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Pointers

We will continue our discussion on the pointers and how we put this concept to use.         

 We have already seen  the representation of  pointers in a program, i.e. how  do we declare  pointers and how to get the address to which it is pointing to. For example to declare a pointer  "num" of the type "integer" we used "int *num;".  we also saw that, to assign or to point this pointer to another integer variable "n" we will use the statement "num=&n;" .

  Here we created a pointer called num  and then directed this num to the address of  the number n.Thats what the use of  "&" operator is . It returns the address of the argument "n" and  then assigns that to "num ".

 Now we will see the use of   *  operator.  We are going to assign  the value stored in the address, to which  "num" is pointing , to another variable "m" . The way that is done is through the staement,  "m=* num;" . Here  * returns  the data to which "num " is pointing to.

Remember that "*" and "&" are two operators that is closely associated with a pointer.Then we come to arrays.  The syntax to declare an integer array "m" of four elements is  int x[4]; The type here is  integer and  array  dimension is 4 . Now elements of the array would be x[0],x[1],x[2] and x[3].  Another important thing to note is that  we use square brackets  instead of  () brackets to mark the elements of the  array. This is because we use () to represent functions.

Now we will see how one can use arrays and pointers interchangeably.  Let us look at the following code.

int *num;

int x[4];

num=x;

 Here we have an integer pointer declared num, which is of type int and an integer array x . By saying num=x  we are point num to the base address of  the array  x. i.e to the base element of x.

 If we now write  num++ it will point to the next element of the array x i.e. x[1]. 

There are two important things to take note of.  We can point num to  the " i^th"element of the array by stating  num=x[i]; .  num++; will then point to the "(i+1)^th " element.  The second point is that num++;  can be used recursively to point to the successive elements of the array. i.e.,

num=x;

for(i=0;i<=3;i++)

 {num++;}

will make num  point to all the elements of the array successively and we can get the value stored in the array elements by using "*num". However if we look at the address of "num" it will always be  the base address of "x".

 Printing the address of array elements using pointers:

        Pointers can be used to get the address of each element of the array.  Let is say we declare an array x in the following manner;

 

int x[ ]={10,20,30,40,50};

As you may have noticed this way of declaring array is different from that we used earlier. Here, while declaring the array, we have also assigned values to the elements.

 So here "x" is an array and we have given 10,20,30,40,50 as the elements of the array.

That is x[0]=10,x[1]=20,x[2]=30,x[3]=40 and x[4]=50.

 We will  now  define another integer  pointer " j " using the statement  int *j;  and assign j=x; this statement basically  mean "j" would  point to the base address of  an array "x".

Now we can read as the address of  num[0] from j[0] and  the address of num[1] from j[1] etc.. We see this in the program given below .

 

(pointer-4.c)

 #include<math.h>

#include<stdio.h>

         main()

        {

        int x[ ]={10,20,30,40,50};

        int i,*j;

        j=x;

        for(i=0;i<=4;i++)

         {

           printf("%u %u %d %d \n",&j,&j[i],*j,x[i]);

           j++;

         }

  }

  

This program implements the idea we just discussed to  print out the  addresses of the array elements. It also points out many features of  pointers. Let us look at it in a little more detail.

 After the usual include statements  the main program has the array declaration we just discussed.  Then we declare an  integer i and integer pointer j. Then the assignment j=x, which make j point to the base address of  the array x.  Then we get into the loop which execute the printf statement five times by incrementing "i" one at a time. Each time it goes through the loop the pointer "j" is incremented by one.

The printf  statement prints out the addresses of "j" and "j[i]".  As we had discussed earlier the operator "&" will return the address of the variable or array following it. We are also printing out the value stored in the address to which "j" is pointing and the value of the array element "x[i]".

  This will demonstrate the difference between the addresses of "j" and "j[i]. What we want to see here is that while the address given by  "&j" remains the same the value given by "*j" changes, indicating that  each time  we increment "j" it is  pointing to the next element of array "x". but the operator "&" acting on "j" always return its base address. So to get the address of the array elements we should use "&j[i]".

  Note that we use % u as the format to print out the addresses and %d  to print the value of an integer.

 If we now compile and run the program using the commands,

 gcc pointer-4.c

./a.out

 the following lines will be printed on the screen;

 4277678728   4277678736  10   10

4277678728   4277678744   20   20

4277678728   4277678752   30   30

4277678728   4277678760   40   40

4277678728   4277678768  50   50

 As we expected  the first column is always printing the  base address of j, even though we are  incrementing  "j" by "j++". But "&j[i]", in the second column, is  printing  different numbers i.e it is giving us the address of the different elements in array "x". So j[i] is pointing to the different  elements of the array.  The third column is "*j" and the fourth is "x[i]".  You can see that there are the same. That is what we expect to find.

