C Programming

Class 3

Objective

c3.tar

Pointers

ex1.c
Pointers are a very powerful feature of the C programming language, they are also somewhat tricky to deal with as well. Don't worry if you don't understand pointers after this class especially if you don't really have any programming experience.

Pointer Operators:   &  and  *

The & is called the address operator and when placed in front of a variable returns its memory address. Notice that this is used in the scanf() function.

Example:

int a = 7;
aPtr = &a;


This statement assigns the address of a to the integer pointer aPtr. The variable aPtr is now said to point toa.
The * is called the dereferencing operator and when placed in front of a pointer returns the value that the pointer points to.

Example: (Assume all previous statements.)

printf("%d", *aPtr);  This statement will output 7.

Arrays and Pointers

ex2.c
In C, there is a strong relationship between pointers and arrays, any operation which can be achieved by array subscripting can also be done with pointers.

The declaration int a[10]; defines an array a of size 10, that is a block of 10 consecutive objects named a[0], a[1], ... a[9]. The notation a[i] means the element of the array i positions from the beginning.

If pa is a pointer to an integer, declared as
            int *pa;
then the assignment
            pa = & a[0];
sets pa to point to the zeroth element of a;  that is,  pa contains the address of a[0] . Now the assignment
            int x = *pa ;
will copy the contents of a[0] into x;

If pa points to a particular element of an array a, then by definition, pa + 1 points to the next element, and in general pa - i points i elements before pa. Thus, if pa points to a[0], then *(pa + 1) refers to the contents of a[1].

Question: What is the difference between  *(pa + 1)  and  (*pa + 1) ?

The array name a is actually a pointer to the first element of the array, so a is equvilent to &a[0]. We could also have the statement:

pa = a;


The following statements are equivalent:

The second and fourth statement offset the pointer by 2 and then use the dereferencing operator to get its value. (Note: offsetting a pointer will not change its initial value.)

Memory Allocation

ex3.c
Space is automatically set aside for variables when they defined, but sometimes you do not know beforehand how many variables you'll need or just how long an array might need to be. The malloc command creates space, returning a pointer to this new area.

Example (allocating space for an one dimensional array):

int i, row;
float *sum;
printf("Enter the row number: ");
scanf("%d", &row);
sum = (float *)malloc(row * sizeof(float));
for (i = 0; i < row; i++) {
    sum[i] = i;
}
Example (allocating space for a two dimensional array):
int row, col;
float **zrand;
printf("Enter the row number: ");
scanf("%d", &row);
printf("Enter the Col number: ");
scanf("%d",&col);
zrand=(float **)malloc(row*sizeof(float *));
for (i=0; i< row; i++) {
  zrand[i]=(float*)malloc(col*sizeof(float));
}

What is a function?

ex4.c
    So far we have already seen some prepackaged functions in C such as the printf(), scanf(), exp() and main() just to name a few. C also allows us to write our own functions to define other specific tasks.
    Having functions allows the programmer to break up a large computer problem into smaller more manageable pieces (or modules). This allows for easier debugging, better manageability of your program and easier reuse.

Example:
    Consider the program in the previous class.
 

Function Prototypes

int comp(int src, int des);


This line from the beginning of the program are called function prototypes. A function prototype tells the compiler the number of arguments a specific function gets and what kind of value to expect when that function is done executing. The compiler also uses to function prototype to check that all calls to a specific function contain the correct number of arguments and the correct argument types.
 

Calling Functions

    When a program reaches the name of a function that function is called or invoked. At this point program control is passed to that function. After the function is done, either by reaching the end of all of its statements or reaching a return,program control is then passed back to the line occurring right after the call to that function.
 

Pass by value and simulated pass by reference (very important!!!)

    All arguments that are passed to a function in C are passed by value, that is a copy of the argument's value is made and passed to the function. Using pass by value does not allow the function to change the value of the original variable. Although C allows arguments to be passed only by value we can simulate what is called pass by reference. Pass by reference does allow for a function to modify the value of the original variable. Simulated pass by reference is done by using pointers. Note that in the program the function modifies the original array sum.
 


Note: These web pages were written with the reference text C How to Program by Deitel and Deitel.