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Comparison between "Pointer and Integer"


I'm trying to compare between pointer and integer in an "IF" statement

how do I make this work?

if(patient[index].id != NULL){

}

Thanks
Chris

chu@gmail.com wrote:
> I'm trying to compare between pointer and integer in an "IF" statement

> how do I make this work?

> if(patient[index].id != NULL){
> }

Why do you think its doesn't?

--
Ian Collins.

"chu@gmail.com" <chu@gmail.com> writes:
> I'm trying to compare between pointer and integer in an "IF" statement

> how do I make this work?

> if(patient[index].id != NULL){
> }

Why would you want to compare a pointer and an integer?

What is "id"?  If it's a pointer, the above should work.  If it's an
integer, comparing it to NULL makes no sense.

It's helpful to post a small, complete program that illustrates your
problem.  If you're having a compilation program, show us the exact
error message your compiler gives you.  Otherise, your sample should
be compilable by itself.

--
Keith Thompson (The_Other_Keith) k@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

On 27 May, 22:45, Keith Thompson <k@mib.org> wrote:

ok..

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

#define MAXPATIENTS 20

struct details {
        int id;
        char forename[20];
        char initial;
        char surname[20];
        int day_of_entry;
        int max_wait;

};

//Lists individual patient information -Work in Progress
int list_patient(int index){
        int time_left = 0;
        int x = 0;
        int i;

        //Should check if index points to a record,
        //Or else it should return -1!!
        if( patient[index].id != NULL){
                printf("Patient Details: \n");
                printf("ID: %d |",patient[index].id);
                printf("Name: %s ",patient[index].forename);
                printf("%c ",patient[index].initial);
                printf("%s |",patient[index].surname);
                printf("Days on queue %d |",day_now()- patient[index].day_of_entry);
                printf("Maximum waiting days %d \n",time_left =
patient[index].max_wait - (day_now() - patient[i].day_of_entry));
        }
        else {
                x = -1;
        }
        return x;

}

int main(void){
    list_patient(0);
    return 0;

On 27 May, 22:45, Keith Thompson <k@mib.org> wrote:

ok..

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

#define MAXPATIENTS 20

struct details {
        int id;
        char forename[20];
        char initial;
        char surname[20];
        int day_of_entry;
        int max_wait;

};

//Lists individual patient information -Work in Progress
int list_patient(int index){
        int time_left = 0;
        int x = 0;
        int i;

        //Should check if index points to a record,
        //Or else it should return -1!!
        if( patient[index].id != NULL){
                printf("Patient Details: \n");
                printf("ID: %d |",patient[index].id);
                printf("Name: %s ",patient[index].forename);
                printf("%c ",patient[index].initial);
                printf("%s |",patient[index].surname);
                printf("Days on queue %d |",day_now()- patient[index].day_of_entry);
                printf("Maximum waiting days %d \n",time_left =
patient[index].max_wait - (day_now() - patient[i].day_of_entry));
        }
        else {
                x = -1;
        }
        return x;

}

int main(void){
    list_patient(0);
    return 0;

On 27 May, 22:45, Keith Thompson <k@mib.org> wrote:

ok..

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

#define MAXPATIENTS 20

struct details {
        int id;
        char forename[20];
        char initial;
        char surname[20];
        int day_of_entry;
        int max_wait;

};

//Lists individual patient information -Work in Progress
int list_patient(int index){
        int time_left = 0;
        int x = 0;
        int i;

        //Should check if index points to a record,
        //Or else it should return -1!!
        if( patient[index].id != NULL){
                printf("Patient Details: \n");
                printf("ID: %d |",patient[index].id);
                printf("Name: %s ",patient[index].forename);
                printf("%c ",patient[index].initial);
                printf("%s |",patient[index].surname);
                printf("Days on queue %d |",day_now()- patient[index].day_of_entry);
                printf("Maximum waiting days %d \n",time_left =
patient[index].max_wait - (day_now() - patient[i].day_of_entry));
        }
        else {
                x = -1;
        }
        return x;

}

int main(void){
    list_patient(0);
    return 0;

chu@gmail.com wrote:
> I'm trying to compare between pointer and integer in an "IF" statement

> how do I make this work?

You don't.  Comparing pointers and integers is meaningless.

> if(patient[index].id != NULL){

If patient[index].id is an integer, compare it with 0, if that's what
you mean.
If patient[index[.id is a pointer, compare it with NULL (as long as one
of the headers defining it is included) or with 0.

