rand() is weird?

I am trying to use random numbers for a program, but they are not working as expected.

I am initializing the random seed like this:



I am using rand() in the program like this:



The odd thing about it is, when I used it in my program, it was always turning up the same number - it occasionally changed, but this was not often enough..

So I investigated and made a small program that does this:



What happens, is that in the loop, the numbers after the first one seem to be increasingly more and more random. Is this normal? Here are some sample outputs.

7 2 4 6 2
7 9 5 1 8
7 2 0 9 2

The first number rarely changes. I figure it has something to do with how the time is brought in, but what? To loop and use the last obtained number doesn't seem to make a lot of sense to me, though if nobody has any other solutions I suppose I could use it.

Ahhh. I found that even the non-random rand() % maxNum works fine for my purposes, though I still don't understand why what I stated earlier is giving me such strange results.

This is because you have to use srand() EVERY time before you use rand.... oftentimes I just write a function like my_rand() that automatically seeds using srand(time(NULL)) and gerates the random number and mods it by the arguement that I pass....

also, look at srand(time(NULL)) for a second, srand() will only seed rand() differently if it is provided with different input, but in a tight loop (like yours) the loop will execute in just a split second, and time(NULL) will never increase since it returns a number of whole seconds....

if you need to do a tight count like that, I recommend using a different seed for srand(), like maybe GetTickCount() (if you're on Windows)....

Pardon, but I'm not sure you understood...

I am not using rand() in a loop... I am using it once when my program starts up. But every time I run my program, the number is the same - it does not act random. i.e. for about 10 minutes or so, it is 9, then it's 1 for just as long...

I decided to experiment, so I wrote a program that would put it in a loop, and lo and behold, the numbers following the first number were more frequently randomated. The first number never changed. As you can see in my examples.

I also heard that setting srand() more than once is a bad idea... it's not like I could, though, because my program does rand() only ONCE.. hope you understand what I'm saying. The program that runs it 5 times was only a test, you could say it's a debug, I was trying to figure out what it's actually doing.

My program is just a guessing game - it picks a random number and you guess. It gives an indication of whether it is higher or lower - here's the code, hope you understand now.




Please ignore the other errors in the program, I'm sure they've got nothing to do with why it is doing this...

hm, that code works fine for me....

try setting your target this way:

Um, that's what I said

Okay, this is what I'm trying to say...

These are examples of the unwanted results that are occurring. (each line represents a new run of the program, and each space is a new iteration of the loop)

rand() % num :

0 6 8 0 8
3 7 2 5 2
3 7 2 5 2
6 5 8 0 9
6 5 8 0 9

num * rand()/RAND_MAX:

7 2 4 6 2
7 9 5 1 8
7 2 0 9 2
7 5 2 8 1

DESIRED result is something like this:

7 2 4 6 2
5 9 5 1 8
3 2 0 9 2
2 5 2 8 1

random even within 1 second of running the program last...

Is this clearer?

I heard that num * rand()/RAND_MAX was the best way to do it because rand() % num isn't actually random, or something... but rand() % num is closer to what I want.... num * rand()/RAND_MAX is really odd because it keeps giving me the same initial value when I run the program within around 10 minutes of running it last.

Maybe is there a way to get time() to give milliseconds too so that the seed is updated faster than 1 second...

On windows you can use GetTickCount() to return the seconds down to the thousandth of a second.... otherwise look into QueryPerformanceCounter()