()%7;
move( pos[i]);
if (i<5)
break;
else
continue;
.......
.......
}
This will randomly choose the position until you get an obstacle and also there will be no need to reset it as the loop will automatically break on encountering an obstacle.