Lecture 3: C Pointers, Arrays, Strings¶

True and False¶

False
- 0 (integer, all bits are 0)
- NULL (pointer)
True
- Anything else

Tips

Now, True and False are provided in stdbool.h

Memory is a single huge array¶

Consider memory to be a 'byte-addressed' array

Each cell of this array has an address associated with it
Each cell also stores some value

Pointers store addresses

Pointer: A variable that contains the address of another variable

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Pointers Syntax

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Warning

In this scenario, when we change *p, actually we are change the value in "What p points to"

So if we print x, we can find it into 5 correspondingly

Pointers are useful when passing parameters¶

Passing by value¶

C passes parameters by value

A function parameter gets assigned a copy of the argument value.

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Passing in pointer¶

Review: when we change *p, actually we are change the value in "What p points to"

So we can get a function to change a value, pass in a pointer.

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Common Bug: Garbage Addresses¶

The situation is that: Declaring a pointer just allocates space to hold the pointer, but it does not initialize the pointer to point to anything.

It does not allocate something to be pointed to !!!

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What is `NULL` pointer¶

The "NULL" pointer, all bits are 0.

Note

It's a very easy definition :)

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If we define char* p = NULL, what contained in p is 0x00000000

Pointers and `Struct`¶

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We can review this process:

C
typedef struct {
    int x;
    int y;
} Coord; // Define a new type called Coord

Coord coord_1, coord_2; // Declare 2 variables in type Coord
Coord* ptr_1, *ptr_2;   // Declare 2 pointers to type Coord
/*
ptr is a variable(pointer) pointing to an entity of type Coord
 ________                 __________
|  ptr   | _____________ |   Coord  |
|________|               |__________|
*/

// instantiations go here …

/* dot notation */
int h = coord1.x; 
coord2.y = coord1.y;

/* arrow notation */ 
int k; 
k = ptr1->x; 
k = (*ptr1).x; // equivalent

/*
ptr1->x == (*ptr1).x
*/

ptr1 = ptr2; // ptr1 now (points to the same thing)/(contains the same address) as ptr2

Pointer Arithmetic¶

a[i] = *(a+i)
pointer + n == add n*sizeof("whatever pointer is pointing to") to memory address
pointer - n == subtract n*sizeof("whatever pointer is pointing to") to memory address

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