Yet another brute force sudoku solver

[Boon]

Hello group,

I've been toying with a simple sudoku[1] solver. I meant for the code to
be short and easy to understand. I figured "brute force is simple" --
who needs finesse, when you've got muscle, right?

[1] http://en.wikipedia.org/wiki/Sudoku

Thus, the strategy is to test every legal "move" and to backtrack when
stuck. A recursive function seemed appropriate. I'd like to hear
comments and suggestions regarding the implementation.

Regards.

#include <stdio.h>

static int grid[9][9];

void read_grid(FILE *fp)
{
int i,j;
for (i=0; i < 9; ++i)
{
char buf[16];
fgets(buf, sizeof buf, fp);
for (j=0; j < 9; ++j)
{
char c = buf[j];
if ('1' <= c && c <= '9') grid[j] = c - '0';
}
}
}

void write_grid(void)
{
int i, j;
for (i=0; i < 9; ++i)
{
for (j=0; j < 9; ++j) printf("%d ", grid[j]);
putchar('\n');
}
}

int is_legal(int row, int col, int val)
{
int ii = row - row%3;
int jj = col - col%3;
int i, j, k = 0, res = 1;
for (i=0; i < 3; ++i)
for (j=0; j < 3; ++j, ++k)
res = res && (grid[row][k] != val) && (grid[k][col] != val) &&
(grid[ii+i][jj+j] != val);
return res;
}

void solve(int pos)
{
if (pos == 9*9) write_grid();
else
{
int row, col, val;
row = pos / 9;
col = pos % 9;
if (grid[row][col] != 0) solve(pos+1);
else
{
for (val=1; val <= 9; ++val)
{
if (is_legal(row, col, val))
{
grid[row][col] = val;
solve(pos+1);
}
}
grid[row][col] = 0;
}
}
}

int main(void)
{
read_grid(stdin);
solve(0);
return 0;
}

 

[Boon]

I've been toying with a simple sudoku[1] solver. I meant for the code
to be short and easy to understand. I figured "brute force is simple"
-- who needs finesse, when you've got muscle, right?

[1] http://en.wikipedia.org/wiki/Sudoku

Thus, the strategy is to test every legal "move" and to backtrack when
stuck. A recursive function seemed appropriate. I'd like to hear
comments and suggestions regarding the implementation.

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I didn't want is_legal() and solve() to require an additional parameter.
(I know this is not a very convincing reason.)

void read_grid(FILE *fp)
{
int i,j;
for (i=0; i < 9; ++i)
{
char buf[16];
fgets(buf, sizeof buf, fp);

Wait, you're reading 16 characters in 9 times? At most you will have
eleven (9 characters plus a newline and '\0').

I'm reading *at most* 16 characters (which should never happen).
NB : I didn't focus on the I/O routines, they may have holes.

BTW, in cygwin, the line ends with 0x0d 0x0a 0x00.
<pedantic mode>
Is it possible for a platform to "define" '\n' as a sequence of 10
characters ? Thereby making my buf too small to hold 9 characters,
newline, and nul.

for (j=0; j < 9; ++j)
{
char c = buf[j];
if ('1' <= c && c <= '9') grid[j] = c - '0';

Why bother changing each character (range '1' to '9') to an int (range 1
to 9)? It isn't really necessary -- you only need to test if two
characters are equal, not do any kind of math with them. Not with the
brute force attack, anyway.

You're right. I'd just have to change the for loop in solve() to
for (val='1'; val <= '9'; ++val)

But this wouldn't buy much, either in simplicity or performance, don't
you agree ?

It's probably better to return a code indicating success than to print
out the result from solve(), in case you want to do something with that
in main(), like print out a different message depending on the result.

I see your point.

else
{
int row, col, val;
row = pos / 9;
col = pos % 9;
if (grid[row][col] != 0) solve(pos+1);
else
{
for (val=1; val <= 9; ++val)
{
if (is_legal(row, col, val))
{
grid[row][col] = val;
solve(pos+1);

What if this call to solve() succeeds?

What does it mean for a call to succeed ?

