error "sketch_dec05a:18: error: 'counter' was not declared in this scope"

Using Arduino Programming Questions
#1

I am a total noob.

I am reading the book Begining C for Arduino.

I am on page 62 and trying to put in the code as the book has it. Am I doing something Wrong?

Here is what I have:

/* alternate blink */

int ledred = 12;
int ledgreen = 13;

void setup(){
  pinMode (ledred, OUTPUT);
  pinMode (ledgreen, OUTPUT);
}
  
  void loop(){
    
    if (counter % 2 == 1) {
      digitalWrite (ledred, HIGH);
      digitalWrite (ledgreen, LOW);
      delay(1000);}
    
    if (counter % 2 == 0){
      digitalWrite (ledred, LOW);
      digitalWrite (ledgreen, HIGH);
      delay(1000);
    }
   
   counter = counter + 1;
    
  }
#2

You haven't told the computer that "counter" is a variable and what type of variable.
For example you could add it here:-

int ledred = 12;
int ledgreen = 13;
int counter =0;
#3

Thanks, that did it.

This is probably a dumb question, but how does it know that the led’s are pins and the counter is not?

Is it because of the digitalWrite lines?

#4

It doesn't know. The variables ledred, ledgreen, and counter are all just numbers and nothing more. The digitalWrite function takes a number that corresponds to one of the pins. You are substituting a variable for that number. But in the end, it is just a number. The processor knows nothing more about it.

#5

OK I see.

Thanks for the help. 8)

#6