A question about arrays,pointers and structures
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Ami
#1 / 7
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 A question about arrays,pointers and structures Hey!
I have a problem concerning both design and implementation.
I have created a structure i.e.
typedef struct{
...
Quote: } Job;
I need to have an array of these objects, but I do not know the number
until the program is run. This suggests dynamic allocation to me.
However, I'm getting a bit confused as to whether the array needs to
contain pointers to Jobs or something else.
I've read the FAQ with no success. I think I'm getting myself in a
twist. I also wrote code that seemed to work, but then spat the dummy
- so I think I've gotten confused with how this should actually be
implemented.
Could anybody please help?
Many thanks for any help
- amie
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| Sun, 21 Aug 2005 15:09:18 GMT |
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Zoran Cutur
#2 / 7
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A question about arrays,pointers and structures
Quote: > Hey! > I have a problem concerning both design and implementation.
> I have created a structure i.e.
> typedef struct{
> ...
> } Job;
> I need to have an array of these objects, but I do not know the number
> until the program is run. This suggests dynamic allocation to me.
> However, I'm getting a bit confused as to whether the array needs to
> contain pointers to Jobs or something else.
you'll use will be pointing to the beginning of the array and you
better don't change this pointer at anytime, because you'll lose
your reference to the allocated memory and thereby introduce a
memory leak.
You may think of
Job *x = malloc(10 * sizeof Job);
as being equivalent to:
Job x[10];
they are not the same, for example sizeof x will return
different values, but the logical appearance is the same.
Quote: > I've read the FAQ with no success. I think I'm getting myself in a
> twist. I also wrote code that seemed to work, but then spat the dummy
> - so I think I've gotten confused with how this should actually be
> implemented.
--
"LISP is worth learning for the profound enlightenment experience
you will have when you finally get it; that experience will make you
a better programmer for the rest of your days." -- Eric S. Raymond
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| Sun, 21 Aug 2005 16:32:50 GMT |
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King W.Wa
#3 / 7
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A question about arrays,pointers and structures Quote: > typedef struct{ > ... > } Job; > I need to have an array of these objects, but I do not know the number
> until the program is run. This suggests dynamic allocation to me.
> However, I'm getting a bit confused as to whether the array needs to
> contain pointers to Jobs or something else.
Job *jobpt;
......
jobpt = (Job *)malloc(100 * sizeof(Job));
I prefer to use:
struct Job {
......
Quote: };
struct Job *jobpt;
...
jobpt = (struct Job *)malloc(100 * sizeof(struct Job));
Weichao
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| Sun, 21 Aug 2005 22:03:59 GMT |
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bd
#4 / 7
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A question about arrays,pointers and structures Quote:
> Hey! > I have a problem concerning both design and implementation.
> I have created a structure i.e.
> typedef struct{
> ...
> } Job;
> I need to have an array of these objects, but I do not know the number
> until the program is run. This suggests dynamic allocation to me.
> However, I'm getting a bit confused as to whether the array needs to
> contain pointers to Jobs or something else.
> I've read the FAQ with no success. I think I'm getting myself in a
> twist. I also wrote code that seemed to work, but then spat the dummy
> - so I think I've gotten confused with how this should actually be
> implemented.
> Could anybody please help?
> Many thanks for any help
> - amie
You mean like this?
Job *jobs;
/* later... */
void allocatejobs(int n){
if(!jobs = malloc(sizeof *jobs * n)){
fprintf(stderr, "Error mallocing %d jobs.\n", n);
exit(1);
}
Quote: }
/* later... */
allocatejobs(n);
jobs[somenumber].someelement = foo;
/* Somenumber must be less than n and greater then or equal to 0 */
--
Freenet distribution not available
Help Microsoft stamp out piracy. Give Linux to a friend today!
-- From a Slashdot.org post
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| Sun, 21 Aug 2005 23:19:12 GMT |
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Stefano
#5 / 7
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A question about arrays,pointers and structures Hi,
I think that for solving your problem there are two different ways.
You wrote that you are not able to determine the number of elements
until the program runs. What does it mean? Does it mean that, for
example, the user must enter which indicates that number in some way?
Does it mean that the program is going to read a file in which the
first line, for example, contains that number?
If you are in situations like these, the use of an array is the better
way: you must allocate it dynamically (pay attention to the pointer
cast!) but afterwards it is very simple to use.
A code example can be:
#include <stdio.h>
#include <stdlib.h>
typedef struct {
...
Quote: } Job;
int main (void)
{
int n;
Job *list;
printf ("Number of elements: ");
scanf ("%d",&n);
list = (Job *) calloc (n, sizeof(Job));
return 0;
Quote: }
The other case is the one in which you are not able to determine the
number of elements in any way: it appens, for example, when you read a
data file with a not specified number of lines. In this case, the
better data structure are named "linked lists": in every object you
must have a pointer to the following structure of the list. This case
is a little more complicated because the operation on the list need
the use of pointers...
I hope to have helped you!
Stefano B.
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| Sun, 21 Aug 2005 23:57:25 GMT |
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ma
#6 / 7
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A question about arrays,pointers and structures Quote:
> > typedef struct{ > > ... > > } Job; > > I need to have an array of these objects, but I do not know the number
> > until the program is run. This suggests dynamic allocation to me.
> > However, I'm getting a bit confused as to whether the array needs to
> > contain pointers to Jobs or something else.
> I've never used this technique. But I think you must use:
> Job *jobpt;
> ......
> jobpt = (Job *)malloc(100 * sizeof(Job));
> I prefer to use:
> struct Job {
> ......
> };
> struct Job *jobpt;
> ...
> jobpt = (struct Job *)malloc(100 * sizeof(struct Job));
> Weichao
what's the difference here? (apart from using the typdef? - or is that your point?)
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| Mon, 22 Aug 2005 03:22:00 GMT |
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Barry Schwar
#7 / 7
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A question about arrays,pointers and structures
Quote: >Hi, >I think that for solving your problem there are two different ways. >You wrote that you are not able to determine the number of elements >until the program runs. What does it mean? Does it mean that, for >example, the user must enter which indicates that number in some way? >Does it mean that the program is going to read a file in which the >first line, for example, contains that number? >If you are in situations like these, the use of an array is the better >way: you must allocate it dynamically (pay attention to the pointer >cast!) but afterwards it is very simple to use. >A code example can be: >#include <stdio.h>
>#include <stdlib.h>
>typedef struct {
> ...
>} Job;
>int main (void)
>{
> int n;
> Job *list;
> printf ("Number of elements: ");
> scanf ("%d",&n);
> list = (Job *) calloc (n, sizeof(Job));
can hurt. For example, if you forget to include stdlib.h, there is
no prototype for calloc. Under the C89 standard, the compiler is
obliged to assume that calloc returns int.
Without the cast, the compiler is required to produce a diagnostic
informing you that you are not allowed to assign an int to a pointer.
This is your clue to remember stdlib.h
With the cast, this diagnostic is suppressed. Since there are no
diagnostics, you assume your code is reasonably correct. If your
system treats ints and pointers differently, you could be invoking
undefined behavior.
Quote: > return 0;
>}
<<Remove the del for email>>
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| Tue, 23 Aug 2005 10:45:59 GMT |
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