Using Pointers

USING POINTERS

This example program shows a pointer being used to swap between two floating point values. The actual manipulation of the floating point values is done within the function get_value, which doesn't have any parameters. I decided that I would do it by reassigning a pointer within the main program.

#include <stdio.h>

/* The pointer has to be declared here so that */

/* the function get_value can get access to it. */

float *my_point;

/* The floating point variables have to be */

/* declared here so that function display_values */

/* can access them.*/

float x = 36.3, y = -100.44;

int main ()

 { display_values();

   my_point = &x;    // Points to x

   get_value();

   my_point = &y;    // Points to y

   get_value();

   display_values();

   return 0;

 }

void get_value ()

 { float decimal_number;

   printf(\n "Please enter a number : \n");

   scanf(“%f”,decimal_number);

   *my_point = decimal_number;

 }

void display_values ()

 { printf("\nThe value of x is %f \n",x);

   printf("\nThe value of y is %f \n",y);

  

 }

The value of x is 36.3

The value of y is -100.44

Please enter a number : 2.1

Please enter a number : 3.1

The value of x is 2.1

The value of y is 3.1

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I thought it might be interesting to see what the address of a variable is. Here I have declared three unsigned long values together with a pointer that moves between them. In each case, the address that the pointer is pointed to is displayed.

#include <stdio.h>

int main ()

  { unsigned long a = 1, b = 2, c = 3, *p;

    p = &a;

    printf("\nThe address of a is %X\n",p);

    printf("\nThe value of a is %d\n",*p);

    p = &b;

    printf("\nThe address of b is %X\n",p);

    printf("\nThe value of b is %d\n",*p);

    p = &c;

    printf("\nThe address of c is %X\n",p);

    printf("\nThe value of c is %d\n",*p);

       

    return 0;

  }

The address of a is 0x3c27245e

The value of a is 1

The address of b is 0x3c27245a

The value of b is 2

The address of c is 0x3c272456

The value of c is 3

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This function is used to sort arrays of integers. The array itself is specified as a pointer. However, the routine doesn't know in advance how many elements the array will have, so that must also be passed across as an (ordinary) parameter.

void sort (*int array, int num_elements)

 { int i, j;

   int *temp;  /* Used in swopping array values */

   for (i = 0; i < num_elements; i++)

    for (j = 0; j < num_elements-1; j++)

     if (*(array+j) > *(array+j+1))

      { temp = *(array+j);

        *(array+j) = *(array+j+1);

        *(array+j+1) = temp;

      }

 }

You will see that the pointer to the array is simply declared as a pointer to an integer. You would have no way of knowing just by looking at the first line of the function that the pointer indicated an array rather than a simple integer (other than the fact that the pointer is called array, of course, which is something of a giveaway!)

The array elements are referred to using the pointer. *(array+j) refers to element array[j] etc.

Check out the program without functions:

Sorting an array would involve swapping the elements as necessary, until they are all in ascending or descending order. There are different sorting methods (called algorithms) and this worked example demonstrates what is probably the simplest sorting algorithm - Bubblesort.

Bubblesort is so-called as the largest elements of the array move gradually to the end like bubbles rising to the surface in a glass of fizzy drink. It is usually the slowest sorting algorithm (although it can take advantage of array elements which have been partially sorted already). The algorithm goes as follows:

Assume N is the number of elements in the array.

Go through the elements from 1 to N-1

   Look at the current element and the next one.

   Are they out of order? If so, swop them.

   At the end of the run, the largest element will have

        "bubbled" to the end of the array.

Repeat this process N times.

Here is the code. Go through it matching it line-by-line with the algorithm above.

#include <stdio.h>

int main ()

 { int x[11];     /* Set up an array of 11 values */

   x[0] = 74;  x[1] = 23;  x[2] = 91;  x[3] = 58;

   x[4] = 52;  x[5] = 60;  x[6] = 30;  x[7] = 19;

   x[8] = 28;  x[9] = 46;  x[10] = 37;

   int i,j;   /* These are used as loop variables */

   int temp;  /* Used in swopping the values */

   printf("\nBefore the sort, the values are \n");

   for (i = 0; i <= 10; i++)

     printf(“x[%d] = %d\n”,i,x[i]);

   for (i = 0; i < 11; i++)  /* Go through 10 times */

     for (j = 0; j < 10; j++)  /* Check all elements */

       if (x[j] > x[j+1])       /* except last */

         { temp = x[j];

           x[j] = x[j+1];

           x[j+1] = temp;

         }

   printf("\n\nAfter the sort, the values are \n");

   for (i = 0; i <= 10; i++)

     printf(“x[%d] = %d\n”,i,x[i]);

  

   return 0;

 }

Before the sort, the values are

74 23 91 58 52 60 30 19 28 46 37

After the sort, the values are

19 23 28 30 37 46 52 58 60 74 91