LESSON 08 – RANDOM NUMBER

RANDOMNESS IN PROGRAMS

It is often useful to generate a random number in order to give your program/circuit a random behavior or starting point.

BUILT-IN FUNCTION

One can use the random function to get a random number. There are two versions of this function.

VERSION 1 – MAX ONLY

In this version, one can generate a random number between 0 and max (exclusive).

Here is an example of generating a number between 0 and 99.

int randomNumber;

randomNumber = random(100);

VERSION 2 – MIN & MAX

In this version, one can generate a random number between min and max.

Here is an example of generating an numberbetween 20 and 29:

int anotherRandomNumber;

anotherRandomNumber = random(20,30);

EXCLUSIVENESS OF MAX

Notice that the max is excluded from the possible range of numbers. So if you wanted to generate a number from 0 to 100 that could include 100, then you’d use

int randomNumber = random(101);

RANDOM SEED

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

EXAMPLE

Inside setup(), you would use:

randomSeed(analogRead(0));

where 0 is any pin that is floating as it will give a fairly unpredictable value.

REPEATED RANDOMNESS

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

EXAMPLE

Inside setup(), use

randomSeed(1000);

where 1000 could be any number.

TRY THIS…

PRACTICE 10

Write a program that will continuously generate a random number between 0 and 10 and output it to screen. Include a reasonable delay.

ALTERATIONS

Create two global variables called belowFive and aboveFive. They will count the number of random numbers that are either below (0 to 4) or above five (5-9). Their totals should be outputted evere 100^th number.