Newbie question on time()
 

I am learning C from a tutorial book. The author explains how srand(time())
is a good way to generate a random number, and I have used it in a few
small example programs. I have come to a larger program that I am supposed
to modify, and in this sample code the author has used srand(time(NULL))
and in other places in the code has assigned time(NULL) to a couple of
variables. What does the NULL do in the time() function? It is not
necessary for me to know to complete this "assignment", but the author
fails to explain this sudden change in the code example and I am keen to
understand exactly what it is that I am looking at.


--
Todd Stephens
ICQ# 3150790
"A witty saying proves nothing." -Voltaire

Re: Newbie question on time()
 

"Todd Stephens" <Huzzah!@Huzzah.com> wrote in message
news:llpfb.2763$qw.367764@twister.tampabay.rr.com. ..
> I am learning C from a tutorial book. The author explains how
srand(time())

The 'time()' function has one parameter, for which
an argument must be supplied by the caller. The call
you show above should not compile.

> is a good way to generate a random number,

'srand()' does not generate a random number. It 'seeds' the
random number generator used by the 'rand()' function.

The value generated by 'time()' is often used as an argument
to 'srand()', as it produces the closest thing to a 'random'
value available to a standard C program.

> and I have used it in a few
> small example programs. I have come to a larger program that I am
supposed
> to modify, and in this sample code the author has used srand(time(NULL))
> and in other places in the code has assigned time(NULL) to a couple of
> variables. What does the NULL do in the time() function?

The time function returns a type 'time_t' object. Additionally
the address of a type 'time_t' object can be supplied as
an argument, in which case the same value which is returned
is also stored at that address. If this is not needed or
wanted, a NULL value can be passed instead, in which case
the parameter is ignored. It's a redundancy. I suppose
some might find a use for this behavior, but I never have.


>It is not
> necessary for me to know to complete this "assignment", but the author
> fails to explain this sudden change in the code example and I am keen to
> understand exactly what it is that I am looking at.

Change from what to what? 'time()' is a function in its
own right with various uses. The author was using its
return value as a seed for the random number generator.

I think you've mistakenly got the idea that 'srand()'
and 'time()' have some special relationship which does
not exist.

-Mike

Re: Newbie question on time()
 

Mike Wahler wrote in article
<zwqfb.682$Qy2.200@newsread4.news.pas.earthlink.ne t>:

> Change from what to what? 'time()' is a function in its
> own right with various uses. The author was using its
> return value as a seed for the random number generator.

Yes, but in earlier code examples, the author omitted the NULL assignment
and used simply srand(time()). I was confused when, in the next example,
he included it. You indicated that this would not compile without NULL,
but it did and the program ran fine. Here is the code from a silly
dice-rolling emulator:

#include <stdio.h>

main()
{

int intDie1 = 0;
int intDie2 = 0;
int intResult = 0;
srand(time());

intDie1 = (rand() % 6) +1;
intDie2 = (rand() % 6) +1;
intResult = (intDie1 + intDie2);

if (intResult == 7 || intResult == 11){
printf("\nYOU WIN!!\n");
printf("First roll was %d and second was %d\n", intDie1, intDie2);
}
else{
printf("\nYOU LOSE!!\n");
printf("First roll was %d and second was %d\n", intDie1, intDie2);
}
}

This code compiled and the resulting binary worked fine using gcc 3.2.3

> I think you've mistakenly got the idea that 'srand()'
> and 'time()' have some special relationship which does
> not exist.

I wasn't really asking about any relationship between them. Just wondering
what the difference between using time(NULL) and time() is. So,
realistically I should use NULL whenever I call time() without wanting any
specific value to time()?


