srand function question

**keyboard** · 06-30-2012

I am studying from a book that was written in 2008. In the book they use srand like this:

Code:

#include <stdio.h>

main()
{

int var = 0;

srand(time());

rand(time() % 10 + 1;
...

Non of that would run on my machine. I had to write it like this:

Code:

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

main()
{

  int var = 0;

  srand(time(NULL));

  rand(time() % 10 + 1;
...

1. Why did I have to add the extra header files #include <stdlib.h>
and #include <time.h>?
2. Why did I have to add NULL as an argument to the rand function?
3. Why is 'rand(time(NULL))' not written with the 's' like this: 'srand(time(NULL))'?

**stahta01** · 06-30-2012

Originally Posted by keyboard

Code:

  rand(time() % 10 + 1;

Above likely type setting bug.

Likely they wanted this; to get a random value from 1 to 10.

Tim S.

Code:

  var = rand() % 10 + 1;

**keyboard** · 06-30-2012

That was my fault for not adding the "var =" to "rand() % 10 + 1;" but what about the other things?

**AndiPersti** · 06-30-2012

Originally Posted by keyboard

I am studying from a book that was written in 2008.

I think you should throw away the book

1. Why did I have to add the extra header files #include <stdlib.h>
and #include <time.h>?

Because "srand()" and "rand()" are defined in <stdlib.h> and "time()" is defined in <time.h>. You have to tell the compiler where it will find definitions of functions/variables if you don't define them yourself.

2. Why did I have to add NULL as an argument to the rand function?

Did you mean time(NULL)?
time() returns the current time. If you call it with a time_t pointer it also stores this time at that address, if you call it with NULL it just returns the current time.

3. Why is 'rand(time(NULL))' not written with the 's' like this: 'srand(time(NULL))'?

The two are different functions: srand() initializes the pseudorandom-number generator and rand() returns one random number generated by the generator.

Bye, Andreas

**keyboard** · 06-30-2012

AndiPersti: Thanks. That all makes sense to me. The book I am reading is garbage. I'll get another one. Any recommendations?

**laserlight** · 06-30-2012

By the way, AndiPersti made a typographical error in the second option for declaring main. It should be:

Code:

int main(int argc, char *argv[])

**keyboard** · 06-30-2012

Thanks laserlight!

**AndiPersti** · 06-30-2012

@laserlight: Sorry, I've edited my post because I wasn't really sure what the OP meant. Thanks anyway for the correction :-)

**AndiPersti** · 06-30-2012

Originally Posted by keyboard

I'll get another one. Any recommendations?

I personally like "C Programming: A Modern Approach" by K.N.King

Here is a whole thread about book recommendations.

Bye, Andreas

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