"function" was not declared in this scope (called in setup) I have this bit of code I'm trying to execute only once. but I seem to get this error every time I try to compile: "calcOppervlakte' was not declared in this scope" when it's being called in the setup.

I've looked at code examples where calling functions like this works, and I cannot wrap my head around this problem.

google isn't really helpful either since the search for something-something not declared in this scope gets me thousands of results.

double pi = 3.14159265359;

void setup() {
  Serial.begin(9600);
  for(int r = 1; r &lt;11; r++){
    Serial.print(calcOppervlakte(r)); &lt;----
  }
}

void loop() { 

}

double calcOpppervlakte(float radius){
  double result = pi * pow(radius, 2);
  return result;
}
The function name you're calling is calcOppervlakte but the function name in the definition is calcOpppervlakte. If you look closely you'll see there is an extra p in the function definition. If you change the names to match the error will be resolved.