Random Numbers

[bella09]

i am trying to generate a random number between 0 and 1. i thought the code would look like:

rand() % 0 + 1;

i keep getting a warning "potential mod by 0"

another method i thought of was:

(double)rand() / (double)RAND_MAX;

but that doesn't seem to work either...

does anybody have any ideas? thanks.

[Bazzy]

rand() % 0 returns the reminder of the division rand() / 0 which isn't possible

(double)rand() / (double)RAND_MAX; should work, why do you say that it doesn't work?

[bella09]

(double)rand() / (double)RAND_MAX

seems to be giving me the same number everytime..am i using it wrong?

double getrand () 
{
   double probability, r; 
   cout << "How likely is the virus to spread to a neighbor (0 = never, 1 = certain)? "; 
   cin >> probability;

   while (probability > 1 || probability < 0) 
{
   cout << "please enter a number between 0 and 1 "; 
   cin >> probability;
}

srand (probability);
r = (double)rand () / (double) RAND_MAX; 
return r;

}

[Bazzy]

You should call srand only once on your program

[Disch]

srand seeds the RNG. In order to get random numbers, you need to provide a seed that is, itself, somewhat random. Typically this is done by using the current time:

srand( time(0) );

Since srand() takes a unsigned integer as its seed, and not a double, your 'probability' is being truncated to either 0 or 1 (usually 0). So your above srand is not very effective.

+ what Bazzy said. Only call srand once. Like when your program first starts up.

[bella09]

ok, so srand() should be in main, not in my function. thank you.

what about the seed? if the user is entering a probability, should that probability be the seed?

srand(probability)?

[bella09]

sorry..didn't see your post disch..thanks

[robman8855]

rand() % x is calling for random integers within a range of x from 0 up. rand()%3 gives you

random numbers from 0-2. rand()%2 should give you 0s and 1s. rand() % 1 will give you

just 0s. so rand() % 0 is basically asking for nothing. personally i like that notation better

than(double)rand () / (double) RAND_MAX


side note (rand()%2)+1 is essentially a) getting random number 0 or 1 then b) adding

one to that number. in your rand()%0 +1 you

were using the +1 to add one to a nonexistant number. for 0s and 1s just use rand()%2


so you can stick with (double)rand () / (double) RAND_MAX or use rand%2 doesnt really

just good to understand how they both work