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Слайды и текст к этой презентации:
№1 слайд
Содержание слайда: Lecture 18-20
Pointers
№2 слайд
Содержание слайда: Outline
Defining and using Pointers
Operations on pointers
Arithmetic
Logical
Pointers and Arrays
Memory Management for Pointers
№3 слайд
Содержание слайда: Pointer Fundamentals
When a variable is defined the
compiler (linker/loader actually)
allocates a real memory address
for the variable
int x;
When a value is assigned to a variable, the value is actually placed to the memory that was allocated
x=3;
№4 слайд
Содержание слайда: Recall Variables
№5 слайд
Содержание слайда: Recall Variables
Recall a variable is nothing more than a convenient name for a memory location.
The type of the variable (e.g., int) defines
how the bits inside that memory location will be interpreted, and
what operations are permitted on this variable.
Every variable has an address.
Every variable has a value.
№6 слайд
Содержание слайда: The Real Variable Name is its Address!
There are 4 billion (232) different addresses, and hence 4 billion different memory locations.
Each memory location is a variable (whether your program uses it or not).
Your program will probably only create names for a small subset of these “potential variables”.
Some variables are guarded by the operating system and cannot be accessed.
When your program uses a variable the compiler inserts machine code that calculates the address of the variable.
Only by knowing the address can the variables be accessed.
№7 слайд
Содержание слайда: Pointers
When the value of a variable is used, the contents in the memory are used
y=x; will read the contents in the 4 bytes of memory, and then assign it to variable y
&x can get the address of x (referencing operator &)
The address can be passed to a function:
scanf("%d", &x);
The address can also be stored in a variable ……
№8 слайд
Содержание слайда: Pointer: Reference to Memory
№9 слайд
Содержание слайда: Pointers
To declare a pointer variable
type * PointerName;
For example:
int x;
int * p; //p is a int pointer
// char *p2;
p = &x; /* Initializing p */
№10 слайд
Содержание слайда: Pointer: Declaration and Initialization
№11 слайд
Содержание слайда: Addresses and Pointers
int a, b;
int *c, *d;
a = 5;
c = &a;
d = &b;
*d = 9;
printf(…,c, *c,&c)
printf(…,a, b)
№12 слайд
Содержание слайда: Addresses and Pointers
A pointer variable is a variable!
It is stored in memory somewhere and has an address.
It is a string of bits (just like any other variable).
Pointers are 32 bits long on most systems.
№13 слайд
Содержание слайда: Using Pointers
You can use pointers to access the values of other variables, i.e. the contents of the memory for other variables
To do this, use the * operator (dereferencing operator)
Depending on different context, * has different meanings
№14 слайд
Содержание слайда: * has different meanings in different contexts
a = x * y; multiplication
int *ptr; declare a pointer
* is also used as indirection or de-referencing operator in C statements.
ptr = &y;
a = x * *ptr;
№15 слайд
Содержание слайда: Using Pointers
You can use pointers to access the values of other variables, i.e. the contents of the memory for other variables
To do this, use the * operator (dereferencing operator)
Depending on different context, * has different meanings
For example:
int n, m = 3, *p;
p = &m; // Initializing
n = *p;
printf("%d\n", n); // 3
printf("%d\n", *p); // 3
*p = 10;
printf("%d\n", n); // 3
printf("%d\n", *p); // 10
№16 слайд
Содержание слайда: An Example
int m = 3, n = 100, *p, *q;
p = &m;
printf("m is %d\n", *p); // 3
m++;
printf("now m is %d\n", *p); // 4
p = &n;
printf("n is %d\n", *p); // 100
*p = 500;
printf("now n is %d\n", n); // 500
q = &m;
*q = *p;
printf("now m is %d\n", m); // 500
№17 слайд
Содержание слайда: An Example
int i = 25;
int *p;
p = &i;
printf("%x %x", &p, &i);
printf("%x %p", p, p);
printf("%d %d", i, *p);
№18 слайд
Содержание слайда: Pointer Assignment
int a = 2, b = 3;
int *p1, *p2;
p1 = &a;
p2 = &b;
printf("%p %p", p1 ,p2);
*p1 = *p2;
printf("%d %d", *p1, *p2);
p2 = p1;
printf("%p %p", p1, p2);
printf("%p %p", &p1, &p2);
№19 слайд
Содержание слайда: Value of referred memory by a pointer
№20 слайд
Содержание слайда: Exercise: Trace the following code
int x, y;
int *p1, *p2;
x = 3 + 4;
Y = x / 2 + 5;
p1 = &y;
p2 = &x;
*p1 = x + *p2;
*p2 = *p1 + y;
printf(…,p1,*p1,&p1)
printf(…,x,&x,y,&y)
№21 слайд
Содержание слайда: Pointer Fundamentals
When a variable is defined the
compiler (linker/loader actually)
allocates a real memory address
for the variable
int x; // &x = 22f54;
&x = 22f54; // Error
When a value is assigned to a variable, the value is actually placed to the memory that was allocated
x = 3; // * (&x) = 3;
*x = 3; // Error
№22 слайд
Содержание слайда: Allocating Memory for a Pointer
№23 слайд
Содержание слайда: Characteristics of Pointers
We’ve seen that pointers can be initialized and assigned (like any variable can).