 To summarize what we have  seen here is that  there are two ways to get the elements of an array. One can point an integer pointer to the array, then  increment that integer pointer and  pick up the values using the "*" operator. Or  one could use array elements straightaway. If you want to get the address of the array elements  there is only one way of doing that.  We the integer pointer is "j", printing  &j we will not  get the address of each elements. You could get only the base address while "&j[i]" will give us the addresses of each elements.

  

Pointer to Pointers:

 

              We can make a pointer to point to   another pointer i.e pointer to pointers. It is useful when we want to read a character array and also a two dimensional array as we will see later.

 

Let us look at a sample code.

 char **marker;

char course[0]="numerical",course[1]="methods";

marker=&course[0];  \* pointer to course[0] which points to "numerical" */

marker++;                  \* points to course[1] which points to "m" in  "methods" */

(*marker)++              \* will then point to "e" */

  

Here "marker" is a pointer to a pointer,   we use  " ** marker " to declare this. In the sample code given above we also have character array  "course" . The first element of this character array is "course [0]" which is storing the string "numerical"  and the second element of the array is "course[1]" which is storing "methods".

 In the third line of the sample code we point "marker"  to "course[0]" . If we now increment  "marker", by using "marker++", it will point to "course[1]".

 Question:How do one read out each of the characters in a string ?

 Here is a program that does it.

 (pointer-5.c)

 #include<math.h>

#include<stdio.h>

        main()

        {

          int i;

          char **marker,*course[2];

          course[0]="numerical";

          course[1]="methods";

          marker=&course[0];

        for(i=0;i<=8;i++)

         {

           printf("%u %u %c \n",&marker[i],&course[i],*(*marker));

           (*marker)++ ;

         }

        }

  This is the way we can do it. We have  (*marker)++  which will move the pointer through "numerical", since "marker" is set to "course[0]".  The bracket is very important here. Instead if we use "marker++" the pointer will shift to the next element of the array "course".

 We have used "%c" to print each characters of the string to which (*marker) is pointing.

The printf statement also prints out the address to which marker is pointing and also the address of the array "course". You can compare these addresses.

  Here is the out put of the program.

 4277434920  4277434920  n

4277434924  4277434924  u

4277434928  4277434928  m

4277434932  4277434932  e

4277434936  4277434936  r

4277434940  4277434940  i

4277434944  4277434944  c

4277434948  4277434948  a 

4277434952  4277434952  l

 What happens if you change the increment of the pointer to "course++" ?

  

Passing variables to functions:

 

A good programmer does most of the calculations in sub functions. When the "main" function (or other sub functions ) invoke a particular sub function it may want to pass and receive some variables. In this section we will look at how this is carried out.

 We have seen the use of a function earlier but haven't looked at how variables are passed.

(see below for general rules on calling a function). So now we will see how arrays and variables are  passed from one function to another.

 In this context the most important  thing to remember is the following.  In C all variables are passed by value except arrays which are passed by reference. What do one means by "by value" and by "reference" ? 

There are two things a function can do, when you pass variable to it. The function can  make a copy of the variable and work with that copy or it can directly work on the variable. In a C program the former method is adopted when we pass only the value of that variable to the function.  That is the function gets a copy of the variable, stores it in a different location in the memory,  and it works with that copy. So if you make a change in that variable value inside that function it doesnt change the variable in the master function.  This is what we refer to as pass by value.

As opposed to this, in  pass by reference , we pass the address of the memory location where the variable is stored. And now in the function if you change the value stored in that location it will of course change it in the main program too. So the main difference is that if you pass by ref it will not make a copy  but if you pass by variable  it will make a copy.

Passing an array to a function:

We do something quite different when we pass the content of  an array from one function to another. This is because, as we had discussed earlier, the array name is also the pointer to the first element of the array. So when you are passing an array to another function, using the array name, you are passing pointer to the first element of  the array, that is we are passing by reference.

 In this case  any, the function that received this array is directly working with it, not with a copy of it. So any change that is made in the array by the subfunction will also change the value of the array in  the program from which its passed.

  

Summary:

All functions have the following form

     type function name (arguments passed to the funtion)

         variable declaration of the arguments

      {

          local variable declarations

          statements

      }

  

Argument passing:  Two ways of passing variables are

 

      (1)  call by value

       Here  the function work with a copy of the variable

        changes made in the function will not effect its value in

      the calling function

 

       (2)  call by reference

           Here the address of the variable is passed to the function.

            changes made in the function will change its value in the

            calling function.