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HELP w/ Random Numbers

Posted on 2003-10-22
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Write a single C statement that will print a random number from the set 2, 4, 6, 8, 10.

I know how to write the rand() function for consecutive numbers and scaling those numbers into a certain range, but for a set??  Help please!

Thanks!!
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Question by:billjuniors
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LVL 45

Expert Comment

ID: 9604405
keep the set sorted ...
generate random number as usual
bump the random number to to the next highest element of the set

e.g. suppose your random number is in r and set is in array set[]
i=0;
while ( i < MAX && r < set[i] )
i++;

r = set[i];

MAX is the cardianlity of the set
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Expert Comment

ID: 9604449
int num = random();

while ((num & 7) > 4)      num >> 1;

return (((num & 7) + 1) << 1);

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Expert Comment

ID: 9604474
sorry, previous post not a single statement

return (((random() % 5) + 1) << 1);

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Expert Comment

ID: 9604584
bret   thats  ingenious...
though random should be rand:
return (((rand() % 5) + 1) << 1);

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Expert Comment

ID: 9604618
Here set is an array which contains your random numbers

first you generate the random number and then map it your Set

int n = rand()%SET_MAX ;

return Set[n] ;

Dennis
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Expert Comment

ID: 9604679
> though random should be rand:

I explicitly used random() rather than rand() because rand() is a very poor
random number generator.  Extracts from the man pages:

RANDOM(3)               System Library Functions Manual              RANDOM(3)

NAME
random, srandom, srandomdev, initstate, setstate - better random number
generator; routines for changing generators

------------

RAND(3)                 System Library Functions Manual                RAND(3)

NAME
rand, srand, sranddev, rand_r - bad random number generator
...

DESCRIPTION
These interfaces are obsoleted by random(3).
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LVL 45

Expert Comment

ID: 9605874

There are several good suggestions already.  Here's another solution:

return ((random () %10) & 0xE);

Of course, if the result set is irregular, Sunnycode wins.  :)

Kent
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LVL 6

Expert Comment

ID: 9606200
re: num >> 1 and similar tricks for multiplying and dividing by two:

Most good compilers, with optimization turned on, will automatically convert i*2 or i/2 into an appropriate shift.  So keep your code lucid by writing num/2 or num * 2, and let the compiler do its job.

Gary
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LVL 23

Expert Comment

ID: 9608119
Kdo,

((random () %10) & 0xE) can produce 0 and won't produce 10.

Notice that with  (((random() % 5) + 1) << 1),

(random() % 5) produces one of { 0, 1, 2, 3, 4 }

+ 1  maps that set to { 1, 2, 3, 4, 5 }

<< 1 (or * 2)  then produces { 2, 4, 6, 8, 10 }

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LVL 45

Expert Comment

ID: 9608162

That's what I get for trying to write application code.  ((random () %10) & 0xE) + 1  does the trick.....

The world SHOULD think relative 0 and in octal.  ;)

Kent
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Accepted Solution

brettmjohnson earned 30 total points
ID: 9608601
((random () %10) & 0xE) + 1  produces only odd numbers

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Expert Comment

ID: 9608723

See?  I'm still trying to think like an application's coder.

((random () %10) & 017) + 2

It makes sense once that "0xE" got removed.  Who ever heard of putting letters where numbers are supposed to go????

;)

0

LVL 1

Expert Comment

ID: 9613243
if you get the number as an int in the range 1 - 5, you could just mulitply by 2
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Expert Comment

ID: 9886751
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