Comparison between "Pointer and Integer"

Discussion in 'C Programming' started by chutsu@gmail.com, May 27, 2007.

  1. Guest

    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
    , May 27, 2007
    #1
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  2. Ian Collins Guest

    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.
    Ian Collins, May 27, 2007
    #2
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  3. "" <> 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) <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"
    Keith Thompson, May 27, 2007
    #3
  4. Guest

    On 27 May, 22:45, Keith Thompson <> wrote:
    > "" <> 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) <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"


    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.day_of_entry));
    }
    else {
    x = -1;
    }
    return x;
    }

    int main(void){
    list_patient(0);
    return 0;
    }
    , May 27, 2007
    #4
  5. Flash Gordon Guest

    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
    Flash Gordon, May 27, 2007
    #5
  6. Guest

    On 27 May, 22:45, Keith Thompson <> wrote:
    > "" <> 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) <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"


    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.day_of_entry));
    }
    else {
    x = -1;
    }
    return x;
    }

    int main(void){
    list_patient(0);
    return 0;
    }
    , May 27, 2007
    #6
  7. Guest

    On 27 May, 22:45, Keith Thompson <> wrote:
    > "" <> 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) <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"


    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.day_of_entry));
    }
    else {
    x = -1;
    }
    return x;
    }

    int main(void){
    list_patient(0);
    return 0;
    }
    , May 27, 2007
    #7
  8. 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!
    Martin Ambuhl, May 27, 2007
    #8
  9. "" <> writes:
    > From: "" <>
    > Subject: Re: Comparison between "Pointer and Integer"
    > Newsgroups: comp.lang.c
    > Date: 27 May 2007 15:00:04 -0700
    > Organization: http://groups.google.com
    >
    > On 27 May, 22:45, Keith Thompson <> wrote:
    >> "" <> 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.

    >
    > 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


    "//" 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) <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"
    Keith Thompson, May 27, 2007
    #9
  10. Guest

    On 27 May, 23:20, Martin Ambuhl <> wrote:
    > 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!


    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
    , May 27, 2007
    #10
  11. wrote:
    > On 27 May, 22:45, Keith Thompson <> wrote:
    >> "" <> 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.



    [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.
    Martin Ambuhl, May 27, 2007
    #11
  12. In article <>,
    <> 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.
    Richard Tobin, May 27, 2007
    #12
  13. "" <> writes:
    > On 27 May, 23:20, Martin Ambuhl <> wrote:
    >> 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!

    >
    > 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.


    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) <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"
    Keith Thompson, May 27, 2007
    #13
  14. blufox Guest

    On May 28, 2:14 am, "" <> 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

    > Thanks
    > Chris
    blufox, May 28, 2007
    #14
  15. blufox <> writes:
    > On May 28, 2:14 am, "" <> 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) <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"
    Keith Thompson, May 28, 2007
    #15
  16. psr Guest

    On May 28, 9:53 am, Keith Thompson <> wrote:
    > blufox <> writes:
    > > On May 28, 2:14 am, "" <> 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 :)
    >
    > --
    > Keith Thompson (The_Other_Keith) <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"
    psr, May 28, 2007
    #16
  17. Joe Wright Guest

    wrote:
    > On 27 May, 23:20, Martin Ambuhl <> wrote:
    >> 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!

    >
    > 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
    >

    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 ---
    Joe Wright, May 28, 2007
    #17
  18. Rui Maciel Guest

    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, May 28, 2007
    #18
  19. Rui Maciel Guest

    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, May 28, 2007
    #19
  20. Rui Maciel <> writes:
    > 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) <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"
    Keith Thompson, May 28, 2007
    #20
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