Strange Struct Array Behavior

Projects Programming
1

Hi All,

I'm seeing a strange behavior in a struct array I created. The code looks like below, when I input an index beyond what is formed in the instantiation I am getting a (1,0,0,0) result.

For example when I have 4 objects in the array (index 0-3), the 5th (non-existent) object (index 4) is (1,0,0,0). When I have 5 objects (index 0-4), the 6th (index 5) is (1,0,0,0). Any calls to an object beyond that is (0,0,0,0). I was expecting errors, nulls, etc, possibly all 0s, but definitely not (1,0,0,0).

Any insight or assistance in eliminating this behavior and/or creating a "default" or "doesn't exist" case would be appreciated.

Below is my struct and array instantiation and usage:

//Declare a struct to contain Directions
struct MotorDirection{
  int m1;
  int m2;
  int m3;
  int m4;
};

//Directions setup
MotorDirection forward = {1,1,1,1};
MotorDirection left = {-1,1,1,-1};
MotorDirection backward = {-1,-1,-1,-1};
MotorDirection right = {1,-1,-1,1};

//Compile Directions into an array
MotorDirection Directions[] {forward,left,backward,right};

//Print a string of all of the array values
void loop()
{
      printDirection(Directions[0], 0);
      printDirection(Directions[1], 1);
      printDirection(Directions[2], 2);
      printDirection(Directions[3], 3);
      printDirection(Directions[4], 4);
      printDirection(Directions[5], 5);
  }

//Parse out a string to give feedback on the array values
void printDirection(MotorDirection md, int index)
{
      Serial.print("Index: ");
      Serial.print(index);
      Serial.print(", Motor Directions: ");
      Serial.print(md.m1);
      Serial.print(", ");
      Serial.print(md.m2);
      Serial.print(", ");
      Serial.print(md.m3);
      Serial.print(", ");
      Serial.println(md.m4);
}
2

if you have just 5 instances just print 5. If you print out of the boundaries you get "any" result.

btw: range based for loop comes very handy when dealing with arrays.

3

if you overrun arrays you get unpredictable behaviour depending on what memory locations are corrupted
programs may give incorrect results, crash immediatly, run for years without problems then one day with a different data set crash
in C++/Java/etc use vectors or similar containers with bounds checking (in fact Java arrays have built in bounds checking to prevent the type of problems you identified)

4

Why do you expecting almost anything, but not (1,0,0,0) ? :slight_smile: Is this value has specific meaning for you?

If you try to print array element beyond limits, you do not get a random result. On the contrary, you will get strictly defined value - contains of the memory address just after array end.

5

Don't go beyond the bounds of the array... simple.

6

Not sure if serious… When you access out of bounds you read random memory value, it’s called undefined behaviour, there is nothing strange about that

7

Simple solution, have the function check for an out of bounds value.

8

Thanks all, it totally makes sense that I should have an out of bounds check... I'm just new to coding in this environment that I was expecting it to be a blank (as I could add add'l objects) or err'd case (as I think VS would throw an error). I'll just put in that check... much appreciation!