rand() seeded with time but values don't change

I've used the system time to seed the std random number generator in the past, but for some reason now it is not working. This is on a Mac running OS 10.6.1, compiling with gcc 4.2.1.

Code:

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#include <iostream>
#include <stdlib.h>
#include <time.h>

int main()
{
        unsigned int current_time = time(NULL);
          srand( current_time );
        std::cout << "\nTime: " << current_time << "\tRandom seed: " << srand  << std::endl;

        return 0;
}

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The output of this is that the time value changes, but the random seed does not! Here are two example outputs:

Time: 1253489088 Random seed: 1
Time: 1253489712 Random seed: 1

Surely I'm missing something obvious! But what?

You need to call rand().

Quote:

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Originally Posted by mmfuller

I've used the system time to seed the std random number generator in the past, but for some reason now it is not working. This is on a Mac running OS 10.6.1, compiling with gcc 4.2.1.

Code:

---

#include <iostream>
#include <stdlib.h>
#include <time.h>

int main()
{
    unsigned int current_time = time(NULL);
      srand( current_time );
    std::cout << "\nTime: " << current_time << "\tRandom seed: " << srand  << std::endl;

    return 0;
}

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The output of this is that the time value changes, but the random seed does not! Here are two example outputs:

Time: 1253489088 Random seed: 1
Time: 1253489712 Random seed: 1

Surely I'm missing something obvious! But what?

---

You're confused.

Passing the name of a function to an ostream effectively converts it to a function pointer, which an ostream will print '1' if the pointer is non-null, otherwise '0' (and since any given function is guaranteed to exist (indeed, it wouldn't compile otherwise), the value printed is *always* going to be '1').

At any rate, the actual seed value in this case is the time value that you passed to the srand function. After that point the implementer of the random library may even discard the value and resort to using some arbitrary (and obscure, as far as your program is concerned) data structure used to generate the next successive value. In other words, if you really need the seed at some point in the future,you'll need to store it in a variable somewhere.

Calling rand() with the same seed will produce the same value. Perhaps I should have stated my question better: why is the value of srand not changing when I give it different values of time?

Quote:

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Originally Posted by mmfuller

Calling rand() with the same seed will produce the same value. Perhaps I should have stated my question better: why is the value of srand not changing when I give it different values of time?

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Read post #3.

Quote:

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Originally Posted by mmfuller

Code:

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        std::cout << "\nTime: " << current_time << "\tRandom seed: " << srand  << std::endl;

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Quote:

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Originally Posted by Sebastiani

You're confused.

Passing the name of a function to an ostream effectively converts it to a function pointer, which an ostream will print '1' if the pointer is non-null, otherwise '0' (and since any given function is guaranteed to exist (indeed, it wouldn't compile otherwise), the value printed is *always* going to be '1').

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Quote:

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Originally Posted by mmfuller

why is the value of srand not changing when I give it different values of time?

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Now maybe you can see what you did wrong.

Sebastiani,
Thank you. You posted your initial reply at almost the same moment that I posted my reply to Dino. My reply was not a response to your post.

Your advice was very helpful. I did not realize that sending a function to an ostream would yield a 1.

Quote:

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Originally Posted by mmfuller

Sebastiani,
I did not realize that sending a function to an ostream would yield a 1.

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Oh, I thought it was more a case of you didn't even realise that you weren't actually even calling a function there, AND that it's the wrong function anyway. So you've changed

Code:

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<< srand <<

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to

Code:

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<< rand() <<

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now?