Safely working out how many rows in an array?

[pkirk25]

Assume an array of structs that is having rows added at random. By the
time it reaches your function, you have no idea if it has a few hundred
over over 10000 rows.

When your function recieves this array as an argument, is there a safe
way to establish how many rows that are or should I iterate over a
field I know will always be used and use the final value of the
iterator as the value of the array?

 

[Richard Heathfield]

pkirk25 said:

Assume an array of structs that is having rows added at random. By the
time it reaches your function, you have no idea if it has a few hundred
over over 10000 rows.

When your function recieves this array as an argument, is there a safe
way to establish how many rows that are or should I iterate over a
field I know will always be used and use the final value of the
iterator as the value of the array?

The trick is to keep track of how many elements the array has, and to pass
that information to functions that need it. Objects are relatively cheap.
Something as small as a size_t is practically free! So don't be squeamish
about using them.

 

[Keith Thompson]

Assume an array of structs that is having rows added at random. By the
time it reaches your function, you have no idea if it has a few hundred
over over 10000 rows.

When your function recieves this array as an argument, is there a safe
way to establish how many rows that are or should I iterate over a
field I know will always be used and use the final value of the
iterator as the value of the array?

You need to rethink your question.

It's not possible to pass an array as an argument in C. What you can
do is, for example, pass the address of (equivalently: a pointer to)
an array's first element as an argument. This pointer can then be
used to access the elements of the array, but it doesn't tell you how
many there are.

The language does a few things that seemingly conspire to make it
*look* like you're passing the array itself, but you're really not.

As Richard Heathfield wrote, the most straightforward way to do this
is to keep track of the size yourself and pass it to your function as
an extra argument.

Other solutions exist.

You should also read section 6 of the comp.lang.c FAQ, available at
<http://www.c-faq.com>.

 

[pkirk25]

Richard Heathfield wrote:
[snip]

The trick is to keep track of how many elements the array has, and to pass
that information to functions that need it. Objects are relatively cheap.
Something as small as a size_t is practically free! So don't be squeamish

[snip]

A file has over 300k rows of which less than 100 are likely to be of
interest.

int ScanFile(const FILE *srcFile,
struct realm_structure *rlm,
int *i)
{
/*
1. do something very clever to find a match
2. update the struct so its a useful matrix
3. i++
*/
return 0;
}

Is this what you were thinking?

If i get the iteration of the count wrong in any 1 function, I could
end up confused. But as a way of dealing with the problem, it does
look very good.

 

[Michael Mair]

pkirk25 said:


The trick is to keep track of how many elements the array has, and to pass
that information to functions that need it. Objects are relatively cheap.
Something as small as a size_t is practically free! So don't be squeamish
about using them.

Indeed.

@OP:
An alternative not using less but more space is using a data structure
different from an array if it is more suitable to your problem. Arrays
do not grow rows magically, so you have to deal with reallocation,
keeping track of the number of rows etc.
If you, for example, use linked lists, then insertion of new rows
is comparatively cheap and iteration costs (but for a constant factor)
the same as for an array.

Cheers
Michael

 

[William Ahern]

pkirk25 said:


The trick is to keep track of how many elements the array has, and to pass
that information to functions that need it. Objects are relatively cheap.
Something as small as a size_t is practically free! So don't be squeamish
about using them.

Put another way, don't ever discard information. Ideally, such information
is kept from the very beginning, and not derived at some intermediate
point.

 

[Andrea Laforgia]

When your function recieves this array as an argument, is there a safe
way to establish how many rows that are [...]

Usually, you pass the "dimension" of the array to your function.
A prototype of that function could be the following:

void yourFunction(int rows[], int rowCount);

Alternatively, you may want to use a "sentinel value".

An example of a an array terminated by a sentinel value is argv, one
of the two parameters of the function main():

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

The standard prescribes that argv[argc] shall be a null pointer, so
the following code is perfectly legal:

#include <stdio.h>

int main(int argc, char *argv[])
{
int count;
for (count=0; argv[count]; count++)
;
printf("argc=%d\n", argc, count);
return 0;
}

 

[Andrea Laforgia]

printf("argc=%d\n", argc, count);

Of course, it is:

printf("argc=%d\n", count);

I would demonstrate that argc == count.

 

[Andrew Poelstra]

Assume an array of structs that is having rows added at random. By the
time it reaches your function, you have no idea if it has a few hundred
over over 10000 rows.

When your function recieves this array as an argument, is there a safe
way to establish how many rows that are or should I iterate over a
field I know will always be used and use the final value of the
iterator as the value of the array?

I'm not sure if I understand your question, but the usual method for
doing this is to pass the length of the array into the function:

int my_function (char array[], size_t len);

 

[Keith Thompson]

Assume an array of structs that is having rows added at random. By the
time it reaches your function, you have no idea if it has a few hundred
over over 10000 rows.

