Evaluate if a value is increasing or decreasing (arduino)

Question

I'm trying to learn how to evaluate if a value is increasing or decreasing. In this case I'm using a potentiometer mapped from 0 - 14. Basically I need it to look at the current value, and if the current value is increasing print one thing, and if the value is decreasing print something else.

Here's what I have so far, I know its not right, but its a start.

Thoughts?

Thank you.

void setup() { 
  Serial.begin(9600);  
}


void loop() {

  int val = analogRead(A0);  // read the input on analog pin 0:
  val = map(val, 0, 1023, 0, 14); // map the vlaues to new values
  Serial.println(val); // print those values
  delay(1); // delay by a second.

  // sudo code

  if (val++) {

    Serial.println("up");

  } else if (val--){

    Serial.print("down");

  }else{

    // do nothing
  }
}// end loop

Desko27 · Accepted Answer · 2013-11-15T14:28:08.947

Declaration of variables

int val = analogRead(A0);  // read the input on analog pin 0:

I don't recommend you to declare variables in the loop() function when you can declare them outside once:

// declaration here!
int val = 0;

void setup() { 
  Serial.begin(9600);  
}

void loop() {

  // just set the value
  val = analogRead(A0);  // read the input on analog pin 0:
  val = map(val, 0, 1023, 0, 14); // map the vlaues to new values

      ...

Increase / decrease checks

In C++ you can make an assignment even in a conditional statement. if (val++) and if (val--) won't do what you expect. The if statement checks if val is not false or 0, and then ++ increases it by 1. Same for val-- but decreasing by -1.

What you can do is to keep the previous value in another variable (let's say prev) to compare it later like this:

// declaration here!
int val = 0;
int prev;

    ...

void loop() {

    // keep the previous value
    prev = val;

    // just set the value
    val = analogRead(A0);  // read the input on analog pin 0:

    ...

    // compare the previous value
    if (val > prev) {
        Serial.println("up");
    } else if (val < prev) {
        Serial.println("down");
    }

    // and no need to leave empty an `else` block

}

Declaring the variables outside the loop declares global variables. This is commonly considered bad practice. Declaring them inside the loop has no speed penalties whatsoever. GCC is smart enough to figure out how to compile it best. — Udo Klein, Nov 19 '13 at 20:24

score 0 · Answer 2 · answered Nov 14 '13 at 23:03

You have to compare the current value with the old value to determine whether it is increasing or decreasing

void setup() { 
  Serial.begin(9600);  
  int oldvalue = 0;

}


void loop() {

  int val = analogRead(A0);  // read the input on analog pin 0:
  if(val > oldvalue)
  {
     // increasing
  }
  else if (val < oldvalue)
  {
     // decreasing
  }

  // get current value into oldvalue for next comparison
  oldvalue = val;

  // other code follows here...
}

score 0 · Answer 3 · answered Nov 15 '13 at 10:56

if (val++) {

means compare the value of val to 0, and if it is not 0 execute the if block. Also increment val (whether or not it was 0 to start with).

You seem to be thinking that that statement checks if val has changed, but it doesn't.

If you want to do that, (as other answers suggest) you need another variable to store the previous value of val, and then compare the two:

if (val > oldval) 
{ 
   Serial.println("Up\n");
}

Evaluate if a value is increasing or decreasing (arduino)

3 Answers3

Declaration of variables

Increase / decrease checks