Activity bot - parallax - moving the robots with sensors - C Programming

Question

This is my code for moving an activity bot from its starting point to the ending point. It detects obstacles on both sides and turns away from them.

I save the sequence of moves in an array so that I can return to the starting point without using any sensors. **(Which is the code after the "//return" comment)

#include "simpletools.h"
#include "abdrive.h"
#include "ping.h"

int back[200];
int i = 0;

int main() {  
  int distance;

  int irLeft = 0, irRight = 0;

  low(26);
  low(27);

  while (1) {
    if (ping_cm(8) < 5) {
      break;
    }

    freqout(11, 1, 38000);  
    irLeft = input(10); 

    freqout(1, 1, 38000);                     
    irRight = input(2);

    if (irLeft == irRight == 1) {
      drive_goto(10,10);
      back[i] = 10;
      ++i;
      back[i]= 10;
      ++i;
    }

    if (irLeft == 0) {
      drive_goto(20,10);
      back[i] = 20;
      ++i;
      back[i] = 10;
      ++i;
    }

    if (irRight == 0) {
      drive_goto(10,20);
      back[i]= 10;
      ++i;
      back[i]= 20;
      ++i;
    }
  }    
  drive_goto(51, 0);   // Make a 180 degree turn
  drive_goto(51, 0); 

//return
  while (1) {
    if (i == 0) {
      break;
    }
    drive_goto(back[i], back[--i]);
    --i; 
  }  

  return 0;
}        

The robot successfully moves to the goal, but it does not move back to the starting point. What can the problem be?

Answer 1

You appear to have an off-by-one error. As you move forward, you track in variable i the index of the next available position in your movement history array. When you start the trip back, you use the current value of i as if it were instead the index of the last value recorded. On the way back you need to decrement i before each read, mirroring the behavior on the forward path of incrementing it after each write.