Does it stop recursing? No, it
just returns /and continues testing all the other possibilities/. This
could mean that you get more than one printout if there is more than one
legal solution. Could be good, could be bad.

This is by design. I want to see whether a problem has multiple solutions.

I'm really not liking this implementation because there's no way for the
calling function to know whether the call to solve() succeeded.

By "succeed" do you mean find one solution or find all solutions or
something else ? Failing would mean not finding any solution ?

In solve() this is bad because when it does succeed the previous level of
recursion has no way of knowing that and continues searching for
solutions (which may or may not exist), and prints them out as it comes
to them. In main() this is bad because you have no way of discerning in
the code whether the puzzle is solvable or not.

I'll keep your comments in mind.

It occurs to me that you may want to know all the possible solutions of
a particular puzzle; in that case you could let each call return the
number of ways it found to solve the problem. This would need a
variable that increments each time a solution is found. In any case,
this function would be much more practical if you could tell whether it
succeeded or not.

If it were to be used as a library function for example ?

Regards.

 

[dj3vande]

I didn't want is_legal() and solve() to require an additional parameter.
(I know this is not a very convincing reason.)

The important thing to remember about global variables is that *every*
global is part of the interface to *every* function in your program.
(Even if it doesn't use it now, there's nothing to stop it from
deciding to use it at some point in the future.)

If the program is small and self-contained, and the information you're
making global is something that the whole program needs, this will
probably be what you want. But small and self-contained programs have
an annoying habit of not wanting to stay small and self-contained.
Sooner or later you'll find yourself wishing that a program that was
written that way had "all the stuff everybody wants" arguments to its
functions so you could put it in a larger program as a module and use
that module for two things at once. So if you're going to use global
variables, at least keep that in mind, and make it easy to convert it
into that form later.

When I use globals, I like to wrap them all up in a small number of
structs. This has two benefits:
(1) It makes it easier to transform globals into pass-around-to-
everybody non-global data; just have everything that needs it (or,
transitively, calls something that needs it) take a pointer to a
struct of the appropriate type. Finding functions that need it and
converting them to use the non-global form is easy - just do a
search for the global struct name, and make the appropriate changes
everywhere it shows up.
(2) It makes it obvious everywhere you use a global variable that this
is a global you're using. That gives you clues about where to look
for more information about it, and reminds you to be careful about
changes there that may affect other chunks of code in completely
different places.

BTW, in cygwin, the line ends with 0x0d 0x0a 0x00.
<pedantic mode>
Is it possible for a platform to "define" '\n' as a sequence of 10
characters ? Thereby making my buf too small to hold 9 characters,
newline, and nul.
</mode>

No.
Well, maybe, but not in a way that affects you.
The platform is allowed to define "end of line in text files" however
it wants; but if you open the file in text mode, it's required to
convert each line from whatever the "native" format is into "sequence
of characters followed by '\n'", which is the internal representation
that a C program uses.

(Note that, on the other side of the stdio library, newline doesn't
even need to be represented by a (sequence of) character(s);
f'rexample, an implementation can use fixed-length space-padded lines,
and that implementation's stdio library would need to read the line,
trim trailing space padding, and insert '\n' between lines before
passing the data on to a C program.)

(This means that if you haven't opened the file in binary mode and
cygwin is giving you "\r\n" at the end of your lines, then either it's
failing to do end-of-line translation properly or it's defining
end-of-line differently than the rest of the system it lives in and
refusing to play nicely.)



dave

 

[Default User]

Boon wrote:

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.




Brian

 

[Richard]

Boon wrote:

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.




Brian

Frankly I think that is nonsense.

If efficiency was the issue he could simply malloc one bunch of result
containers or use in function statics to effectively hide things.

And file scope IS global (as anyone without a rod up their arse knows
it) since anyone can declare an extern and then see it.

 

[user923005]

Hello group,

I've been toying with a simple sudoku[1] solver. I meant for the code to
be short and easy to understand. I figured "brute force is simple" --
who needs finesse, when you've got muscle, right?

