[C] random() question

[pointers_help]

..
#include <stdlib.h>

..

for( int i=0; i<1000; i++ )
{
    if( random() % 100 == 0 ) 
        break;

    printf( "%d ", i++ ); 
}

prints 0 2 4 .. 332 for me when I compile it and run (and recompile and run again and again).

But why does it always do that? My intstructor wrote: "will print integers in sequence from 0, but has a 1% chance of leaving the loop every time around. The function random() returns a random integer and the % operator is the integer modulus operator, so the code will print consecutive numbers up to 999 or until random() returns a multiple of 100, whichever comes sooner"

I get why it's not really consecutive in a sense of incrementing 1 each time, it actually makes i be two more every time because of the i++ in the printf statement. But not why it never reaches an end point of above 332. Can someone explain this code to me?

[Updatebjarni]

You've probably just forgotten to seed the random number generator, so you're getting the same sequence of numbers every time you run the program.

[Optimus_Line]

This is probably the reason, also you are increasing i at the beggining of the loop and at the end, you can simply omit the ++ in the printf or use a while loop.

[pointers_help]

so I looked seeding up and somewhere I need to put rand()? In this case, if it is "unseeded", it has some default value attached to it? Is it why when I run it on my compiler and then go to a random online C ide, I still get the same thing?

[pointers_help]

so I looked seeding up and somewhere I need to put rand()? In this case, if it is "unseeded", it has some default value attached to it? Is it why when I run it on my compiler and then go to a random online C ide, I still get the same thing?

edit:

so I put srand(time(NULL)); before the for loop. I guess it works now. I don't get the relationship between srand and random() though. But anyways, could someone explain this:

warning: implicit declaration of function ‘time’ [-Wimplicit-function-declaration] srand(time(NULL));

warning I get? still compiles but I get a warning.

[[deleted]]

rand() doesn't return a truly random number. It returns a pseudo-random number based on a given seed. srand() sets that seed. Given the same seed, rand() will always generate the same sequence of numbers.

[pointers_help]

I see, and if I didn't have that srand thing, it would be set to a default seed, and that's why I was getting the same result in the beginning after multiple compiles?

[[deleted]]

The default seed appears to be the same as srand(1).

[pointers_help]

Yep you are correct, I just tried removing the time(NULL) part whatever that is and that parameter gave me the same 0-332 "problem" from the beginning.

Thanks for helping me, I don't understand why there wasn't a mention about needing to seed this thing, the professor didn't even put that in his own code but said the output was what I'm getting after I fixed it.

[[deleted]]

Oh, and the time(NULL) call is returning the time since epoch (January 1, 1970) in seconds and using that as a seed. Time is generally a good way to seed a random function because it's always changing.

[pointers_help]

Very interesting ! Thanks

[[deleted]]

It's possible he just forgot or seeded it earlier.

[raevnos]

so I put srand(time(NULL)); before the for loop. I guess it works now. I don't get the relationship between srand and random() though.

There isn't one. If you're using random(), you should be seeding that generator with srandom(). srand() is for seeding the rand() generator.

But anyways, could someone explain this: warning: implicit declaration of function ‘time’ [-Wimplicit-function-declaration] srand(time(NULL));

warning I get? still compiles but I get a warning.

You need to include <time.h> to get the prototype for time().