I'm rather new to Arduino programming. I have a problem compiling the following bit of code:

const int relay1 = 10;  //Power Relay 1
const int relay2 = 11;  //Power Relay 2
const int relay3 = 12;  //Toggle Relay
const int button1 = 3;  
const int button2 = 4;
const int button3 = 5;

//---Button States---\\
int button1State;   //Current state of Button 1
int button2State;   //Current state of Button 2
int button3State;   //Current state of Button 3
int button1State_prev = LOW;  //Previous state of Button 1
int button2State_prev = LOW;  //Previous state of Button 2
int button3State_prev = LOW;  //Previous state of Button 3

//---General Variables---\\
int userSelection = 0;
int interlockState = 0;
int platformState = 0;

//---Interval-Tracking Variables---\\
unsigned long lastTime_Debounce1 = 0;   //Button 1 debounce time
unsigned long lastTime_Debounce2 = 0;   //Button 2 debounce time

//---Activity Delays---\\
const unsigned int relayDelay = 10;           //Delay between relay actions (ms)
const unsigned int debounceDelay = 60;        //Delay for button de-bouncing (ms)

void setup() {
  //Configure Pins
  pinMode(relay1, OUTPUT);
  pinMode(relay2, OUTPUT);
  pinMode(relay3, OUTPUT);
  pinMode(button1, INPUT);
  pinMode(button2, INPUT);
  pinMode(button3, INPUT);

  digitalWrite(relay1, LOW);
  digitalWrite(relay2, LOW);
  digitalWrite(relay3, LOW);

void loop() {
  //Read value of each input pin
  int button1Reading = digitalRead(button1);  //Current reading of Button 1
  int button2Reading = digitalRead(button2);  //Current reading of Button 2
  int button3Reading = digitalRead(button3);  //Current reading of Button 3

  //Debounce Button1
  if (button1Reading != button1State_prev) {
    lastTime_Debounce1 = millis();
  button1State_prev = button1Reading;
  if ((millis() - lastTime_Debounce1) > debounceDelay) {
    if (button1Reading != button1State) {
      button1State = button1Reading;

  //Debounce Button2
  if (button2Reading != button2State_prev) {
    lastTime_Debounce2 = millis();
  button2State_prev = button2Reading;
  if ((millis() - lastTime_Debounce2) > debounceDelay) {
    if (button2Reading != button2State) {
      button2State = button2Reading;

For some reason, the compiler is convinced that the variable lastTime_Debounce1 in the second IF-statement, on Line 54, has not been declared in-scope. I don't understand how this is possible, since the variable in question is a defined and initialized global.

If I comment-out the first trio of IF-statements (handling Button 1), the second trio (handling button 2) has no problem compiling, even though it does exactly the same thing in exactly the same way.

I checked all the usual suspects: spelling, braces, semicolons, even commented-out blocks of code one at a time, but I can't find the source of the problem. I'm using the Arduino 1.8.2 IDE.

Can someone please point out the mistake I'm missing?

  • 1
    I tagged your question with the C++ tag to see if the syntax highlighting would make the problem more obvious, but unfortunately not.
    – Nick Gammon
    Commented Jan 21, 2018 at 21:02

1 Answer 1


The backslash in your comments are the problem. As per C++ language definitions, a \ at the end of the line is interpreted as a "line continuation". Thus, your comment gets continued in the next line and your variable declaration and initialization is commented out.

This is apparent when opening your code with a good syntax highlighting program, like Notepad++.

enter image description here

Removing all \\ from the end of the comment line solves your problem.

Side note: For more info, look at https://stackoverflow.com/a/30290133/5296568. Backslashes at the end of a line are actually useful when used in multi-line macro definitions.

  • 1
    I'm surprised the code highlighter here doesn't catch that.
    – WernerCD
    Commented Jan 22, 2018 at 3:59
  • 2
    @WernerCD Stack Exchange code markup tries to intelligently detect what language is being used in the code block, but sometimes it fails to do so and thinks it's something it isn't. you can fix it by specifying <!-- language: insert-lang-here --> before the snippet.
    – Nzall
    Commented Jan 22, 2018 at 11:05
  • 1
    @Nzall see NickGammon comment above; even the StackOverflow syntax highlighter, when put in C/C++ mode (by adding C++ tag), doesn't highlight it correctly. Commented Jan 22, 2018 at 11:29
  • Why does that even work? Shouldn't the first \ demask the second one and thus not comment out the new line?
    – Nova
    Commented Jan 22, 2018 at 12:50
  • I am not aware of a \ in a comment being able to escape the next character as if it the `\` was placed in a character string. Commented Jan 22, 2018 at 12:51

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