[1]http://en.wikipedia.org/wiki/Sudoku

Thus, the strategy is to test every legal "move" and to backtrack when
stuck. A recursive function seemed appropriate. I'd like to hear
comments and suggestions regarding the implementation.

Regards.

#include <stdio.h>

static int grid[9][9];

void read_grid(FILE *fp)
{
   int i,j;
   for (i=0; i < 9; ++i)
   {
     char buf[16];
     fgets(buf, sizeof buf, fp);
     for (j=0; j < 9; ++j)
     {
       char c = buf[j];
       if ('1' <= c && c <= '9') grid[j] = c - '0';
     }
   }

}

void write_grid(void)
{
   int i, j;
   for (i=0; i < 9; ++i)
   {
     for (j=0; j < 9; ++j) printf("%d ", grid[j]);
     putchar('\n');
   }

}

int is_legal(int row, int col, int val)
{
   int ii = row - row%3;
   int jj = col - col%3;
   int i, j, k = 0, res = 1;
   for (i=0; i < 3; ++i)
     for (j=0; j < 3; ++j, ++k)
       res = res && (grid[row][k] != val) && (grid[k][col] != val) &&
(grid[ii+i][jj+j] != val);
   return res;

}

void solve(int pos)
{
   if (pos == 9*9) write_grid();
   else
   {
     int row, col, val;
     row = pos / 9;
     col = pos % 9;
     if (grid[row][col] != 0) solve(pos+1);
     else
     {
       for (val=1; val <= 9; ++val)
       {
         if (is_legal(row, col, val))
         {
           grid[row][col] = val;
           solve(pos+1);
         }
       }
       grid[row][col] = 0;
     }
   }

}

int main(void)
{
   read_grid(stdin);
   solve(0);
   return 0;



}

Compare your method with this:

/* Program resolution of sudoku grids by backtracking,
with propagation of the constraints and variable
selection the most constrained (mrv: minimum remaining value).
The file is read from stdin. Try for example with:

53..7....
6..195...
.98....6.
8...6...3
4..8.3..1
7...2...6
.6....28.
...419..5
....8..79

Bernard Helmstetter, 2006
Translated to English by Dann Corbit
*/

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

#define UNKNOWN 0

int grid[81];
int domain[81][10]; /* The list of possibilities */
int domain_cuts[81]; /* domain cuts */
int computer_nodes;

/* Read the grid from standard input, using the characters '0'-'9' and
'.' */
void read_grid()
{
int k = 0;

while (k < 81) {
char c = getchar();
if (c >= '1' && c <= '9')
grid[k++] = c - '0';
else if (c == '.')
grid[k++] = UNKNOWN;
else if (c == EOF) {
fprintf(stderr, "Bad format reading grid\n");
exit(1);
}
}
}

/* print the grid to standard output */
void print_grid()
{
int c,
l;

for (l = 0; l < 9; l++) {
for (c = 0; c < 9; c++) {
int k = l * 9 + c;
if (grid[k] == UNKNOWN)
printf(".");
else
printf("%c", grid[k] + '0');
}
putchar('\n');
}
putchar('\n');
}

/* remove all the values of the domains that are incompatible for case
'i' */
void restrict_domain(int i)
{
int l = i / 9,
c = i % 9;
int lb = l / 3;
int cb = c / 3;
int l2,
c2,
lr2,
cr2,
i2;

/* column constraints for case 'i' */
for (l2 = 0; l2 < 9; l2++) {
i2 = l2 * 9 + c;
if (domain[i2][grid]) {
domain[i2][grid] = 0;
domain_cuts[i2]--;
}
}

/* row constraints for case 'i' */
for (c2 = 0; c2 < 9; c2++) {
i2 = l * 9 + c2;
if (domain[i2][grid]) {
domain[i2][grid] = 0;
domain_cuts[i2]--;
}
}

/* verify the region containing case 'i' */
for (lr2 = 0; lr2 < 3; lr2++)
for (cr2 = 0; cr2 < 3; cr2++) {
i2 = (lb * 3 + lr2) * 9 + (cb * 3 + cr2);
if (domain[i2][grid]) {
domain[i2][grid] = 0;
domain_cuts[i2]--;
}
}
}

