C pointer and arrays

In this lesson, we discuss C pointer and arrays. Pointers are one of the most important concepts in C language, their use in various cases makes programming easier, and sometimes the only solution is to use pointers.
Pointers and arrays are closely related, and any operation that is done through array indexing; can also be done with pointers. The advantage of using a pointer over using an index is to increase the speed of operation, although it is easier for beginners to use an index.

Assign arrays to pointers

By defining an array, several bytes of memory are assigned to it, and the starting address of these bytes is assigned to the name of the array, which is also called the base address of the array. This name is a constant pointer because it is assigned a fixed address from memory, which cannot be changed. Pay attention to the following example:

int key[10];
int *pt, *pr;

• Key is a constant pointer, and pt and pr are pointer variables.
• Regarding the key array, it is interesting to state that &key[0] and key both indicate the address of the array or the address of the first element of the array.
• For example, &key[3] is the address of the fourth element of the array, which can also be written as key + 3.

Now we want to assign the address of the array to the pointers. Both of the following statements are true:

pt = key;
pr = key + 2;

• On line one, the first address of the key array is assigned to the pt pointer.
• On line two, the address of the third element of the key array is assigned to the pr pointer.

Note that writing a statement such as key = pt; is wrong because the key is a constant pointer, and its value cannot be changed.

Accessing arrays with pointers

When a pointer contains the address of an array, that pointer is addressable, like the array’s name. For example, expressions like key[3], pt[3], and pr[3] mean the fourth element of the array. To access the content of the array, you can use statements like *(key + 3), *(pt + 3), and *(pr+1) used.
Note that because the pt and pr pointers point to different array elements, different values ​​have been added to them to access the content of the fourth array’s element.

Now let’s pay attention to the details of this example. Suppose that the key points to cell number 3200 in memory; Because each cell of this array is of int type, it occupies two bytes. Therefore, the address of the second element is 3202, which can be accessed as key+1 or key[1]. In the key+1 expression, we added one to the key, but in the address conversion, two are added to the real address of the key. This is because the compiler knows that the key is a pointer of type int and must add two units to it in each increment.

Look at the following example:

float *x, *y, *z, total[]={5,10,15,20,25};
int num[10] = {2,4,6,8,10,12,14,16,18,20};
int p, q;
p = num;
q = num + 4;
x = total + 1;
y = &total[0];
++x;
z = x - 1;
z--;

1. On line one, pointers x, y, z, and the total array of float types are defined.
2. On line two, the num array of int type is defined.
3. On line three, pointers p and q are defined as int type.
4. Lines 4 to 10 include the assignment of the addresses of the elements of these two arrays to pointers of the same type.
5. Try to understand which pointer points to which location.

C pointer and strings

As mentioned before, pointers contain addresses, and the name of any string is a constant address(pointer). The string’s name specifies its first element address. Consider the following example:

char table[5];
char *p;
p = &table;

1. String “table” with 5 elements and pointer “p” are defined.
2. The string’s name is a constant pointer.
3. The second line represents a variable pointer.
4. P points to the first element of the “table.”