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C++ Random Numbers

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Join Date: Aug 2006

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uu666

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Re: C++ Random Numbers

#21

Nov 2nd, 2006

i found something these days that looks like this:

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#include <iostream.h> //for cout
#include <conio.h> //for clrscr() and getch()
#include <stdlib.h> //for randomize() and rand()
 
void main()
{
clrscr();
 
randomize();
cout << rand() << endl;
 
getch();
}
#include <iostream.h> //for cout
#include <conio.h> //for clrscr() and getch()
#include <stdlib.h> //for randomize() and rand()

void main()
{
clrscr();

randomize();
cout << rand() << endl;

getch();
}

hope it helps

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Join Date: Feb 2005

Posts: 84

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evilsilver evilsilver is offline

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Re: C++ Random Numbers

#22

Nov 4th, 2006

•

Originally Posted by brahle

BTW, why are you people using halfly C and halfly C++?
Why do you use cin and cout and normal pointers? You see, cin and cout are much slower than scanf() and printf(), and when you have to input/output more than 10k of data, you see the difference. That is why I use scanf() and printf(). Normal pointers aren't used much in C++ because there is a templated conatiner vector that you can use to easily manipulate arrays.

i know that is a faster way of doing it but remember that not everyone is dealing with that much memory need and may only know about cout and cin which are simpler to use and are first thing that people learn about (hello world program always uses this, that i see anyways)

this way the code snipet can be used by new programmers and by those that may not know this code yet

just thought i would point that out lol

o and by the way i am using microsoft cisual studio 2005 now, works great and has a lot more functionality than borland, but borland was good to learn on from it's simplicity too lol kk well there wasn't much help in this post so i gonna shut up now lol

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Join Date: Nov 2006

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bmanoman

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Re: C++ Random Numbers

#23

Nov 19th, 2006

How would i create more than one random number, between 1, and 3, kind of like the slots [#],[#],[#] each filled with a random number...i tried using the examples here but they would just give me the same random numbers and would bring up the same numbers every time i ran it. if i got [3],[2],[3]. it would keep repeating those every time i ran it...id really appreciate the help

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Join Date: Feb 2005

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evilsilver evilsilver is offline

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Re: C++ Random Numbers

#24

Nov 21st, 2006

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Originally Posted by bmanoman

How would i create more than one random number, between 1, and 3, kind of like the slots [#],[#],[#] each filled with a random number...i tried using the examples here but they would just give me the same random numbers and would bring up the same numbers every time i ran it. if i got [3],[2],[3]. it would keep repeating those every time i ran it...id really appreciate the help

the problem with rand is that it isn't truely random, it is a psuedo random, or fake random. it has a list of numbers that it follows and though is alright for some very light things, is very predictable and will repeat it's self eventually.

if you read the rest of the snipet look for srand() it basically has the compiler pick a spot in the string of numbers it has for psuedo random so that it looks more random everytime you run the program

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Join Date: Dec 2006

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draclich

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Re: C++ Random Numbers

#25

Dec 14th, 2006

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Originally Posted by Bob

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C++ Syntax (Toggle Plain Text)
#include <iostream> 
#include <ctime> 
#include <cstdlib>
 
using namespace std;
 
int main() 
{ 
    srand((unsigned)time(0)); 
    int random_integer; 
    int lowest=1, highest=10; 
    int range=(highest-lowest)+1; 
    for(int index=0; index<20; index++){ 
        random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0)); 
        cout << random_integer << endl; 
    } 
}
#include <iostream> #include <ctime> #include <cstdlib> using namespace std; int main() { srand((unsigned)time(0)); int random_integer; int lowest=1, highest=10; int range=(highest-lowest)+1; for(int index=0; index<20; index++){ random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0)); cout << random_integer << endl; } }

Greetings
I'm new to C++ and started playing with the above code. I edited the code a bit to allow for a wider range, and a smaller range, but no matter what range i specified my first number was always a 1 or a 2. Is there a way around that, or is it really random and the 20 some times I ran the code just kept giving me the same result out of bad luck? I'm using Microsoft Visual C++.

Thanks
-Drac

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Join Date: Oct 2006

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venomlash

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Re: C++ Random Numbers

#26

Dec 14th, 2006

Like evilsilver said, rand isn't really random. It uses a really complex algorithm on the seed number (which you set with srand), so every time the program is run, it gives you the same output from the same algorithm with the same input. Try putting a srand([something]) inside the for loop and try again.

