Randomizing

I'd like to make a program that chooses randomly. Say, either Potato or Apple. To do this I figured I'd choose randomly from the numbers 1 to 10, and then decide which of the vegetables I'd choose. 1-5 = Apple, 6-10 = Potato. How do I do this? I know its a piece of cake for you experienced programmers...

regards,
Morph

First you seed the random number generator so that the next time you run the program it's not the same, then just use the rand() function.

Code:

#include <iostream>
#include <cstdlib>
#include <ctime>

using namespace std;

int main()
{
  srand(time(0));

  int r = rand() % 10 + 1;
  cout<<"Random number -- "<< r <<endl;
}

It seems I'm missing a couple of the library files. I'm using Borland C++ 5, and it can't find the cstdlib.h and ctime.h files.

>>I'm using Borland C++ 5, and it can't find the cstdlib.h and ctime.h files.
If you're using them with the .h at the end then it won't work, remove the extension and it should compile fine. If it doesn't then change them to stdlib.h and time.h.

It works now. Thanks a lot Cela

If you only want your program to choose between two random values, you need only do this:

Code:

rand() % 2;

That will evaluate to either zero or one. That should save you a little room and time.

>>That will evaluate to either zero or one.
Yea, but how random will it be? In my experience not very, that's why if I need a random value between 0 and 1 I do this :-)

Code:

(rand() < RAND_MAX/2) ? 0 : 1;

They should both be just as random. The only reason why yours would possibly be better would be if for some reason the random number generated more odds than evens or vice versa. I've never had a problem with that and all the tests I've done where it gets millions of numbers has always shown a 50/50 spread.

>> I've never had a problem with that
It works for you, that's good. It works well on one compiler I use too, but only that one. Read the link Salem posted, just because your generator is good enough to handle rand() % 2 well doesn't mean everyone else who uses your code will be as lucky. You're not just programming for yourself, but for other people to, so be nice to them and program portably. :-)

Ahhhh... that makes sense

if you want it to randomize every time you run it you need to put a
"randomize();"
function in it or it i think will come up with the same random every time it is runned

The alpha way.

Code:

#ifndef _MAIN_CPP_
#define _MAIN_CPP_

// C++ Includes
#include <iostream>

// C Includes
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

using std::cout;
using std::endl;

char* Fruit(void);         // returns a string

int main(int argc, char **argv)
{
 cout << "Today you'll be eating " << Fruit() << endl;

 return( cin.get() ? EXIT_SUCCESS:EXIT_FAILURE);
}

char* Fruit(void)
{
 srand( (int)time(0) );
 char* lpFruit;
 int result=1 + (rand() % 5);

 if(result== 1) { lpFruit= "Apples"; }
 else if(result== 2) { lpFruit= "Grapes"; }
 else if(result== 3) { lpFruit= "Banana"; }
 else if(result== 4) { lpFruit= "Peach"; }
 else if(result== 5) { lpFruit= "Urine"; }

 return(lpFruit);
}

#endif

Why does the rand() function work differently on different compilers or computers? Shouldn't it be reliable if it's part of the standard library? I read the posted link, but it didn't provide as much information on the subject as I wanted. Please explain the discrepancy.

Random numbers are a ***** to get right, and you can't expect everyone to use the best algorithm. rand is standard and every implementation meets the requirements, but that doesn't mean they're all the same.

Originally posted by Crimpy
Random numbers are a ***** to get right, and you can't expect everyone to use the best algorithm. rand is standard and every implementation meets the requirements, but that doesn't mean they're all the same.

Why doesn't the standard require everyone to use one algorithm that is determined the best? What are the current standards on the subject, and why is the current standard not uniform? I always thought the point of having a standard was to make things standard.