/* using the initial state, initialize the domain table */
void init_domain()
{
int i,
v;

for (i = 0; i < 81; i++) {
domain_cuts = 9;
for (v = 1; v <= 9; v++)
domain[v] = 1;
}

/* restrict the domain by backtracking */
for (i = 0; i < 81; i++)
if (grid != UNKNOWN)
restrict_domain(i);
}

void backtrack()
{
int i,
i_min = -1;
int cuts_min = 10;
int domain2[81][10];
int domain_cuts2[81];

/* look for a smaller domain */
for (i = 0; i < 81; i++) {
if (grid == UNKNOWN && domain_cuts < cuts_min) {
i_min = i;
cuts_min = domain_cuts;
}
}

/* Are all UNKNOWN fields resolved? */
if (i_min == -1) {
print_grid(); /* solution found */
return;
}
computer_nodes++;
for (grid[i_min] = 1; grid[i_min] <= 9; grid[i_min]++) {
if (domain[i_min][grid[i_min]] == 0)
continue;
/* Save the state between recursive calls */
memcpy(domain2, domain, sizeof(domain));
memcpy(domain_cuts2, domain_cuts, sizeof(domain_cuts));
restrict_domain(i_min);
backtrack();
memcpy(domain, domain2, sizeof(domain));
memcpy(domain_cuts, domain_cuts2, sizeof(domain_cuts));
}
grid[i_min] = UNKNOWN;
}

int main()
{
read_grid();
print_grid();
init_domain();
backtrack(0);
printf("%d nodes searched\n", computer_nodes);
return 0;
}

 

[Ben Bacarisse]

Boon wrote:

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.

Frankly I think that is nonsense.

And file scope IS global (as anyone without a rod up their arse knows
it) since anyone can declare an extern and then see it.

No. No matter how rude you are, you can't alter the fact that, in C,
scope and linkage are disconnected. The term "global" (usually)
conflates them.

Brian talks about encapsulation and non-global file scope data so I
conclude he is talking about static objects declared at file scope
which can not be seen using an "extern".

Even if this not what he meant, file scope is /not/ global (rod or no
rod).

 

[Richard]

Trent Josephsen wrote:

Boon wrote:

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.

Frankly I think that is nonsense.

And file scope IS global (as anyone without a rod up their arse knows
it) since anyone can declare an extern and then see it.

No. No matter how rude you are, you can't alter the fact that, in C,
scope and linkage are disconnected. The term "global" (usually)
conflates them.

No. Forget linkaqe. We do not discuss Linkage. A file variable is
available globally to the program which is being compiled. You can play
word games if you wish. There was no mention of static.

Brian talks about encapsulation and non-global file scope data so I
conclude he is talking about static objects declared at file scope
which can not be seen using an "extern".

You assume. But he did not mention static. I did. And there is no need
for them to file scope either. This was my point.

Without the mention of static then it can possibly be deduced that he
was doing a Heathfield and suggesting no such thing as Globals in C.

Even if this not what he meant, file scope is /not/ global (rod or no
rod).

Where do YOU declare your globals Ben? In something that is NOT a
file?

What part of you can access via an extern is wrong? In the obvious and
non clever understanding of what a global is?

 

[jameskuyper]

Boon wrote:

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.




Brian

Frankly I think that is nonsense.

If efficiency was the issue he could simply malloc one bunch of result
containers or use in function statics to effectively hide things.

And file scope IS global (as anyone without a rod up their arse knows

it) since anyone can declare an extern and then see it.

That statement implies that you're unfamiliar with the meaning of
'static' when applied at file scope. Contrary to what you're
suggesting, such declarations have internal linkage, not external
linkage. As a result, declaring the same identifier with external
linkage in a different translation unit will not make this definition
visible in that translation unit. You need to understand the
distinction between scope and linkage.

This is not a pedantic quibble, since 'grid' was in fact declared with
the keyword 'static'.

