Arrays in Arduino - arrays

Question

I am trying to create an array and be able to compare the second to last and last item in an array. It needs to be constantly adding and comparing to work correctly. This is just a function I am trying to get running to help control a stepper motor function. I have a loop that is going to fast for me to be able to compare directly. I do know that some of it is wrong but as I haven't coded in C very much I can't figure out how to use an array correctly. Thank you in advance.
int P[10],V[10],i,x,y;
Serial.print("checkvalue = ");
Serial.print(checkvalue);Serial.print("\n");
Serial.print("P = "); Serial.print(P[i]); Serial.print("\n"); //attempting to print array
Serial.print("V = "); Serial.print(V[i]); Serial.print("\n"); //to see if it is collecting
//data correctly
//these variables are declared above in my code, just didn't copy in
Dgreadpb = digitalRead(13);
PBcheck = Dgreadpb;
//Serial.print("Button in = ");Serial.print(Dgreadpb); Serial.print("\n");
Dgreadvls = digitalRead(12);
VLScheck = Dgreadvls;
//Serial.print("Photo in = ");Serial.print(Dgreadvls); Serial.print("\n");
for (i = 0; i < 10; i++){
x = Dgreadpb;
y = Dgreadvls;
P[i] = x;
V[i] = y;
if (P[i-1] == P[i] && V[i-1] == V[i]){ //trying to compare second to
checkvalue == 0; //last term to the last term
return;
}
else if(P[i-1] != P[i] || V[i-1] != V[i]){
checkvalue == 1;
return;
}
}
delay (1000);

Answer 1

By "trying to compare second to last term to the last term", do you mean "Trying to compare second to last term with their previous"? If that's the case your indices are wrong, it should be for(i = 1; i<10; i++).
Also both conditions are opposite (Either both are equal or AT LEAST one of them is different), there is no need for else if. Even more, cause both conditions are opposite it will never complete the loop. I think that's not the intention, if you're trying to say that only one of them are different you should do:
if (P[i-1] == P[i] && V[i-1] == V[i]){ //If both are equal
checkvalue == 0;
return;
}
else if(P[i-1] == P[i] || V[i-1] == V[i]){ //If only one is equal
checkvalue == 1;
return;
}

Answer 2

Ok, then I'd make a different thing.
Looking at your comment, it looks like you want to do something like this: read the value of two pins, compare them to the last value you read and, if they are differernt, start the motor, otherwise stop it.
Now, a lot of info are missing (e.g. how do you check the motor? how often do you want to check the sensor? what sensor?) but IMHO you should do something like this.
In this code I suppose that
you want to check the sensor every 100 milliseconds
if the values differ, you want to turn on the motor for the next 100 ms
the motor is a DC motor turned on by setting the corresponding pin (e.g. 10)
the sensors have a binary output on pins 12 and 13, since you wrote that in the code
BTW I used the millis() function because I hate the delay, since it blocks the uC. Using my function you'll be able to perform other operations while it is idle.
const byte motorPin = 10;
const byte sensorPPin = 12;
const byte sensorVPin = 13;
#define LOOP_TIME_MS 100
unsigned long lastLoopTime;
boolean lastPval, lastVval;
void setup()
{
pinMode(motorPin, OUTPUT);
pinMode(sensorPPin, INPUT);
pinMode(sensorVPin, INPUT);
lastPval = digitalRead(sensorPPin);
lastVval = digitalRead(sensorVPin);
lastLoopTime = millis();
}
void loop()
{
if ((millis() - lastLoopTime >= LOOP_TIME_MS)
{
boolean Pval = digitalRead(sensorPPin);
boolean Vval = digitalRead(sensorVPin);
if ((Pval != lastPval) || (Vval != lastVval))
{
digitalWrite(motorPin, HIGH);
}
else
{
digitalWrite(motorPin, LOW);
}
lastLoopTime += LOOP_TIME_MS;
}
/* Here you can do something else */
}
EDIT: If, on the other side, you want to use arrays (because you want to test the last N values instead of just the previous one) please provide further info on what are the changing conditions (or better provide examples)