Arduino error: 'was not declared in this scope' (4 causes and fixes)

You press Verify and the Arduino IDE answers with a red line: 'digitalwrite' was not declared in this scope. It sounds like a deep problem. It almost never is. It means the compiler met a name it has not been introduced to. This post walks through the four reasons that cause it in beginner sketches, and shows what Mokxi says for each one when you build the same code in the browser.

One thing first. The Arduino IDE uses a compiler called GCC, and GCC says "was not declared in this scope". Mokxi builds with clang, which words the same mistake as use of undeclared identifier. The two messages mean the same thing, so everything below applies to both.

Open this circuit in Mokxi

What the message means

In C++, every name has to be declared before you use it. A declaration tells the compiler "this name exists, and this is what it is". int ledPin = 13; declares a variable. #include <Servo.h> brings in a whole set of declarations from a library. When a line uses a name the compiler has no declaration for, at that point in the file, you get the error.

The word "scope" matters too. A name only exists inside the curly brackets where you declared it, and below the line where you declared it.

Cause 1: a capital letter is wrong

C++ is case sensitive. digitalWrite works. digitalwrite does not. Here is a blink sketch with two typos:

int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
}

void loop() {
  digitalwrite(ledPin, HIGH);
  delay(500);
  digitalWrite(ledpin, LOW);
  delay(500);
}

I built this in Mokxi's headless runner (the same compiler and simulator as the editor, driven from a terminal). It stopped with two errors:

e1.ino:8:3: error: use of undeclared identifier 'digitalwrite'; did you mean 'digitalWrite'?
e1.ino:10:16: error: use of undeclared identifier 'ledpin'; did you mean 'ledPin'?

Clang even guesses the right spelling. In the Mokxi editor, the build card puts it in a sentence: "Arduino spells it digitalWrite, not digitalwrite. Names in C++ must match exactly, capital letters included." It also offers a button that reads Change it to digitalWrite. For ledpin, where clang's guess differs only in case, there is a matching Change it to ledPin button.

The fix: match the capitals exactly. Library names count too: Serial has a capital S, and Servo has a capital S.

Cause 2: the variable lives in a different function

This one trips almost everyone once. You declare a variable inside setup() and then use it in loop():

void setup() {
  int ledPin = 13;
  pinMode(ledPin, OUTPUT);
}

void loop() {
  digitalWrite(ledPin, HIGH);
}

ledPin only exists between the curly brackets of setup(). When setup() ends, the name is gone. The same sketch fails in the headless runner with use of undeclared identifier 'ledPin' on every line of loop() that uses it.

In the editor, Mokxi reads your sketch and explains it in your own terms. The card says the variable is declared inside setup() on a certain line, so only lines in there can use it and loop() cannot see it. It suggests declaring it once at the top of the sketch, outside every function.

The fix: move int ledPin = 13; to the top of the file, above setup(). If you want to set its value inside setup(), declare it at the top and assign it later without the int.

Cause 3: the name is used before it is declared

C++ reads from the top down. If line 5 uses count and int count = 0; is on line 9, line 5 fails. Mokxi's card for this says the name is declared below the line that uses it, and that you should move the declaration above its first use. Variables that more than one function uses belong at the top, outside every function.

Functions are a little kinder. Like the Arduino IDE, Mokxi writes a prototype for each function in your .ino tabs, so loop() can call a function you define further down. That rescue does not cover everything, though. Per Mokxi's help page on this error, functions inside a class or namespace, and functions whose parameter types are declared further down, get no prototype. For those, put the function or the type above the code that calls it.

Cause 4: a library name without its include

A servo is the classic case:

Servo myServo;

void setup() {
  myServo.attach(9);
}

void loop() {
  myServo.write(90);
}

Servo is not part of the core Arduino language. It comes from a library, and the sketch has to ask for it. In the Arduino IDE this fails with a message that mentions 'Servo' does not name a type. In Mokxi's runner it stops with unknown type name 'Servo', and the card says: "Servo comes from a library the sketch does not include. Add #include <Servo.h> at the top of the sketch." The fix button reads Add #include <Servo.h>, and one click puts the line in the right place.

The fix: add #include <Servo.h> as the first line. This is true for any library class: LiquidCrystal, DHT, Adafruit_SSD1306 and the rest each need their own include. Mokxi's help lists the headers it carries, including Servo.h, Wire.h, SPI.h, EEPROM.h, DHT.h and Adafruit_SSD1306.h.

A short checklist

When you see this error, go through these in order:

1. Read the first error only. The later ones are often fallout from the first.
2. Compare the name letter by letter with the one in the reference, including capitals.
3. Check where the name is declared. Is it above the line that uses it, and is it outside every function if two functions share it?
4. Is the name from a library? Add the include.
5. Fix, build again, and repeat. A fixed first error often clears five others.

Check that the fixed version runs

After fixing, the blink sketch is simply this, with the variable at the top and the right capitals:

int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
}

void loop() {
  digitalWrite(ledPin, HIGH);
  delay(500);
  digitalWrite(ledPin, LOW);
  delay(500);
}

In the headless runner it built in about a second, and pin 13 changed state 8 times in 4 seconds of simulated time. That is two seconds for a full blink: on for half a second, off for half a second, and each blink is two changes. In the editor you would see the LED on the Uno's pin 13 flash.

Keep going

For the longer list of messages, see Arduino compile errors explained and Build errors in plain words. You can try every example above on the Arduino simulator with an Arduino Uno and an LED. Breaking a sketch on purpose in a simulator is a calm way to learn what each message looks like. For the exact behavior of the call itself, the digitalWrite reference is the official source.

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