--
Todd Stephens
ICQ# 3150790
"A witty saying proves nothing." -Voltaire

Re: Newbie question on time()
 

Todd Stephens wrote:
> Mike Wahler wrote in article
> <zwqfb.682$Qy2.200@newsread4.news.pas.earthlink.ne t>:
>
>
>>Change from what to what? 'time()' is a function in its
>>own right with various uses. The author was using its
>>return value as a seed for the random number generator.
>
>
> Yes, but in earlier code examples, the author omitted the NULL assignment
> and used simply srand(time()). I was confused when, in the next example,
> he included it. You indicated that this would not compile without NULL,
> but it did and the program ran fine. Here is the code from a silly
> dice-rolling emulator:
>
> #include <stdio.h>

You are missing needed includes:
#include <time.h>
#include <stdlib.h>
>
> main()
int main(void)
> {
>
> int intDie1 = 0;
> int intDie2 = 0;
> int intResult = 0;
> srand(time());
>
> intDie1 = (rand() % 6) +1;
> intDie2 = (rand() % 6) +1;
> intResult = (intDie1 + intDie2);
>
> if (intResult == 7 || intResult == 11){
> printf("\nYOU WIN!!\n");
> printf("First roll was %d and second was %d\n", intDie1, intDie2);
> }
> else{
> printf("\nYOU LOSE!!\n");
> printf("First roll was %d and second was %d\n", intDie1, intDie2);
> }

return 0;
> }
>
> This code compiled and the resulting binary worked fine using gcc 3.2.3
>

You did not get an error because you omitted the header, time.h, that
prototypes function time.

>>I think you've mistakenly got the idea that 'srand()'
>>and 'time()' have some special relationship which does
>>not exist.
>
>
> I wasn't really asking about any relationship between them. Just wondering
> what the difference between using time(NULL) and time() is. So,
> realistically I should use NULL whenever I call time() without wanting any
> specific value to time()?
>

The synopsis for function time.
#include <time.h>
time_t time(time_t *timer);

So you see, function time takes an argument. The argument can
be a pointer to a time_t object or it can be a null pointer, time(NULL).
In the code, you omitted an argument and you also failed to include the
needed header which would have informed the compiler the correct
prototype for the function. Had you included time.h then the compiler
would have notified you with an error message.

Again, try the code with the includes. You should get an diagnostic.

--
Al Bowers
Tampa, Fl USA
mailto: xal@abowers.combase.com (remove the x)
http://www.geocities.com/abowers822/

Re: Newbie question on time()
 

Al Bowers wrote in article <bllgjm$cu5c0$1@ID-169908.news.uni-berlin.de>:

> Again, try the code with the includes. You should get an diagnostic.

OK. That code was basically the same as the author gave in the book with a
few things I threw in (mostly in the print statements). Why does the code
compile and the binary function if the code contains errors? Maybe I need
a different book. Any suggetions on a good 'Intro to C' book? I'm not too
interested in C++ at this point. I have some knowledge of VB and have done
a small bit of work in Python and Perl, so it need not be at the *most*
basic level.

I will try your suggestions and see what happens.

> Al Bowers
> Tampa, Fl USA

Hey, I'm in Lakeland.

--
Todd Stephens
ICQ# 3150790
"A witty saying proves nothing." -Voltaire

Re: Newbie question on time()
 

"Todd Stephens" <Huzzah!@Huzzah.com> wrote in message
news:%Mqfb.67085$eS5.50173@twister.tampabay.rr.com ...
> Mike Wahler wrote in article
> <zwqfb.682$Qy2.200@newsread4.news.pas.earthlink.ne t>:
>
> > Change from what to what? 'time()' is a function in its
> > own right with various uses. The author was using its
> > return value as a seed for the random number generator.
>
> Yes, but in earlier code examples, the author omitted the NULL assignment
> and used simply srand(time()).

If that is the standard library's 'time()' function,
then that is invalid statement.

> I was confused when, in the next example,
> he included it.

Perhaps it was a typographical error. Check to see
if an errata document is available for your book.

> You indicated that this would not compile without NULL,
> but it did and the program ran fine.

Then the 'time()' function you're calling is not the
one from the standard library, or your standard library
implementation is broken.

>Here is the code from a silly
> dice-rolling emulator:
>
> #include <stdio.h>
>
> main()

int main()

> {
>
> int intDie1 = 0;
> int intDie2 = 0;
> int intResult = 0;
> srand(time());

No declaration of 'time()' in scope. A C90 compiler
will assume a return type of 'int'. A C99 compiler
must issue a diagnostic. A linker will complain about
undefined symbol 'time'.

