                            Chapter 8 - Pointers


                             WHAT IS A POINTER?

             Simply  stated,  a pointer is an address.   Instead  of
        being  a  variable,  it  is a pointer to a  variable  stored
        somewhere in the address space of the program.  It is always
        best to use an example so load the file named POINTER.C  and
        display  it on your monitor for an example of a program with
        some pointers in it.

             For  the moment,  ignore the data declaration statement
        where we define "index" and two other fields beginning  with
        a star.   It is properly called an asterisk, but for reasons
        we  will see later,  let's agree to call it a star.   If you
        observe  the  first statement,  it should be clear  that  we
        assign the value of 39 to the variable "index".   This is no
        surprise,  we  have been doing it for several programs  now.
        The  next  statement  however,  says to assign  to  "pt1"  a
        strange looking value,  namely the variable "index" with  an
        ampersand in front of it.   In this example, pt1 and pt2 are
        pointers,  and  the  variable "index" is a simple  variable.
        Now we have a problem.  We need to learn how to use pointers
        in a program, but to do so requires that first we define the
        means of using the pointers in the program.

             The  following two rules will be somewhat confusing  to
        you at first but we need to state the definitions before  we
        can  use  them.   Take your time,  and the whole thing  will
        clear up very quickly.

                          TWO VERY IMPORTANT RULES

             The  following two rules are very important when  using
        pointers and must be thoroughly understood.

        1.  A variable name with an ampersand in front of it defines
            the  address of the variable and therefore points to the
            variable.  You can therefore read line seven as "pt1  is
            assigned the value of the address of index".

        2.  A  pointer  with a "star" in front of it refers  to  the
            value of the variable pointed to by the  pointer.   Line
            ten  of the program can be read as "The stored (starred)
            value  to which the pointer "pt1" points is assigned the
            value  13".   Now  you can see why it is  convenient  to
            think of the asterisk as a star,  it sort of sounds like
            the word store.

                                  MEMORY AIDS

            1. Think of & as an address.
            2. Think of * as a star referring to stored.


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             Assume for the moment that "pt1" and "pt2" are pointers
        (we will see how to define them shortly).  As pointers, they
        do not contain a variable value but an address of a variable
        and  can  be  used to point to a variable.  Line  7  of  the
        program  assigns the pointer "pt1" to point to the  variable
        we have already defined as "index" because we have  assigned
        the address of "index" to "pt1".  Since we have a pointer to
        "index",  we  can manipulate the value of "index"  by  using
        either the variable name itself, or the pointer.

             Line 10 modifies the value by using the pointer.  Since
        the  pointer  "pt1"  points to the  variable  "index",  then
        putting  a star in front of the pointer name refers  to  the
        memory location to which it is pointing.  Line 10  therefore
        assigns to "index" the value of 13. Anyplace in the  program
        where it is permissible to use the variable name "index", it
        is  also permissible to use the name "*pt1" since  they  are
        identical in meaning until the pointer is reassigned to some
        other variable.

                              ANOTHER POINTER

             Just  to add a little intrigue to the system,  we  have
        another  pointer  defined in this  program,  "pt2".    Since
        "pt2" has not been assigned a value prior to statement 8, it
        doesn't point to anything, it contains garbage.  Of  course,
        that is also true of any variable until a value is  assigned
        to it. Statement 8 assigns "pt2" the same address as  "pt1",
        so  that now "pt2" also points to the variable "index".   So
        to continue the definition from the last paragraph, anyplace
        in  the program where it is permissible to use the  variable
        "index",  it  is  also permissible to use  the  name  "*pt2"
        because  they   are  identical in  meaning.   This  fact  is
        illustrated  in  the  first "printf"  statement  since  this
        statement  uses  the  three means of  identifying  the  same
        variable to print out the same variable three times.

                         THERE IS ONLY ONE VARIABLE

             Note carefully that,  even though it appears that there
        are three variables, there is really only one variable.  The
        two  pointers  point  to  the  single  variable.    This  is
        illustrated in the next statement which assigns the value of
        13  to  the  variable "index",  because that  is  where  the
        pointer  "pt1"  is pointing.   The next  "printf"  statement
        causes  the  new value of 13 to be printed out three  times.
        Keep  in mind that there is really only one variable  to  be
        changed, not three.




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                            Chapter 8 - Pointers


             This is admittedly a very difficult concept,  but since
        it  is  used  extensively  in all but  the  most  trivial  C
        programs,  it  is  well  worth your time to stay  with  this
        material until you understand it thoroughly.

                        HOW DO YOU DECLARE A POINTER?