In either case, a simple
   if(patient[index].id) {
would do.

> }

No matter where you got in the very bad habit of treating pointers and
integers as things of the same sort, it is time for you to break with
those ways.  Repent!

"//" comments are not recommended on Usenet.  They're not legal in C90,
and they can create syntax errors when Usenet software causes long lines
to wrap around.  If a long "/* ... */" comment wraps around to two
lines, it's still a valid comment.

> int list_patient(int index){
>    int time_left = 0;
>    int x = 0;
>    int i;

>    //Should check if index points to a record,
>    //Or else it should return -1!!
>    if( patient[index].id != NULL){

Here's your problem.  You didn't show us your compiler's error
message, as I requested, but here's what I got:

c.c: In function `list_patient':
c.c:25: error: `patient' undeclared (first use in this function)

That's not the error you described originally.  Assuming that
"patient" is supposed to be either an array of "struct details", or a
pointer to "struct details", the member "patent[index].id" is of type
int.

NULL is a null pointer constant.  Why are you trying to compare an int
value to a null pointer constant?

(Incidentally, if your compiler complained about comparing an integer
to a pointer, you're lucky.  A legal definition of the NULL macro is
0; that's both an integer expression and a null pointer constant.  If
your implementation chose to use that definition, your compiler
wouldn't have diagnosed the error.  See section 5 of the comp.lang.c
FAQ, <http://www.c-faq.com/>.)

I'm guessing that you want "id" to be able to hold some distinguished
value that means "this structure does not refer to a patient; ignore
it".  If "id" were a pointer, using a null pointer would make sense.
But there is built-in null value for integer types.

You probably want to define some specific value to serve this purpose.
0 might be a good choice, unless 0 is a valid id.  -1 might also be a
good choice.  In either case, I suggest defining symbolic constant
rather than using "magic numbers" in your code, something like:

    #define NOT_A_PATIENT (-1)

    ...

    if (patient[index].id != NOT_A_PATIENT) {
    ...

This is assuming that a distinguished id value is the way to handle
this; it may not be.

--
Keith Thompson (The_Other_Keith) k@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

On 27 May, 23:20, Martin Ambuhl <mamb@earthlink.net> wrote:

I'm not too sure you get what I'm trying to achieve here...I'm trying
to compare the database "patient", inside patient I need to know its
not NULL (Empty)....if you compare it with 0, your saying that the id
should not be 0.

chris

[quoted .sig suppressed.  Don't quite .sigs unless you are commenting on
them.]

> ok..

Not ok.  You code is not compilable.  You have no declaration for
patient visible in list_patient.  The function day_now() is used without
a declaration or definition.  In list_patient you use the variable i
without initialization.

Further, the defintion of struct details, with the member id being an
int, and of index in list_patient being an int, makes this

>    //Should check if index points to a record,
>    //Or else it should return -1!!
>    if( patient[index].id != NULL){

nonsense.

index is an int, and does not point to a record, and you don't check it
anyway.
patient[index].id is an int, does not point, and should be compared to
0, not NULL.

If you what patient[index].id to be a pointer, declare it so.

In article <1180305018.724298.13@o5g2000hsb.googlegroups.com>,

chu@gmail.com <chu@gmail.com> wrote:
>I'm not too sure you get what I'm trying to achieve here...I'm trying
>to compare the database "patient", inside patient I need to know its
>not NULL (Empty)....if you compare it with 0, your saying that the id
>should not be 0.

In C, NULL is a special value a pointer can have, which doesn't point
to anything.  There's no similar value for integer integer variables -
they always have an integer value.  Perhaps you're used to databases,
where any field can be null.  C doesn't work like that.  If there's
some integer value you are sure you will never normally use (0 or -1
perhaps) you can use that instead.  But if the value really might be
any integer, you'll need another variable to indicate whether it's
valid.

-- Richard
--
"Consideration shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

Then you don't understand what NULL means.

You have an object of a struct type.  That object cannot be "null"
unless you define your own convention for marking it as "null (say,
setting id to -1 or whatever).

NULL is a macro that expands to a null pointer constant; a null
pointer constant evaluates to a null pointer value.  More briefly,
NULL is used *only* for pointer types.  Each pointer type has a single
distinct value that indicates that it doesn't point to anything; we
call that a "null pointer" or NULL.

The language provides this distinctive null value for pointer types.
There is no null integer or null structure value.  There is no "empty"
value for a structure type; each object of that type contains all its
members.