 

[Richard]

Trent Josephsen wrote:

Boon wrote:

#include <stdio.h>

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.




Brian

Frankly I think that is nonsense.

If efficiency was the issue he could simply malloc one bunch of result
containers or use in function statics to effectively hide things.

And file scope IS global (as anyone without a rod up their arse knows

it) since anyone can declare an extern and then see it.

That statement implies that you're unfamiliar with the meaning of
'static' when applied at file scope. Contrary to what you're

No one mentioned static. I mentioned static.

suggesting, such declarations have internal linkage, not external
linkage. As a result, declaring the same identifier with external
linkage in a different translation unit will not make this definition
visible in that translation unit. You need to understand the
distinction between scope and linkage.

I dont know where you got that impression of my meaning. I suppose its
that word "declare" again. I should have posted an example.

So you are telling me that "extern int c" in a header declares a new
object? Wow. My C is rusty ....

This is not a pedantic quibble, since 'grid' was in fact declared with
the keyword 'static'.

My point is valid. He is as well to maintain internal statics "in
function" or a malloc result set..

Littering stuff at file scope is poor practice in most cases.

 

[jameskuyper]

....

You appear to have found the "static" keyword in the following
declaration invisible:

static int grid[9][9];

Generally you want to avoid global variables, as you probably know.

I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.

Frankly I think that is nonsense.

And file scope IS global (as anyone without a rod up their arse knows
it) since anyone can declare an extern and then see it.

No. No matter how rude you are, you can't alter the fact that, in C,
scope and linkage are disconnected. The term "global" (usually)
conflates them.

No. Forget linkaqe. We do not discuss Linkage. ...

Who's "we", kemo-sabe? I discuss linkage when it's relevant, as it is
to the question of whether or not something is global.

... A file variable is
available globally to the program which is being compiled.

Unless explicitly defined as static, in which case it has only
internal linkage, and cannot be accessed by name from any translation
unit of the program other than the one in which it was so defined.

... You can play
word games if you wish. There was no mention of static.

The declaration under discussion, which you yourself have quoted, says

static int grid[9][9];

That looks like a mention of "static" to me.

You assume. But he did not mention static.

The relevant use of the word "static" was the one written by Boon in
the very first message of this discussion. Not by you, and not by
Brian.

Without the mention of static then it can possibly be deduced that he
was doing a Heathfield and suggesting no such thing as Globals in C.

Since Boon did mention static, you can drop that line of thought.

Where do YOU declare your globals Ben? In something that is NOT a
file?

He's not saying that globals don't have file scope. He's saying that
not all things declared with file scope are globals. Some things
declared at file scope have internal linkage, and therefore cannot, by
any reasonable definition of the term, be called globals. This doesn't
have anything to do with Richard Heathfield's definition of global; I
am in agreement with you that his definition is an unreasonably strict
definition.

What part of you can access via an extern is wrong?

The part where an extern declaration in one translation unit will NOT
make visible an object (or function) declared 'static' at file scope
in a different translation unit.

 

[jameskuyper]

The presence of the word "static" on the following line seems to
somehow have repeatedly escaped your attention:

static int grid[9][9]; ....
it) since anyone can declare an extern and then see it.

That statement implies that you're unfamiliar with the meaning of
'static' when applied at file scope. Contrary to what you're

No one mentioned static. I mentioned static.

I bed to differ. In the section that you yourself have quoted above,
Boon definitely mentioned 'static', and did so long before you did.

<nit-pick>A strict interpretation of your last two sentences leads to
the deduction that you are "no one". I think you meant to say "no one

I dont know where you got that impression of my meaning. I suppose its
that word "declare" again. I should have posted an example.

No, it's not your use of declare (unless you actually meant "define"
in the following citation, in which case the sentence becomes
irrelevant to the subject under discussion, since the relevant object
was defined with 'static', not 'extern'). The problem is your use of
extern in the following:

That's true only of identifiers with external linkage, not identifiers
with internal linkage; file scope alone is not sufficient to make that
true.

So you are telling me that "extern int c" in a header declares a new
object? Wow. My C is rusty ....

