Generating Random Numbers

[tiktik]

Hey,
I had school exercise in which i needed to generate some numbers within the range of 70 to 90. I'd like to know whether there exists an exact formula which allows the numbers to be generated within a particular range.


When I had to generate numbers from 10 to 20, the correct code was :

float a = (float) (Math.random()*10) +10;

but I can't establish a general rule for all the ranges...

Any ideas pls??

[Nepomuk]

OK, let's have a look at what that formula does.

float a = (float) (Math.random()*10) +10; // Your formula
 
double x = Math.random; // create a pseudo-random number with 0.0 <= x < 1.0
x = x * 10;             // multiply that random number with 10, so you'll have a random number 0.0 <= x < 10.0
float a = (float) x;    // cast that double (64 bit) to float (32 bit)
a = a + 10;             // add 10 to a, so you'll have a variable 10.0 <= a < 20.0

With that explanation, can you write code to create a random number between 70 and 90?

Greetings,
Nepomuk

[JosAH]

If you have to generate numbers in the range [lo,hi) (inclusive and exclusive), you
have to generate numbers in the range lo+[0, hi-lo), so if R is a random number
in the range [0, hi-lo) then lo+R is the wanted random number. If you can only
generate random numbers in the range [0,1) == R' then R == (hi-lo)*R'

kind regards,

Jos

[tiktik]

OK, let's have a look at what that formula does.

float a = (float) (Math.random()*10) +10; // Your formula
 
double x = Math.random; // create a pseudo-random number with 0.0 <= x < 1.0
x = x * 10;             // multiply that random number with 10, so you'll have a random number 0.0 <= x < 10.0
float a = (float) x;    // cast that double (64 bit) to float (32 bit)
a = a + 10;             // add 10 to a, so you'll have a variable 10.0 <= a < 20.0

With that explanation, can you write code to create a random number between 70 and 90?

Greetings,
Nepomuk

Thanks a lot ... i think i've found it :

int a = (int) (Math.random()*20) +70;

isn't it?...
(it does work)

thanks again =)

[Nepomuk]

int a = (int) (Math.random()*20) +70;

Yes, well done! :-) (Although you're using an integer here and a float in your example - any specific reason for that?)

Greetings,
Nepomuk