rand() gives only one number...

J0nathan · 07-15-2009, 09:18 AM

Hey.
Im a new C++ programmer and having trouble using rand() in programs.
the only number rand returns is 41.
Is there anything I can do to make it generate other numbers?
Or is there any better generators?
Thank you for reading this thread and have a good day =).

cpjust · 07-15-2009, 09:26 AM

How are you using it? Are you calling srand() first?
Post an example.

Sebastiani · 07-15-2009, 09:29 AM

>> Is there anything I can do to make it generate other numbers?

Of course not. Why would you want any other number? How useful would that be?

>> Or is there any better generators?

The implementation of rand is not dictated by the standard, so really just depends on how it is implemented on your platform.

>> Im a new C++ programmer and having trouble using rand() in programs.

You need to seed your generator before you first use it. Look up srand.

J0nathan · 07-15-2009, 09:42 AM

Okay i have looked srand up.
But i still don't understand its syntax :S
srand(time(NULL))

Sebastiani · 07-15-2009, 09:47 AM

That has nothing to do with it's syntax, but one particular usage. You should probably get a hold of some decent CRT API documentation. Here's what mine says:

Quote:

Header File

stdlib.h

Category

Math Routines

Syntax

#include <stdlib.h>
void srand(unsigned seed);

Description

Initializes random number generator.

The random number generator is reinitialized by calling srand with an argument value of 1. It can be set to a new starting point by calling srand with a given seed number.

Return Value

None.

Exercise: Now look up the documentation for the 'time' function. Things will be much clearer then.

Elysia · 07-15-2009, 09:58 AM

Remember that the seed must be initialized with a unique number, because a randomizer is basically just an algorithm applied to a unique number.
This is why we see this particular line of code very often.
Once you read the documentation on time, it should be clear.

J0nathan · 07-15-2009, 09:58 AM

okay. can you give me a example of rand() where it gives a totally random number and im not sure
what time has to do with random numbers.

Elysia · 07-15-2009, 09:59 AM

Code:

int main()
{
    std::srand( (unsigned int)std::time(NULL) );
    for (int i = 0; i < 10; i++)
        std::cout << std::rand() << std::endl;
}

It's still your job to figure out how this works.

Sebastiani · 07-15-2009, 10:22 AM

>> im not sure what time has to do with random numbers.

Random number generators are generally not really random, but fully deterministic and thus "psuedo-random". That simply means that if you give a particular implementation the value 3114 it will generate the same sequence of numbers everytime. Naturally, you'll probably want to get different values in most cases, so you need a function that can produce an ever-changing flow of numbers, and the 'time' function is one such source. But there are other possibilities, of course. You could access the CPU temperature, for instance, and produce a seed from that.

J0nathan · 07-15-2009, 10:40 AM

Ahh i get it now, Thanks a lot!

07-15-2009, 09:18 AM	#1
J0nathan Registered User Join Date: Jul 2009 Posts: 4	rand() gives only one number... Hey. Im a new C++ programmer and having trouble using rand() in programs. the only number rand returns is 41. Is there anything I can do to make it generate other numbers? Or is there any better generators? Thank you for reading this thread and have a good day =).
J0nathan is offline

07-15-2009, 09:26 AM	#2
cpjust and the hat of sweating Join Date: Aug 2007 Location: Toronto, ON Posts: 3,120	How are you using it? Are you calling srand() first? Post an example. __________________ "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
cpjust is offline

07-15-2009, 09:29 AM	#3
Sebastiani Guest Join Date: Aug 2001 Posts: 4,923	>> Is there anything I can do to make it generate other numbers? Of course not. Why would you want any other number? How useful would that be? >> Or is there any better generators? The implementation of rand is not dictated by the standard, so really just depends on how it is implemented on your platform. >> Im a new C++ programmer and having trouble using rand() in programs. You need to seed your generator before you first use it. Look up srand.
Sebastiani is offline

07-15-2009, 09:42 AM	#4
J0nathan Registered User Join Date: Jul 2009 Posts: 4	Okay i have looked srand up. But i still don't understand its syntax :S srand(time(NULL))
J0nathan is offline

07-15-2009, 09:58 AM	#7
J0nathan Registered User Join Date: Jul 2009 Posts: 4	okay. can you give me a example of rand() where it gives a totally random number and im not sure what time has to do with random numbers.
J0nathan is offline

07-15-2009, 10:22 AM	#9
Sebastiani Guest Join Date: Aug 2001 Posts: 4,923	>> im not sure what time has to do with random numbers. Random number generators are generally not really random, but fully deterministic and thus "psuedo-random". That simply means that if you give a particular implementation the value 3114 it will generate the same sequence of numbers everytime. Naturally, you'll probably want to get different values in most cases, so you need a function that can produce an ever-changing flow of numbers, and the 'time' function is one such source. But there are other possibilities, of course. You could access the CPU temperature, for instance, and produce a seed from that.
Sebastiani is offline

07-15-2009, 10:40 AM	#10
J0nathan Registered User Join Date: Jul 2009 Posts: 4	Ahh i get it now, Thanks a lot!
J0nathan is offline

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