They can be local or global variables (or parameters)
You can have an array of pointers
etc., just like any other kind of variable.
We’ve also seen the dereference operator (*).
This is the operation that really makes pointers special (pointers are the only type of variable that can be dereferenced).
№24 слайд
Содержание слайда: Pointer “Size”
Note: Pointers are all the same size. On most computers, a pointer variable is four bytes (32 bits).
However, the variable that a pointer points to can be arbitrary sizes.
A char* pointer points at variables that are one byte long. A double* pointer points at variables that are eight bytes long.
When pointer arithmetic is performed, the actual address stored in the pointer is computed based on the size of the variables being pointed at.
№25 слайд
Содержание слайда: Constant Pointers
A pointer to const data does not allow modification of the data through the pointer
const int a = 10, b = 20;
a = 5; // Error
const int *p;
int *q;
p = &a; // or p=&b;
*p = 100; // Error : p is (const int *)
p = &b;
q = &a;
*q = 100; // OK !!!
№26 слайд
Содержание слайда: Constant Pointers
int x; /* define x */
int y; /* define y */
/*ptr is a constant pointer to an integer that can be modified through ptr, but ptr always points to the same memory location */
int * const ptr = &x;
*ptr = 7; /* allowed: *ptr is not const */
ptr = &y; /* error: cannot assign new address */
№27 слайд
Содержание слайда: Constant Pointers
int x = 5; /* initialize x */
int y; /* define y */
/*ptr is a constant pointer to a constant integer. ptr always points to the same location; the integer at that location cannot be modified */
const int * const ptr = &x;
*ptr = 7; /* error: cannot assign new value */
ptr = &y; /* error: cannot assign new address */
№28 слайд
Содержание слайда: Pointer to pointer
int main(void)
{
int s = 1;
int t = 1;
int *ps = &s;
int **pps = &ps;
int *pt = &t;
**pps = 2;
pt = ps;
*pt = 3;
return 0;
}
№29 слайд
Содержание слайда: Multiple indirection
int a = 3;
int *b = &a;
int **c = &b;
int ***d = &c;
int ****f = &d;
№30 слайд
Содержание слайда: Pointer Initialization
№31 слайд
Содержание слайда: NULL Pointer
Special constant pointer NULL
Points to no data
Dereferencing illegal
To define, include <stdio.h>
int *q = NULL;
№32 слайд
Содержание слайда: NULL Pointer
№33 слайд
Содержание слайда: NULL Pointer
Often used as the return type of functions that return a pointer to indicate function failure
int *myPtr;
myPtr = myFunction( );
if (myPtr == NULL){
/* something bad happened */
}
Dereferencing a pointer whose value is NULL will result in program termination.
№34 слайд
Содержание слайда: Generic Pointers: void *
void *: a pointer to anything
Lose all information about what type of thing is pointed to
Reduces effectiveness of compiler’s type-checking
Can’t use pointer arithmetic
№35 слайд
Содержание слайда: Operations on Pointers
№36 слайд
Содержание слайда: Arithmetic Operations
When an integer is added to or subtracted from a pointer, the new pointer value is changed by the integer times the number of bytes in the data variable the pointer is pointing to
For example, if the pointer p contains the address of a double precision variable and that address is 234567870, then the statement:
p = p + 2; // 234567870 + 2 * sizeof(double)
would change p to 234567886
№37 слайд
Содержание слайда: Operations on Pointers
№38 слайд
Содержание слайда: Arithmetic Operations
A pointer may be incremented or decremented
An integer may be added to or subtracted from a pointer.
Pointer variables may be subtracted from one another
int a, b;
int *p = &a, *q = &b;
p = p + q ; // Error
p = p * q; // Error
p = p / q; // Error
p = p - q; // OK
p = p + 3;
p += 1.6; // Error
p %= q; // Error
№39 слайд
Содержание слайда: Arithmetic Operations
pointer + number pointer – number
№40 слайд
Содержание слайда: Comparing Pointers
Pointers can also be compared using ==, !=, <, >, <=, and >=
Two pointers are “equal” if they point to the same variable (i.e., the pointers have the same value!)
