A CT&P excursion - beat it up!

[Bob Nelson]

After completing KNK2, my interest in C was reignited, prompting a
re-reading of Traister's dubious book on pointers. Then I dusted off
the fine ``C Traps and Pitfalls'' by Andy Koenig. Nowadays I primarily
work in PHP, Perl and C++. Consequently my C skills have atrophied.

So inspired by CT&P's ``calendar'' example on page 30, I decided to
brush up on some elementary C (although Koenig describes it as walking
``out on the ice'') with the little toy exercise below.

For posting, here I yanked all the comments for brevity. There's a
(too?) liberal use of typedef, pointers for the sake of pointers and
some blatantly obvious asserts. It compiles cleanly with Sun C 5.8
2005/10/13 on a SPARC and lint shows only functions returning values
that are ignored. The output meets expectations for a non-leap year.

So look this over being brutal and merciless with your critique as I
try to rebuild my flabby C muscles:

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

enum { MONTHS = 12, DAYS = 31 };
enum { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC };
static const char *const months[] = { "JAN", "FEB", "MAR", "APR",
"MAY", "JUN", "JUL", "AUG",
"SEP", "OCT", "NOV", "DEC", };

int main(void)
{
typedef int month_type[DAYS];
typedef month_type *month_ptr_type ;
typedef int *day_ptr_type;

int calendar[MONTHS][DAYS];
month_ptr_type month_ptr;
size_t num_months;
size_t num_days_per_mo nth;

memset(calendar , 0, sizeof calendar);
assert(calendar[DEC][DAYS - 1] == 0);

month_ptr = calendar;
assert(sizeof *month_ptr == DAYS * sizeof(int));

num_months = sizeof calendar / sizeof *calendar;
assert(num_mont hs == MONTHS);

num_days_per_mo nth = sizeof *calendar / sizeof **calendar;
assert(num_days _per_month == DAYS);

for(; month_ptr < calendar + num_months; ++month_ptr) {
day_ptr_type day_ptr = *month_ptr;
int month_num = (month_ptr - calendar + num_months) - num_months;
int days_in_month;
int day_num;

switch(month_nu m) {
case FEB:
days_in_month = 28;
break;
case APR:
case JUN:
case SEP:
case NOV:
days_in_month = 30;
break;
default:
days_in_month = 31;
break;
}

for(day_num = 1; day_num <= days_in_month; ++day_num, ++day_ptr)
*day_ptr = day_num;
}

for(month_ptr = calendar; month_ptr < calendar + num_months; ++month_ptr) {
int month_num = (month_ptr - calendar + num_months) - num_months;
int total_days_in_m onth = -1;
day_ptr_type day_ptr;

for(day_ptr = *month_ptr + num_days_per_mo nth - 1;
day_ptr *month_ptr; --day_ptr) {
if(*day_ptr) {
total_days_in_m onth = *day_ptr;
break;
}
}

printf("month %d (%s) has %d days\n",
month_num + 1, months[month_num], total_days_in_m onth);
}

return EXIT_SUCCESS;
}

[Ron Ford]

On Sat, 07 Jun 2008 02:08:07 -0500, Bob Nelson wrote:

[code elided]
0 warnings, 0 errors.

On my machine the output doesn't last long enough to read. Add a getchar()
call somewhere:

>
printf("month %d (%s) has %d days\n",
month_num + 1, months[month_num], total_days_in_m onth);

getchar();

}

return EXIT_SUCCESS;
}

I had to write a perpetual calender for my scientific programming class in
fortran in college. Output was un-fun.
--
The chief value of money lies in the fact that one lives in a world in
which it is overestimated.
H. L. Mencken

[Richard Heathfield]

Ron Ford said:

On Sat, 07 Jun 2008 02:08:07 -0500, Bob Nelson wrote:

[code elided]
0 warnings, 0 errors.

On my machine the output doesn't last long enough to read. Add a
getchar() call somewhere:

Why should he do that? Learn how to drive a command shell.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Keith Thompson]

Ron Ford <ro*@nowhere.ne twrites:

On Sat, 07 Jun 2008 02:08:07 -0500, Bob Nelson wrote:
[code elided]
0 warnings, 0 errors.

On my machine the output doesn't last long enough to read. Add a getchar()
call somewhere:

>>
printf("month %d (%s) has %d days\n",
month_num + 1, months[month_num], total_days_in_m onth);

getchar();

> }

return EXIT_SUCCESS;
}

Don't bother. Just run the program in a way that doesn't cause the
window to close when the program terminates. Not all systems are
Windows; on a non-Windows system, a program that gratuitously waits
for input before terminating would be quite annoying.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Ron Ford]

On Sat, 07 Jun 2008 08:42:01 +0000, Richard Heathfield wrote:

Ron Ford said:

>On Sat, 07 Jun 2008 02:08:07 -0500, Bob Nelson wrote:

[code elided]
0 warnings, 0 errors.