Beware of the Rancor. I'm not kidding.
If it doesn't compile, try saying "By the power of MegaMan!!!" <this has kinda worked for me, actually...>
Scotland is NOT North Britain, Glasgow does NOT rhyme with "cow", and Robbie Burns is...well, if you don't already know who he was, you're kinda screwed.

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Join Date: Jan 2007

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Xakil

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Re: C++ Random Numbers

#27

Jan 27th, 2007

Hi, im kinda new at C++ and this site and I wanted to see if someone could better explain about having a list of numbers (say 50) and then grabing them one by one randomly, but not reusing any of the 50 numbers that have already been used.

Thanks in advanced...

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Join Date: Oct 2006

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venomlash

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Re: C++ Random Numbers

#28

Jan 29th, 2007

Create a

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 C++ Syntax (Toggle Plain Text) 
bool list[50]
bool list[50]

, an array of 50 boolean (yes/no) values and set all of them to true at the start by using a for loop. Then, create the following function:

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int randnum(int num){
    return rand()*num/(RAND_MAX+1);
}
int randnum(int num){
    return rand()*num/(RAND_MAX+1);
}

Then, inside the body of your program, write the following:

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 C++ Syntax (Toggle Plain Text) 
int printnum;
for(int i=0; i<50; i++){
    printnum=randnum(50);
    if(list[printnum]==true){
        list[printnum]=false;
        cout<<printnum+1<<endl;
    } else {
        i--;
    }
}
int printnum;
for(int i=0; i<50; i++){
    printnum=randnum(50);
    if(list[printnum]==true){
        list[printnum]=false;
        cout<<printnum+1<<endl;
    } else {
        i--;
    }
}

If this doesn't work, tell me.
Venomlash

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Join Date: Apr 2007

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ahlongxp

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Re: C++ Random Numbers

#29

Apr 23rd, 2007

•

Originally Posted by Bob

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C++ Syntax (Toggle Plain Text)
#include <iostream> 
#include <ctime> 
#include <cstdlib>
 
using namespace std;
 
int main() 
{ 
    srand((unsigned)time(0)); 
    int random_integer; 
    int lowest=1, highest=10; 
    int range=(highest-lowest)+1; 
    for(int index=0; index<20; index++){ 
        random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0)); 
        cout << random_integer << endl; 
    } 
}
#include <iostream> #include <ctime> #include <cstdlib> using namespace std; int main() { srand((unsigned)time(0)); int random_integer; int lowest=1, highest=10; int range=(highest-lowest)+1; for(int index=0; index<20; index++){ random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0)); cout << random_integer << endl; } }

 Help with Code Tags 
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int(range*rand()/(RAND_MAX + 1.0));
int(range*rand()/(RAND_MAX + 1.0));

always return 0 on g++ (GCC) 4.1.2.
changed into

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int((double)range*rand()/(double)(RAND_MAX + 1.0)) ;
int((double)range*rand()/(double)(RAND_MAX + 1.0)) ;

fixed this problem.

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Join Date: Jul 2007

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Arctic wolf Arctic wolf is offline

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Re: C++ Random Numbers

#30

Jul 21st, 2007

Hello everybody,
I wanted to ask Bob about this piece of code:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int main() 
{ 
    srand((unsigned)time(0)); 
    int random_integer; 
    int lowest=1, highest=10; 
    int range=(highest-lowest)+1; 
    for(int index=0; index<20; index++){ 
        random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0)); 
        cout << random_integer << endl; 
    }
int main() 
{ 
    srand((unsigned)time(0)); 
    int random_integer; 
    int lowest=1, highest=10; 
    int range=(highest-lowest)+1; 
    for(int index=0; index<20; index++){ 
        random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0)); 
        cout << random_integer << endl; 
    }

you said that this formula:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0))
random_integer = lowest+int(range*rand()/(RAND_MAX + 1.0))

is better than using modulu because it solves some kind of compiler problems,I wanted to ask what kind of problems does it solve(why is it so different from the one that uses modulu)
and I also wanted to ask how did you develope this formula(what concept is it based on)?

Thank you.