No, I'm telling you that 'static int c' declared in one translation
unit and 'extern int c' declared in a different translation unit
cannot both refer to the same object, not even if the static
declaration occurs at file scope. The object referred to by the
'extern' declaration must also be defined somewhere, not merely
declared, but the 'static' declaration cannot provide that definition.
If you were not aware of this fact, as seems to be the case, then your
C is indeed rusty.

My point is valid. He is as well to maintain internal statics "in
function" or a malloc result set..

Littering stuff at file scope is poor practice in most cases.

I would generally agree, for objects with external linkage. I think
that file scope objects with internal linkage can be good practice in
many cases. Your statements to the contrary notwithstanding, only
functions in the same translation unit can see them, but all of those
functions can see them, which can be a useful combination in some
contexts. I've used such objects to coordinate the behavior of a call-
back function with the behavior of another function; those were the
only two functions which knew about the file-scope variable that
shared the same translation unit with them; it was invisible to all
the other translation units in the program.

 

[Default User]

No. No matter how rude you are, you can't alter the fact that, in C,
scope and linkage are disconnected. The term "global" (usually)
conflates them.

Riley's a troll and an idiot, so no surprises there. And of course,
I've had him killfiled for a long time. I'm not sure he believes it,
but it's true.

Brian talks about encapsulation and non-global file scope data so I
conclude he is talking about static objects declared at file scope
which can not be seen using an "extern".

Yeppers. I've been encapsulating some of the operations through the use
of file-scope static data, static utility functions, and a small set of
public interface functions. In some ways it's like OO programming writ
large. In other ways it's very different.

Even if this not what he meant, file scope is not global (rod or no
rod).

I think I didn't express my initial thoughts well enough if you're
unsure. I can't draw any conclusions from Riley being confused.




Brian

 

[Ben Bacarisse]

"Scope" is what "linkage" is all about.

Yes, "disconnected" is too strong. Orthogonal? Too technical (and
the technical means has to be bent for it to fit). Independent?
Probably also too strong.

The trouble with the word "global" is that it conflates two ideas that
need to be taken separately to understand what is happening in C
programs.

 

[Ben Bacarisse]

Ben Bacarisse wrote:

Yeppers. I've been encapsulating some of the operations through the use
of file-scope static data, static utility functions, and a small set of
public interface functions. In some ways it's like OO programming writ
large. In other ways it's very different.


I think I didn't express my initial thoughts well enough if you're
unsure.

No, I was clear about what you meant. I simply wanted to cover myself
to cut down on the number of posts exchanged since I knew Just Plain
Richard would accuse me of making unwarranted assumptions.

 

[Boon]

Hello Dann,

I've been toying with a simple sudoku[1] solver. I meant for the code to
be short and easy to understand. I figured "brute force is simple" --
who needs finesse, when you've got muscle, right?

[1]http://en.wikipedia.org/wiki/Sudoku

Thus, the strategy is to test every legal "move" and to backtrack when
stuck. A recursive function seemed appropriate. I'd like to hear
comments and suggestions regarding the implementation.

[snip my implementation]

Compare your method with this:

/* Program resolution of sudoku grids by backtracking,
with propagation of the constraints and variable
selection the most constrained (mrv: minimum remaining value).
The file is read from stdin. Try for example with:

53..7....
6..195...
.98....6.
8...6...3
4..8.3..1
7...2...6
.6....28.
...419..5
....8..79

Bernard Helmstetter, 2006
Translated to English by Dann Corbit
*/

[snip Bernard's implementation]

Thanks Dann. In my opinion, Bernard's method is more complex than mine,
but it is very likely faster. Were there specific differences you wanted
to highlight ?

Regards.

 

[Nick Keighley]

Trent Josephsen wrote:
Boon wrote:
#include <stdio.h>
static int grid[9][9];
Generally you want to avoid global variables, as you probably know.
I disagree. This is a perfect usage for file scope (NOT global) data
constructs. I've been using that sort of thing for my game programming
some lately. It's a sort of data encapsulation model.
Frankly I think that is nonsense.
And file scope IS global <snip unnecessary rudeness> since anyone can
declare an extern and then see it.