On my machine the output doesn't last long enough to read. Add a
getchar() call somewhere:

Why should he do that? Learn how to drive a command shell.

I use Dev-Cpp for quick and dirty C stuff and simply could not find a
means, from within the IDE, to pause a program before windows eats it up.
I ended up putting in a dos-shortcut where the executable lives, and that
works, but it misses the whole point of staying within an IDE during
development and testing.:-(

--
I confess I enjoy democracy immensely. It is incomparably idiotic, and
hence incomparably amusing.
H. L. Mencken

[Flash Gordon]

Bob Nelson wrote, On 07/06/08 08:08:

<snip>

So look this over being brutal and merciless with your critique as I
try to rebuild my flabby C muscles:

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

enum { MONTHS = 12, DAYS = 31 };
enum { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC };
static const char *const months[] = { "JAN", "FEB", "MAR", "APR",
"MAY", "JUN", "JUL", "AUG",
"SEP", "OCT", "NOV", "DEC", };

int main(void)
{
typedef int month_type[DAYS];
typedef month_type *month_ptr_type ;
typedef int *day_ptr_type;

int calendar[MONTHS][DAYS];

Rather than the memset further down initialise calendar here
int calendar[MONTHS][DAYS] = {0};

month_ptr_type month_ptr;
size_t num_months;
size_t num_days_per_mo nth;

memset(calendar , 0, sizeof calendar);
assert(calendar[DEC][DAYS - 1] == 0);

As you say, a someone pointless assert. As are the others below.

month_ptr = calendar;
assert(sizeof *month_ptr == DAYS * sizeof(int));

num_months = sizeof calendar / sizeof *calendar;
assert(num_mont hs == MONTHS);

num_days_per_mo nth = sizeof *calendar / sizeof **calendar;
assert(num_days _per_month == DAYS);

for(; month_ptr < calendar + num_months; ++month_ptr) {

Rather than assigning monthptr several lines up I would assign it in the
for statement.
for (month_ptr = calendar;
month_ptr < calendar + num_months;
++month_ptr) {

Well, actually I would not use a pointer at all but instead in index use
it as an array.

day_ptr_type day_ptr = *month_ptr;
int month_num = (month_ptr - calendar + num_months) - num_months;

<snip>

This looks horrible complex for a simple problem. Having num_months
appear twice is somewhat suggestive of a poor job.

I've not reviewed the rest.
--
Flash Gordon

[Richard Heathfield]

Ron Ford said:

<snip>

I use Dev-Cpp for quick and dirty C stuff and simply could not find a
means, from within the IDE, to pause a program before windows eats it up.

If you're running a console-style program, it makes sense to run it from a
console.

I ended up putting in a dos-shortcut where the executable lives, and that
works, but it misses the whole point of staying within an IDE during
development and testing.:-(

The whole point being... what, exactly?

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Ben Bacarisse]

Bob Nelson <bn*****@nelson be.comwrites:

After completing KNK2, my interest in C was reignited, prompting a
re-reading of Traister's dubious book on pointers. Then I dusted off
the fine ``C Traps and Pitfalls'' by Andy Koenig. Nowadays I primarily
work in PHP, Perl and C++. Consequently my C skills have atrophied.

So inspired by CT&P's ``calendar'' example on page 30, I decided to
brush up on some elementary C (although Koenig describes it as walking
``out on the ice'') with the little toy exercise below.

I don't know what KNK2 nor CT&P are, but I don't think that matters...

<snip>

So look this over being brutal and merciless with your critique as I
try to rebuild my flabby C muscles:

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

enum { MONTHS = 12, DAYS = 31 };
enum { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC };
static const char *const months[] = { "JAN", "FEB", "MAR", "APR",
"MAY", "JUN", "JUL", "AUG",
"SEP", "OCT", "NOV", "DEC", };

int main(void)
{
typedef int month_type[DAYS];

Some people object to typedefs (external declarations) inside
functions. I don't, but you may find people who do.