A pointer p is “less than” some other pointer q if the address currently stored in p is smaller than the address currently stored in q.
It is rarely useful to compare pointers with < unless both p and q “point” to variables in the same array.
№41 слайд
Содержание слайда: Logical Operations
Pointers can be used in comparisons
int a[10], *p, *q , i;
p = &a[2];
q = &a[5];
i = q - p; /* i is 3*/
i = p - q; /* i is -3 */
a[2] = a[5] = 0;
i = *p - *q; // i = a[2] – a[5]
if (p < q) ...; /* true */
if (p == q)...; /* false */
if (p != q) ...; /* true */
№42 слайд
Содержание слайда: Pointers and Arrays
the value of an array name is also an address
In fact, pointers and array names can be used interchangeably in many (but not all) cases
The major differences are:
Array names come with valid spaces where they "point" to. And you cannot "point" the names to other places.
Pointers do not point to valid space when they are created. You have to point them to some valid space (initialization)
№43 слайд
Содержание слайда: Pointers and Arrays
№44 слайд
Содержание слайда: Pointers and Arrays
№45 слайд
Содержание слайда: An Array Name is Like a Constant Pointer
Array name is like a constant pointer which points to the first element of the array
int a[10], *p, *q;
p = a; /* p = &a[0] */
q = a + 3; /* q = &a[0] + 3 */
a ++; /* Error !!! */
№46 слайд
Содержание слайда: Example
int a[10], i;
int *p = a; // int *p = &a[0];
for (i = 0; i < 10; i++)
scanf("%d", a + i); // scanf("%d", &a[i]);
for (i = 9; i >= 0; --i)
printf("%d", *(p + i));
// printf("%d", a[i]);
//printf("%d", p[i]);
for (p = a; p < &a[10]; p++)
printf("%d", *p);
№47 слайд
Содержание слайда: An example
int a[10], *p, *q;
p = &a[2];
q = p + 3;
p = q – 1;
p++;
q--;
*p = 123;
*q = *p;
q = p;
printf("%d", *q); /* printf("%d", a[5]) */
№48 слайд
Содержание слайда: An Example
int a[10], *p;
a++; //Error
a--; // Error
a += 3; //Error
p = a; // p = &a[0];
p ++; //OK
p--; // Ok
P +=3; // Ok
№49 слайд
Содержание слайда: Array Example Using a Pointer
№50 слайд
Содержание слайда: Array of Pointers
int a=1, b=2, c=3, d=4;
int *k[4] = {&a, &b, &c, &d};
printf("%d %d %d %d", *k[0], *k[1],*k[2],*k[3]);
№51 слайд
Содержание слайда: Strings
In C, strings are just an array of characters
Terminated with ‘\0’ character
Arrays for bounded-length strings
Pointer for constant strings (or unknown length)
№52 слайд
Содержание слайда: Strings & Pointers
№53 слайд
Содержание слайда: Strings in C (cont’d)
№54 слайд
Содержание слайда: char Array vs. char *: Example
№55 слайд
Содержание слайда: An Example
char *str, s[] = "ALIREZA";
printf("%s", s); // ALIREZA
printf(s); // ALIREZA
printf("%s", s + 3); // REZA
scanf("%s", s);
scanf("%s", &s[0]);
str = s;
while(* str)
putchar(*str++); // *s++ : Error
№56 слайд
Содержание слайда: Array of Pointers
char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" };
№57 слайд
Содержание слайда: Empty vs. Null
Empty string ""
Is not null pointer
Is not uninitialized pointer
№58 слайд
Содержание слайда: Multi-Dimensional Arrays
int a[row][col];
a[row][col] *(a[row] + col)
a a[0][0] a[0]
№59 слайд
Содержание слайда: Call by value
№60 слайд
Содержание слайда: Call by reference
№61 слайд
Содержание слайда: Pointers in Functions
№62 слайд
Содержание слайда: Swap function (wrong version)
№63 слайд
Содержание слайда: swap function (the correct version)
№64 слайд
Содержание слайда: Now we can get more than one value from a function
Write a function to compute the roots of quadratic equation ax^2+bx+c=0. How to return two roots?
