static

[Variable Scope & Qualifiers]

Description

The static keyword is used to create variables that are visible to only one function. However unlike local variables that get created and destroyed every time a function is called, static variables persist beyond the function call, preserving their data between function calls.

Variables declared as static will only be created and initialized the first time a function is called.

Example Code

/* RandomWalk
  Paul Badger 2007
  RandomWalk wanders up and down randomly between two
  endpoints. The maximum move in one loop is governed by
  the parameter "stepsize".
  A static variable is moved up and down a random amount.
  This technique is also known as "pink noise" and "drunken walk".
*/

#define randomWalkLowRange -20
#define randomWalkHighRange 20
int stepsize;

int thisTime;

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

void loop() {
  //  test randomWalk function
  stepsize = 5;
  thisTime = randomWalk(stepsize);
  Serial.println(thisTime);
  delay(10);
}

int randomWalk(int moveSize) {
  static int place; // variable to store value in random walk - declared static so that it stores
  // values in between function calls, but no other functions can change its value

  place = place + (random(-moveSize, moveSize + 1));

  if (place < randomWalkLowRange) {                               // check lower and upper limits
    place = randomWalkLowRange + (randomWalkLowRange - place);    // reflect number back in positive direction
  }
  else if (place > randomWalkHighRange) {
    place = randomWalkHighRange - (place - randomWalkHighRange);  // reflect number back in negative direction
  }

  return place;
}

See also

  • Language const

  • Language scope

  • Language volatile