>
> intDie1 = (rand() % 6) +1;
> intDie2 = (rand() % 6) +1;
> intResult = (intDie1 + intDie2);
>
> if (intResult == 7 || intResult == 11){
> printf("\nYOU WIN!!\n");
> printf("First roll was %d and second was %d\n", intDie1, intDie2);
> }
> else{
> printf("\nYOU LOSE!!\n");
> printf("First roll was %d and second was %d\n", intDie1, intDie2);
> }
> }
>
> This code compiled and the resulting binary worked fine using gcc 3.2.3

Then you're either using some nonstandard function called
'time', or your standard library is noncompliant.

>
> > I think you've mistakenly got the idea that 'srand()'
> > and 'time()' have some special relationship which does
> > not exist.
>
> I wasn't really asking about any relationship between them.

It was a speculation.

> Just wondering
> what the difference between using time(NULL) and time() is.

The former is a valid call of the standard library function 'time()',
the latter is not.

>So,
> realistically I should use NULL whenever I call time() without wanting any
> specific value to time()?

Please read again my description of the 'time()' function.
I described the 'time()' function from the C standard library.
If you're using something else, it's not standard and thus
not topical here.

-Mike

Re: [welcome msg, FAQ] Newbie question on time()
 

"Todd Stephens" <Huzzah!@Huzzah.com> wrote in message
news:uJrfb.3226$qw.407848@twister.tampabay.rr.com. ..
> Al Bowers wrote in article <bllgjm$cu5c0$1@ID-169908.news.uni-berlin.de>:
>
> > Again, try the code with the includes. You should get an diagnostic.
>
> OK. That code was basically the same as the author gave in the book

"Basically the same" tells us nothing. What was it *exactly*?
If there was no #include of <time.h> and no argument passed to
it, then its simply wrong.

>with a
> few things I threw in (mostly in the print statements). Why does the code
> compile and the binary function if the code contains errors?

Um, nonstandard compiler?

>Maybe I need
> a different book.

Perhaps. Which book do you have?

>Any suggetions on a good 'Intro to C' book?

www.accu.org
See the book review section under category "beginner C"

> I'm not too
> interested in C++ at this point.

No problem. C++ is not C.

>I have some knowledge of VB and have done
> a small bit of work in Python and Perl, so it need not be at the *most*
> basic level.


See above.

Also see the welcome message for comp.lang.c:
http://www.angelfire.com/ms3/bchambl...me_to_clc.html

and the C FAQ:
http://www.eskimo.com/~scs/C-faq/top.html

Much useful information at both links for C programmers
of all levels of experience.



>
> I will try your suggestions and see what happens.

Rather than "try and see", I recommend you get a good
book, and you can *know* what will or will not happen
with a particular piece of code.

HTH,
-Mike

Re: Newbie question on time()
 

Todd Stephens wrote:
> Al Bowers wrote in article <bllgjm$cu5c0$1@ID-169908.news.uni-berlin.de>:
>
>
>>Again, try the code with the includes. You should get an diagnostic.
>
>
> OK. That code was basically the same as the author gave in the book with a
> few things I threw in (mostly in the print statements). Why does the code
> compile and the binary function if the code contains errors? Maybe I need
> a different book. Any suggetions on a good 'Intro to C' book? I'm not too
> interested in C++ at this point. I have some knowledge of VB and have done
> a small bit of work in Python and Perl, so it need not be at the *most*
> basic level.
>
> I will try your suggestions and see what happens.
>

It seems you are on the right tract. Remember, a compiler, does not
define the C Language. Just using your compiler to determine the
validity of code will get you in trouble. The International Standard
ISO/IEC 9899 defines the C Language. So, resources like a copy of
the Standard or books that explain the standard is the right approach.
Personallly, my favorite resources are the Standard itself, 'The C
Progamming Language' (Second Edition), "The Standard C Library', and
finally the FAQ which can be found on the web at:
http://www.eskimo.com/~scs/C-faq/top.html

OK, Getting back to the code you supplied: Here it is again.

