GoogleCodeExporter
commented
8 years ago This was also posted in the arduino forum...
It turns out that *the first* (pseudo) random number *after* seeding may or may
not appear to be random
depending on the arguments given to random().
Here are 2 programs that run 1000 trials for each random(0, max) where max goes
from 2 to 40. One seeds
only once, the other seeds prior to every use of random(). The difference is
quite clear.
(Note: to create nice big noisy seeds I called analogRead 32 times and then
used the 32 low bits to construct
the seed).
Phil
// program to test whether the numbers generated by random()
// are reasonably (pseudo) random. It turns out that regardless
// of the maximum value being used the results appear to be
// reasonably random.
int bins[40];
void setup(){
Serial.begin(9600);
long tempBits = 0; // create a long of random bits to use as seed
for (int i=1; i<=32 ; i++){
tempBits = ( tempBits | ( analogRead( 0 ) & 1 ) ) << 1;
}
randomSeed(tempBits); // seed
}
void loop(){
for (int max=2 ; max<=40 ; max++){ // test 2 through 40 as random maximums
for (int i=0 ; i<40 ; i++) bins[i] = 0; // zero out counting bins
for( int trial = 1 ; trial <= 1000 ; trial++ ){ // for each maximum run 1000 trials
bins[random( 0, max )]++; // and count the random number generated
}
Serial.print( max ); // display the results (counts for each random number)
Serial.print( " *** " );
for (int i=0 ; i<max ; i++){ // display the counts
Serial.print( bins[i] );
Serial.print( " " );
}
Serial.println(" ");
delay(400);
}
}
// program to test whether the *first number* generated by random()
// *after being seeded* is really (pseudo) random. It turns out that depending
// on the maximum value being used the result may or may not be
// reasonably random.
int bins[40];
void setup(){
Serial.begin(9600);
}
void loop(){
for (int max=2 ; max<=40 ; max++){ // test 2 through 40 as random maximums
for (int i=0 ; i<40 ; i++) bins[i] = 0; // zero out counting bins
for( int trial = 1 ; trial <= 1000 ; trial++ ){ // for each maximum run 1000 trials
long tempBits = 0; // create a long of random bits to use as seed
for (int i=1; i<=32 ; i++){
tempBits = ( tempBits | ( analogRead( 0 ) & 1 ) ) << 1;
}
randomSeed(tempBits); // seed
bins[random( 0, max )]++; // and count the random number generated
}
Serial.print( max ); // display the results (counts for each random number)
Serial.print( " *** " );
for (int i=0 ; i<max ; i++){ // display the counts
Serial.print( bins[i] );
Serial.print( " " );
}
Serial.println(" ");
}
}
Original comment by b...@philipgalanter.com on 10 Mar 2010 at 6:45
Original issue reported on code.google.com by
b...@philipgalanter.comon 10 Mar 2010 at 7:16