             Now  to  keep a promise and tell you how to  declare  a
        pointer.   Refer  to the third line of the program  and  you
        will  see  our  old familiar way of  defining  the  variable
        "index",  followed  by  two more  definitions.   The  second
        definition  can  be read as "the storage location  to  which
        "pt1"  points  will be an int  type  variable".   Therefore,
        "pt1" is a pointer to an int type variable.  Likewise, "pt2"
        is another pointer to an int type variable.

             A  pointer  must  be defined to point to some  type  of
        variable.   Following a proper definition, it cannot be used
        to point to any other type of variable or it will result  in
        a  "type incompatibility" error.   In the same manner that a
        "float"  type of variable cannot be added to an  "int"  type
        variable,  a pointer to a "float" variable cannot be used to
        point to an integer variable.

             Compile and run this program and observe that there  is
        only  one  variable  and the single  statement  in  line  10
        changes the one variable which is displayed three times.

                      THE SECOND PROGRAM WITH POINTERS

             In these few pages so far on pointers,  we have covered
        a lot of territory, but it is important territory.  We still
        have  a  lot  of  material to cover so stay in  tune  as  we
        continue  this important aspect of C.   Load the  next  file
        named  POINTER2.C  and display it on your monitor so we  can
        continue our study.

             In  this program we have defined several variables  and
        two pointers.   The first pointer named "there" is a pointer
        to  a "char" type variable and the second named "pt"  points
        to an "int" type variable.  Notice also that we have defined
        two  array variables named "strg" and "list".   We will  use
        them  to show the correspondence between pointers and  array
        names.

                  A STRING VARIABLE IS ACTUALLY A POINTER

             In  the programming language C,  a string  variable  is
        defined to be simply a pointer to the beginning of a string.
        This  will  take  some explaining.   Refer  to  the  example
        program  on  your monitor.   You will notice that  first  we


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                            Chapter 8 - Pointers


        assign a string constant to the string variable named "strg"
        so we will have some data to work with.  Next, we assign the
        value  of the first element to the variable "one",  a simple
        "char" variable.   Next,  since the string name is a pointer
        by  definition  of the C language,  we can assign  the  same
        value to "two" by using the star and the string  name.   The
        result  of  the two assignments are such that "one" now  has
        the same value as "two", and both contain the character "T",
        the  first character in the string.   Note that it would  be
        incorrect  to  write  the ninth line as  "two  =  *strg[0];"
        because the star takes the place of the square brackets.

             For all practical purposes,  "strg" is a  pointer.   It
        does, however, have one restriction that a true pointer does
        not  have.   It cannot be changed like a variable,  but must
        always contain the initial value and therefore always points
        to  its  string.   It  could  be thought  of  as  a  pointer
        constant,  and in some applications you may desire a pointer
        that cannot be corrupted in any way.   Even though it cannot
        be changed, it can be used to refer to other values than the
        one  it is defined to point to,  as we will see in the  next
        section of the program.

             Moving ahead to line 13, the variable "one" is assigned
        the  value of the ninth variable (since the indexing  starts
        at zero) and "two" is assigned the same value because we are
        allowed to index a pointer to get to values farther ahead in
        the string.  Both variables now contain the character "a".

            The C programming language takes care of indexing for us
        automatically  by  adjusting the indexing for  the  type  of
        variable  the  pointer is pointing to.   In this  case,  the
        index  of  8  is simply added to the  pointer  value  before
        looking up the desired result because a "char" type variable
        is  one byte long.   If we were using a pointer to an  "int"
        type variable,  the index would be doubled and added to  the
        pointer  before  looking up the value because an "int"  type
        variable  uses two bytes per value stored.   When we get  to
        the chapter on structures,  we will see that a variable  can
        have  many,   even  into  the  hundreds  or  thousands,   of
        bytes  per  variable,  but  the  indexing  will  be  handled
        automatically for us by the system.

             Since "there" is already a pointer, it can be  assigned
        the  address  of  the  eleventh element  of  "strg"  by  the
        statement  in line 17 of the program.  Remember  that  since
        "there"  is a true pointer, it can be assigned any value  as
        long as that value represents a "char" type of address.   It
        should  be clear that the pointers must be "typed" in  order
        to  allow  the  pointer arithmetic  described  in  the  last



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        paragraph to be done properly.  The third and fourth outputs
        will be the same, namely the letter "c".