You need to re-think your approach.  You can define and enforce your
own convention for marking a structure as being unused.  Or you can
keep track of which members of your array are currently valid, perhaps
just with a simple count.  Or you can use pointers to structures, but
you might not want to tackle that just yet.

The comp.lang.c FAQ, <http://www.c-faq.com/>, has a lot of good
answers, many of them to questions you probably haven't thought to ask
yet.

--
Keith Thompson (The_Other_Keith) k@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

chu@gmail.com wrote:

<snip>

> struct details {
>    int id;

<snip>

>    if( patient[index].id != NULL){

<snip>

NULL if for pointers, why do you think comparing an integer to NULL
makes sense? As id is an int you are probably expected to use a value of
0 to indicate an empty record.
--
Flash Gordon

"chu@gmail.com" wrote:

> I'm trying to compare between pointer and integer in an "IF"
> statement how do I make this work?

> if(patient[index].id != NULL){
> }

I assume you mean that the id field is defined as an integer.  You
don't.  pointers don't convert to integers.  If id is an integer,
try 0 in place of NULL.

--
 <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
 <http://www.securityfocus.com/columnists/423>
 <http://www.aaxnet.com/editor/edit043.html>
 <http://kadaitcha.cx/vista/dogsbreakfast/index.html>
                        cbfalconer at maineline dot net

--
Posted via a free Usenet account from http://www.teranews.com

On May 28, 2:14 am, "chu@gmail.com" <chu@gmail.com> wrote:
> I'm trying to compare between pointer and integer in an "IF" statement

> how do I make this work?

> if(patient[index].id != NULL){

> }

pretty weird here.
Just to help you make it
 if(patient[index].id != !!NULL) {

But this is no excuse for mindlessly comparing an integer with a NULL
value.

Thanks
--psr

blufox <2500.prad@gmail.com> writes:
> On May 28, 2:14 am, "chu@gmail.com" <chu@gmail.com> wrote:
>> I'm trying to compare between pointer and integer in an "IF" statement

>> how do I make this work?

>> if(patient[index].id != NULL){

>> }
> pretty weird here.
> Just to help you make it
>  if(patient[index].id != !!NULL) {

Was that intended to be helpful??

--
Keith Thompson (The_Other_Keith) k@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

On May 28, 9:53 am, Keith Thompson <k@mib.org> wrote:

> blufox <2500.prad@gmail.com> writes:
> > On May 28, 2:14 am, "chu@gmail.com" <chu@gmail.com> wrote:
> >> I'm trying to compare between pointer and integer in an "IF" statement

> >> how do I make this work?

> >> if(patient[index].id != NULL){

> >> }
> > pretty weird here.
> > Just to help you make it
> >  if(patient[index].id != !!NULL) {

> Was that intended to be helpful??

Well ...
a better answer would have been
    if(patient[index].id) {

Its just how not to use NULL and int for comparing, note above reply
very well removes warning you 'll get eventually on compiling the
original code.(at least on gcc)

But still comparing a pointer with int is not wise.

thanks
--same person who wrote shitty code above :-)

You are probably looking for the conceptual NIL which doesn't exist in C.

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
                     --- Albert Einstein ---

chu@gmail.com wrote:
> I'm trying to compare between pointer and integer in an "IF" statement

> how do I make this work?

> if(patient[index].id != NULL){
> }

Don't use integers to store pointers. In some platforms those types may be
identical but in others (intel and AMD's 64 bit platforms, IIRC) the
integers are 32 bit and the pointers are 64 bit.

Rui Maciel

chu@gmail.com wrote:
> I'm trying to compare between pointer and integer in an "IF" statement

> how do I make this work?

> if(patient[index].id != NULL){
> }

Don't use integers to store pointers. In some platforms those types may be
identical but in others (intel and AMD's 64 bit platforms, IIRC) the
integers are 32 bit and the pointers are 64 bit.

Rui Maciel

Rui Maciel <rui.mac@gmail.com> writes:
> chu@gmail.com wrote:

>> I'm trying to compare between pointer and integer in an "IF" statement

>> how do I make this work?

>> if(patient[index].id != NULL){
>> }

> Don't use integers to store pointers. In some platforms those types may be
> identical but in others (intel and AMD's 64 bit platforms, IIRC) the
> integers are 32 bit and the pointers are 64 bit.

He wasn't trying to store integers in pointers.  He was trying to use
the "null" value of type int, analogous to a null pointer but for
integers rather than pointers.  His problem is that there is no such
thing.

--
Keith Thompson (The_Other_Keith) k@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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