When your function recieves this array as an argument, is there a safe
way to establish how many rows that are or should I iterate over a
field I know will always be used and use the final value of the
iterator as the value of the array?

I'm not sure if I understand your question, but the usual method for
doing this is to pass the length of the array into the function:

int my_function (char array[], size_t len);

And keep in mind that, in a parameter declaration (and *only* in a
parameter declaration), "char array[]" really means "char *array".

 

[pkirk25]

Many thanks all.

I curious why people prefer a size_t instead of int? On my PC, a quick
sizeof shows both are same size, though that could be down the the %d
in prinft("%d", sizeof(tSize_t))

 

[=?ISO-8859-1?Q?Une_b=E9vue?=]

As Richard Heathfield wrote, the most straightforward way to do this
is to keep track of the size yourself and pass it to your function as
an extra argument.

i'm still a newb in C, i don't use the same way as Richard Heathfield
wrote, rather i initiate the array with null value, then i can
iterate...

is that way of doing wrong ???

 

[a.laforgia]

pkirk25 ha scritto:

I curious why people prefer a size_t instead of int?

The ISO standard states that "size_t" is the type of sizeof, so you
should use it to specify the "size" of your data. It is not guaranteed
that size_t matches the int type, but only that it is an unsigned type.
Using size_t instead of int is much more correct, since it is portable
and since data size is obviously and unsigned value.

 

[Michael Mair]

pkirk25 ha scritto:

The ISO standard states that "size_t" is the type of sizeof, so you
should use it to specify the "size" of your data. It is not guaranteed
that size_t matches the int type, but only that it is an unsigned type.
Using size_t instead of int is much more correct, since it is portable
and since data size is obviously and unsigned value.

Other examples:
strlen() returns size_t
malloc()'s parameter is of type size_t

There are several functions where one would expect a size_t return
value or parameter; that you find int (or another type) instead, is
mainly for historic reasons.

There is one main disadvantage: size_t is an unsigned integer type.
unsigned types need more care in some places; a classic is the
beloved
for (index = size-1; index >= 0; --index)
way of writing an endless loop.
There is no corresponding signed type which is a pity (POSIX gives
you ssize_t which is useful in many respects).

If I start something new and have the choice, then I go for size_t.


Cheers
Michael

 

[Richard Heathfield]

Une bévue said:

i'm still a newb in C, i don't use the same way as Richard Heathfield
wrote, rather i initiate the array with null value, then i can
iterate...

is that way of doing wrong ???

Show us what you mean, using a small C program that compiles correctly, and
we'll find out together.

 

[Keith Thompson]

i'm still a newb in C, i don't use the same way as Richard Heathfield
wrote, rather i initiate the array with null value, then i can
iterate...

is that way of doing wrong ???

It depends on what you mean. I don't know what "initiate the array
with null value" is supposed to mean.

One way to let a function know how many elements an array has is to
use a sentinal value, i.e., assign some unique value to the last
element of the array. If it happens to be an array of pointers, and a
null pointer is not otherwise a valid value, then marking the end of
the array with a null pointer value is a reasonable approach.

 

[bert]

It depends on what you mean. I don't know what "initiate the array
with null value" is supposed to mean.

I think it means initially filling the whole array with
the end-of-array sentinel value. Then when you add
a new element, it already has a sentinel value after it.
--

 

[=?ISO-8859-1?Q?Une_b=E9vue?=]

I think it means initially filling the whole array with
the end-of-array sentinel value. Then when you add
a new element, it already has a sentinel value after it.

right, this method is ok ?

nb : this is the case when the max number of elements is known in
advance, otherwise i've a minimum count of elements to start with, when
the elements number is over this min the array is arranged such that i
don't have to malloc for it rather for the next i set to null.

 

[bert]

right, this method is ok ?

nb : this is the case when the max number of elements is known in
advance, otherwise i've a minimum count of elements to start with, when
the elements number is over this min the array is arranged such that i
don't have to malloc for it rather for the next i set to null.

Well, there's nothing that anybody could call wrong with
this method. True, some people would find sentinel
values not mixing well with dynamic reallocation, but it
still comes down to a matter of personal taste, and what
style or mixture you find readable and maintainable.
--

 

[bert]

right, this method is ok ?

nb : this is the case when the max number of elements is known in
advance, otherwise i've a minimum count of elements to start with, when
the elements number is over this min the array is arranged such that i
don't have to malloc for it rather for the next i set to null.

Nobody could call it a wrong method. Some people find that
sentinel values do not mix well with dynamic reallocation, but
it's still just a matter of personal taste, and what style
or mixture you have found to be readable and maintainable.
--