No.  No matter how rude you are, you can't alter the fact that, in C,
scope and linkage are disconnected.  The term "global" (usually)
conflates them.

No. Forget linkaqe. We do not discuss Linkage.

yes we do!

A file variable is
available globally to the program which is being compiled. You can play
word games if you wish. There was no mention of static.

it is available at file scope. It could be said to be available
to the Tranaslation Unit but it ain't available to the program!

The trouble with the word "global" is it needs some context.
I used to use it situations like this

char *base_addr;

/* lots of code */

char read_dma_setting (void)
{return *(base_addr + DMA_SETTING_OFFSET);}

/* more code */
base_addr = 0x80001234;
rds = read_dma_setting();

[I *really* don't write code like that anymore!]

I'd describe base_addr as a "global parameter" of
read_dma_setting(). The term "global" would float around as well.
I found it ambiguous and stopped using the term. Some people
used to global to mean visible throughout the program.
Some just meant file visible. Its a confusing term.
Nowadays I use it as a pejorative term meaning "having too
much scope". "This program would be more robust if it used
less globals".

You assume.

pretty reasonable assumption. Note "file scope"

But he did not mention static.

static int grid[9][9];

(I'm not sure if that was what "Brian" posted as he's been snipped

I did. And there is no need
for them to file scope either. This was my point.

Without the mention of static then it can possibly be deduced that he
was doing a Heathfield and suggesting no such thing as Globals in C.

There *is* no such thing as a global in C. See the standard.


why not if you must use the word it's a reasonable usage.
If you mean "global" to ba an alias for "external linkage"
why not use the term?

<snip rudeness>

<snip stuff>

 

[Nick Keighley]

Littering stuff at file scope is poor practice in most cases

depends what "littering" means. I submit most well written
non-trivial C programs have some file scope data.

Good way to start another argument anyway...

 

[James Kuyper]

Nick Keighley wrote:
....

There *is* no such thing as a global in C. See the standard.

The standard uses the term "global variables" only once (F8.1p1) and
does not define the term. However, that one use involves saying that
"The flags and modes in the floating-point environment may be regarded
as global variables...", which seems pretty inconsistent with the idea
that there is no such thing in C. Unlike several of the other Annexes,
Annex F is marked as "normative".

Since the C standard doesn't define what the term means, we have to fall
back on, in order of increasing relevance: ordinary English usage,
common usage in IT contexts, and ISO/IEC 2382−1 (a standard far too
expensive for me to have a copy of). I believe that it is both common
usage and quite reasonable in C contexts to use the term "global" to
refer to identifiers with external linkage.

I've heard arguments that such identifiers are not truly "global",
because they can be hidden by declarations of the same identifier with
internal linkage. While I can appreciate the point being made, I
consider it insufficiently convincing to justify breaking with common
usage. In ordinary English usage, companies (for instance) are said to
be global if they have a presence almost everywhere around the globe,
even if there are a few places where they have no presence.

....

If you mean "global" to ba an alias for "external linkage"
why not use the term?

Because the term "global" is in common usage with this meaning, and is a
lot easer to say than "with external linkage".

 

[jameskuyper]

Trent Josephsen wrote:
Boon wrote:
#include <stdio.h>

static int grid[9][9]; ....
And file scope IS global (as anyone without a rod up their arse knows

it) since anyone can declare an extern and then see it.

....
suggesting, such declarations have internal linkage, not external
linkage. As a result, declaring the same identifier with external
linkage in a different translation unit will not make this definition
visible in that translation unit. You need to understand the
distinction between scope and linkage.

I dont know where you got that impression of my meaning. I suppose its
that word "declare" again. I should have posted an example.

Perhaps you should. I don't know how to make a file scope identifier
with internal linkage (such as the array 'grid' defined above) visible
from another translation unit by "declar[ing] an extern", unless the
identifier with external linkage refers to something different from
what the identifier with internal linkage refers to. If you know how
to do this, could you give us an example?