                             POINTER ARITHMETIC

             Not  all  forms  of  arithmetic are  permissible  on  a
        pointer.   Only  those things that make  sense,  considering
        that a pointer is an address somewhere in the computer.   It
        would  make sense to add a constant to an  address,  thereby
        moving it ahead in memory that number of places.   Likewise,
        subtraction  is permissible,  moving it back some number  of
        locations.   Adding  two  pointers together would  not  make
        sense  because absolute memory addresses are  not  additive.
        Pointer multiplication is also not allowed, as that would be
        a  funny number.   If you think about what you are  actually
        doing,  it will make sense to you what is allowed,  and what
        is not.

                         NOW FOR AN INTEGER POINTER

             The  array named "list" is assigned a series of  values
        from  100  to 199 in order to have some data to  work  with.
        Next  we  assign the pointer "pt" the address  of  the  28th
        element  of the list and print out the same value both  ways
        to  illustrate that the system truly will adjust  the  index
        for the "int" type variable.  You should spend some time  in
        this program until you feel you fairly well understand these
        lessons on pointers.

             Compile and run POINTER2.C and study the output.

             You  may recall that back in the lesson on functions we
        mentioned that there were two ways to get variable data back
        from a function.   One way is through use of the array,  and
        you should be right on the verge of guessing the other  way.
        If your guess is through use of a pointer,  you are correct.
        Load  and display the program named TWOWAY.C for an  example
        of this.

                    FUNCTION DATA RETURN WITH A POINTER

             In  TWOWAY.C,  there  are two variables defined in  the
        main program "pecans" and "apples".   Notice that neither of
        these is defined as a pointer.   We assign values to both of
        these  and print them out,  then call the  function  "fixup"
        taking with us both of these values.   The variable "pecans"
        is  simply  sent  to the function,  but the address  of  the
        variable  "apples" is sent to the function.   Now we have  a
        problem.   The two arguments are not the same, the second is
        a pointer to a variable.  We must somehow alert the function
        to  the  fact  that it is supposed  to  receive  an  integer


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        variable  and a pointer to an integer variable.   This turns
        out   to  be  very  simple.    Notice  that  the   parameter
        definitions in the function define "nuts" as an integer, and
        "fruit"  as a pointer to an integer.   The call in the  main
        program  therefore  is now in agreement  with  the  function
        heading and the program interface will work just fine.

             In  the body of the function,  we print the two  values
        sent  to  the function,  then modify them and print the  new
        values out.   This should be perfectly clear to you by  now.
        The  surprise occurs when we return to the main program  and
        print out the two values again.  We will find that the value
        of  pecans will be restored to its value before the function
        call  because  the C language makes a copy of  the  item  in
        question and takes the copy to the called function,  leaving
        the original intact.   In the case of the variable "apples",
        we  made  a copy of a pointer to the variable and  took  the
        copy of the pointer to the function.  Since we had a pointer
        to  the  original variable,  even though the pointer  was  a
        copy,  we  had  access  to the original variable  and  could
        change  it in the function.   When we returned to  the  main
        program,  we  found  a  changed value in  "apples"  when  we
        printed it out.

             By  using  a pointer in a function call,  we  can  have
        access  to the data in the function and change it in such  a
        way  that when we return to the calling program,  we have  a
        changed  value  of data.    It must be pointed out  however,
        that  if you modify the value of the pointer itself  in  the
        function,  you  will have a restored pointer when you return
        because the pointer you use in the function is a copy of the
        original.  In this example, there was no pointer in the main
        program because we simply sent the address to the  function,
        but  in  many  programs you will use  pointers  in  function
        calls.  One of the places you will find need for pointers in
        function  calls  will be when you request data  input  using
        standard  input/output routines.   These will be covered  in
        the next two chapters.

             Compile and run TWOWAY.C and observe the output.

                           POINTERS ARE VALUABLE

             Even  though  you are probably somewhat intimidated  at
        this point by the use of pointers,  you will find that after
        you  gain experience,  you will use them profusely  in  many
        ways.  You will also use pointers in every program you write
        other than the most trivial because they are so useful.  You
        should  probably  go  over this material  carefully  several
        times until you feel comfortable with it because it is  very



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        important  in the area of input/output which is next on  the
        agenda.


        PROGRAMMING EXERCISES

        1.   Define  a character array  and use "strcpy" to  copy  a
             string  into it.  Print the string out by using a  loop
             with  a  pointer to print out one character at a  time.
             Initialize the pointer to the first element and use the
             double  plus  sign  to increment  the  pointer.  Use  a
             separate  integer variable to count the  characters  to
             print.

        2.   Modify the program to print out the string backwards by
             pointing to the end and using a decrementing pointer.





































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