randomSeed() initializes the pseudorandom number generator, causing it to start at an arbitrary point in its random sequence. This sequence, while very long, and random, is always the same.

Library Documentation

void randomSeed(unsigned int seed)

Initialize the pseudo-random number generator.

  • seed -

    the number used to initialize the seed; cannot be zero.


If it is important for a sequence of values generated by random() to differ, on subsequent executions of a sketch, use randomSeed() to initialize the random number generator with a fairly random input, such as analogRead() on an unconnected pin.

Conversely, it can occasionally be useful to use pseudorandom sequences that repeat exactly. This can be accomplished by calling randomSeed() with a fixed number, before starting the random sequence.


The following sketch initializes the random seed based on an ADC reading of pin 0. If this pin is unconnected, the Sketch should print different values to the serial monitor each time it is run:

long randNumber;

void setup() {
  pinMode(0, INPUT_ANALOG);

void loop() {
  randNumber = random(300);


See Also

License and Attribution

Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.