Array types are odd and although you use them correctly, some people
will consider it obfuscation to have them at all.

typedef month_type *month_ptr_type ;

Pointers to array types are even rarer. I don't think it makes
anything clearer in this program.

typedef int *day_ptr_type;

And finally, there is a school of thought that says the pointerlyness
of type (that it is a pointer) should not be hidden inside a typedef
unless you are making an opaque handle type. You are not, and I think
you've obscured things a little with all these typedefs. Note how you
feel you want to name the type with _ptr_ so you don't get lost. Much
better to let the * in the declaration do that for you.

For example, if you do need to have the pointed-to type named, try
this:

typedef int day_type;

and simply declare your variables:

day_type *dp;

but even that is overkill here, I feel.

int calendar[MONTHS][DAYS];

Having done all of the above, why not

month_type calendar[MONTHS];

If you've gone the effort of having an array type, why not use it?

month_ptr_type month_ptr;
size_t num_months;
size_t num_days_per_mo nth;

memset(calendar , 0, sizeof calendar);
assert(calendar[DEC][DAYS - 1] == 0);

month_ptr = calendar;
assert(sizeof *month_ptr == DAYS * sizeof(int));

num_months = sizeof calendar / sizeof *calendar;
assert(num_mont hs == MONTHS);

num_days_per_mo nth = sizeof *calendar / sizeof **calendar;
assert(num_days _per_month == DAYS);

for(; month_ptr < calendar + num_months; ++month_ptr) {
day_ptr_type day_ptr = *month_ptr;
int month_num = (month_ptr - calendar + num_months) - num_months;

Why not: 'int month_num = month_ptr - calendar;'?

int days_in_month;
int day_num;

switch(month_nu m) {
case FEB:
days_in_month = 28;
break;
case APR:
case JUN:
case SEP:
case NOV:
days_in_month = 30;
break;
default:
days_in_month = 31;
break;
}

I'd do this with an array rather than a switch.

for(day_num = 1; day_num <= days_in_month; ++day_num, ++day_ptr)
*day_ptr = day_num;

Slightly more idiomatic might be:

for(day_num = 0; day_num < days_in_month; ++day_num)
*day_ptr++ = day_num + 1;

}

for(month_ptr = calendar; month_ptr < calendar + num_months; ++month_ptr) {
int month_num = (month_ptr - calendar + num_months) - num_months;

see above.

int total_days_in_m onth = -1;
day_ptr_type day_ptr;

for(day_ptr = *month_ptr + num_days_per_mo nth - 1;
day_ptr *month_ptr; --day_ptr) {
if(*day_ptr) {
total_days_in_m onth = *day_ptr;
break;
}
}

printf("month %d (%s) has %d days\n",
month_num + 1, months[month_num], total_days_in_m onth);
}

return EXIT_SUCCESS;
}

Overall I am a little baffled. What is all the array setting for? Is
this part of a larger program that will do more? If I wanted the
output that you get, I'd just write:

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

enum { MONTHS = 12, DAYS = 31 };
enum { JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC };

static const char *const months[] = {
"JAN", "FEB", "MAR", "APR", "MAY", "JUN",
"JUL", "AUG", "SEP", "OCT", "NOV", "DEC"
};

static const int month_len[] = {
31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};

int main(void)
{
int month_num;
for (month_num = 0; month_num < MONTHS; month_num++)
printf("month %d (%s) has %d days\n",
month_num + 1, months[month_num], month_len[month_num]);
return EXIT_SUCCESS;
}

but I am sure that misses the point of your code.

--
Ben.

[Bartc]

"Richard Heathfield" <rj*@see.sig.in validwrote in message
news:Qd******** *************** *******@bt.com. ..

Ron Ford said:

<snip>

>I use Dev-Cpp for quick and dirty C stuff and simply could not find a
means, from within the IDE, to pause a program before windows eats it up.

If you're running a console-style program, it makes sense to run it from a
console.

Yes, once it's finished. At that point you don't want the pause anymore.

>

>I ended up putting in a dos-shortcut where the executable lives, and that
works, but it misses the whole point of staying within an IDE during
development and testing.:-(

The whole point being... what, exactly?

The point being that it's supposed to be Integrated.

-- bartc

[Richard Heathfield]

Bartc said:

>
"Richard Heathfield" <rj*@see.sig.in validwrote in message
news:Qd******** *************** *******@bt.com. ..

>Ron Ford said:

<snip>

>>

>>I ended up putting in a dos-shortcut where the executable lives, and
that works, but it misses the whole point of staying within an IDE
during development and testing.:-(

The whole point being... what, exactly?

The point being that it's supposed to be Integrated.

I guess we all have different ways of working. My way involves a lot of
xterm sessions. They all appear on the same screen, so I guess you could
call them integrated. No getchar call required, though.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999