/*************** test.c *************/
#include <stdio.h>

main()
{

int intDie1 = 0;
int intDie2 = 0;
int intResult = 0;

srand(time());

intDie1 = (rand() % 6) +1;
intDie2 = (rand() % 6) +1;
intResult = (intDie1 + intDie2);

if (intResult == 7 || intResult == 11){
printf("\nYOU WIN!!\n");
printf("First roll was %d and second was %d\n", intDie1, intDie2);
}
else{
printf("\nYOU LOSE!!\n");
printf("First roll was %d and second was %d\n", intDie1, intDie2);
}
}

You said that when you compiled it with GCC 3.2.3 that it compiled and
the binary worked fine. When you compiled it did the compiler give
you any warning diagnostics? If gcc did not give you any warnings
perhaps you should increase the warning level. I compiled the above
code with gcc 3.2 and with a high level of warning. The command was:
gcc test.c -pedantic -Wall
The code compiled and a binary was built. However, I got five warning
messages. They were:

Line 4: warning, return type defaults to 'int'
In function 'main':
Line 9: warning, implicit declaration of function 'srand'
Line 9: warning, implicit declaration of function 'time'
Line 11: warning, implicit declaration of function 'rand'
Line 23: warning, control reaches end of non-void function.

The line 4 warning is the result of not explicitly declarating
function main returning an integer: int main()

The Line 9 warnings(2) is the result of not including the headers,
stdlib.h for function srand and time.h for function time. These
headers have the declarations for these functions.

The line 11 warning is the same as line 9. You did not include the
header stdlib.h which declares function rand.

The Line 23 warning is the result of function main not returning
an int. So you should return an integer, like 'return 0' at the
end of the function.

So you see, the resource that supplied you with this code has
serious deficiencies. You should seek additional resources like I
mentioned above.

--
Al Bowers
Tampa, Fl USA
mailto: xal@abowers.combase.com (remove the x)
http://www.geocities.com/abowers822/

Re: Newbie question on time()
 

On Fri, 03 Oct 2003 23:59:49 -0400, Al Bowers wrote:

> return 0;

return EXIT_SUCCESS;

Zygmunt

Re: [welcome msg, FAQ] Newbie question on time()
 

Mike Wahler wrote in article
<Qtsfb.926$Qy2.569@newsread4.news.pas.earthlink.ne t>:

>> OK. That code was basically the same as the author gave in the book
>
> "Basically the same" tells us nothing. What was it *exactly*?
> If there was no #include of <time.h> and no argument passed to
> it, then its simply wrong.

No, there was no other include other than <stdio.h>. As commented below, I
only /added/ to the code.

>>with a
>> few things I threw in (mostly in the print statements). Why does the
>> code compile and the binary function if the code contains errors?
>
> Um, nonstandard compiler?

gcc 3.2.3, but I did not have any high warning levels turned on.

>>Maybe I need
>> a different book.
>
> Perhaps. Which book do you have?

"C Programming for the Absolute Beginner" by Michael Vine

>>Any suggetions on a good 'Intro to C' book?
>
> www.accu.org
> See the book review section under category "beginner C"

I'll have to check that out.

>
> Also see the welcome message for comp.lang.c:
> http://www.angelfire.com/ms3/bchambl...me_to_clc.html
>
> and the C FAQ:
> http://www.eskimo.com/~scs/C-faq/top.html
>
> Much useful information at both links for C programmers
> of all levels of experience.

Thanks for the links.

>
>>
>> I will try your suggestions and see what happens.
>
> Rather than "try and see", I recommend you get a good
> book, and you can *know* what will or will not happen
> with a particular piece of code.
>

It is becoming apparant to me that the book I have is using some sort of
hackneyed approach to the code. I know the K&R book is the definitive
guide, but I am led to believe it is not so good for teaching from the
ground-up in C. Still, I shall probably get a copy of it and see how it
feels.

--
Todd Stephens
ICQ# 3150790
"A witty saying proves nothing." -Voltaire