Pointers and Structures in C

Where all the fun starts

https://talks.obedmr.com/

Pointer

A pointer is a variable that contains the address of a variable.

int x = 1, y = 2, z[10];
int *ip;         /* ip is a pointer to int */
ip = &x;         /* ip now points to x */
y = *ip;         /* y is now 1 */
*ip = 0;         /* x is now 0 */
ip = &z[0];      /* ip now points to z[0] */

Pointers in Functions

  • Arguments by value
void swap(int x, int y)
{
   int temp;
   temp = x;
   x = y;
   y = temp;
}
  • Arguments by reference
void swap(int *px, int *py)
{
    int temp;
    temp = *px;
    *px = *py;
    *py = temp;
}

So, what would be used in the main function?

Arrays and Pointers

int a[10];
int *pa;

What's the output of the following code?

a[1] = 132;
pa = a;
printf("%d\n", pa);

How to fix it?

int a[10]; // Defines an array of 10 elements


int *pa;
pa = &a[0];

How to access a's elements from *pa

Then,

*pa == a[0]
*(pa + 1) == a[1]
*(pa + 2) == a[2]
*(pa + n) == a[n]

Multi-dimensional arrays

static char daytab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};


int day_of_year(int year, int month, int day)
{
    int i, leap;
    leap = year%4 == 0 && year%100 != 0 ¦¦ year%400 == 0;

    for (i = 1; i < month; i++)
        day += daytab[leap][i];
    return day;
}

Let's code: month day calculator

  • Add the proper error checking mechanisms

  • Implement month_day function that given a year and a year day, calculate the corresponding month and the month's day numbers. Use pointers for month and month's day.

/* month_day function's prototype*/
void month_day(int year, int yearday, int *pmonth, int *pday);

Pointer Arrays

char *month_name(int n) {

   static char *name[] = {
       "Illegal month",
       "January", "February", "March",
       "April", "May", "June",
       "July", "August", "September",
       "October", "November", "December"
   };

 return (n < 1 ¦¦ n > 12) ? name[0] : name[n];
}

Question: How's the string's lenght being defined?

Pointers vs. multi-dimensional arrays

  • Multi-dimensional array
 int a[10][20];
  • Pointer array
 int *b[10];

Question: What's the difference between a and b?

  • Pointer array
char *name[] = { "Illegal month", "Jan, "Feb", "Mar" };

  • Multi-dimensional array
char aname[][15] = { "Illegal month", "Jan", "Feb", "Mar" };

Questions:

  • Which one is using more memory? name or aname?
  • How strings size are being calucated in name pointer array?
  • Remember argv? what is it?

Pointers to functions

void qsort(void *lineptr[], int left, int right,
           int (*comp)(void *, void *));

Questions:

  • What is *lineptr[]?
  • What is int (*comp)(void *, void *)?
  • What void * means?

Let's code: Generic Sort

More about Structures ...

Read the Chapter 6 - Structures in
the The C Programming Language book.

  • Declaration of structures
  • Usage in function and as parameters
  • Arrays of structures
  • Self-referencing
  • Pointers to structures
  • Table lookup (Hash tables)

Resources and Credits

This material is generated thanks to some extracts from following resources:

  • The C Programming Language (Chapters 5,6) - Brian W. Kernighan

Thanks

  • Obed N Muñoz Reynoso
    • Cloud Software Engineer
    • obed.n.munoz@gmail | tec | intel.com