void comproots(int a,int b,int c,
double *dptr1, double *dptr2)
{
*dptr1 = (-b - sqrt(b*b-4*a*c))/(2.0*a);
*dptr2 = (-b + sqrt(b*b-4*a*c))/(2.0*a);
return;
}
№65 слайд
Содержание слайда: Trace a program
№66 слайд
Содержание слайда: Pointer as the function output
№67 слайд
Содержание слайда: Pointer as the function output
№68 слайд
Содержание слайда: Pointer to constant: const <type> *
№69 слайд
Содержание слайда: Constant pointer: <type> * const
№70 слайд
Содержание слайда: Passing Arrays to Functions
#include <stdio.h>
void display(int a)
{
printf("%d",a);
}
int main()
{
int c[] = {2,3,4};
display(c[2]); //Passing array element c[2] only
return 0;
}
№71 слайд
Содержание слайда: Arrays in Functions
№72 слайд
Содержание слайда: Passing Arrays to Functions
#include <stdio.h>
float average(float a[], int count); // float average(float *a, int count)
int main(){
float avg, c[]={23.4, 55, 22.6, 3, 40.5, 18};
avg=average(c, 6); /* Only name of array is passed as argument */
printf("Average age=%.2f", avg);
return 0;
}
float average(float a[], int count){ // float average(float *a
int I; float avg, sum = 0.0;
for(I = 0;I < count; ++i) sum += a[i];
avg = (sum / 6);
return avg;
}
№73 слайд
Содержание слайда: Passing Arrays to Functions
#include <stdio.h>
void f1(float *a) { a[1] = 100;}
void f2(float a[]){ a[2] = 200;}
void printArray(float a[])
{
int i = 0;
for(; i < 6; i++) printf("%g ", a[i]);
}
int main(){
float c[]={23.4, 55, 22.6, 3, 40.5, 18};
f1(c);
printArray(c);
puts("");
f2(c);
printArray(c);
return 0;
}
№74 слайд
Содержание слайда: Pointer to functions
№75 слайд
Содержание слайда: Pointer to Function
#include <stdio.h>
void f1(float a){ printf("F1 %g", a);}
void f2(float a){ printf("F2 %g", a);}
int main(){
void (*ptrF)(float a);
ptrF = f1;
ptrF(12.5);
ptrF = f2;
ptrF(12.5);
getch();
return 0;
}
№76 слайд
Содержание слайда: Example
№77 слайд
Содержание слайда: Pointer to function
№78 слайд
Содержание слайда:
№79 слайд
Содержание слайда:
№80 слайд
Содержание слайда: Dynamic Memory Allocation
№81 слайд
Содержание слайда: Dynamic Memory Allocation
Memory is allocated using the:
malloc function (memory allocation)
calloc function (cleared memory allocation)
Memory is released using the:
free function
note: memory allocated dynamically does not go away at the end of functions, you MUST explicitly free it up
The size of memory requested by malloc or calloc can be changed using the:
realloc function
№82 слайд
Содержание слайда: malloc
Prototype: void *malloc(size_t size);
function returns the address of the first byte
programmers responsibility to not lose the pointer
Example:
№83 слайд
Содержание слайда: calloc
№84 слайд
Содержание слайда: Example of malloc and calloc
№85 слайд
Содержание слайда: Example
№86 слайд
Содержание слайда: malloc and calloc
№87 слайд
Содержание слайда: malloc vs. calloc
№88 слайд
Содержание слайда: Free
№89 слайд
Содержание слайда: free
Prototype: void free(void *ptr)
releases the area pointed to by ptr
ptr must not be null
trying to free the same area twice will generate an error
№90 слайд
Содержание слайда: #include <stdio.h>
№91 слайд
Содержание слайда: #include <stdio.h>
№92 слайд
Содержание слайда: Reallocation
№93 слайд
Содержание слайда: realloc Example
float *nums;
int I;
nums = (float *) calloc(5, sizeof(float));
/* nums is an array of 5 floating point values */
for (I = 0; I < 5; I++)
nums[I] = 2.0 * I;
/* nums[0]=0.0, nums[1]=2.0, nums[2]=4.0, etc. */
nums = (float *) realloc(nums,10 * sizeof(float));
/* An array of 10 floating point values is allocated, the first 5 floats from the old nums are copied as the first 5 floats of the new nums, then the old nums is released */
№94 слайд
Содержание слайда:
№95 слайд
Содержание слайда: Allocating Memory for a Pointer
There is another way to allocate memory so the pointer can point to something:
#include <stdio.h>
#include <stdlib.h>
int main(){
int *p;
p = (int *) malloc( sizeof(int) ); /* Allocate 4 bytes */
scanf("%d", p);
printf("%d", *p);
// ....
free(p); /* This returns the memory to the system*/
/* Important !!! */
}
№96 слайд
Содержание слайда: Allocating Memory for a Pointer
You can use malloc and free to dynamically allocate and release the memory
int *p;
p = (int *) malloc(1000 * sizeof(int) );
for(i=0; i<1000; i++)
p[i] = i;
p[999]=3;
free(p);
p[0]=5; /* Error! */
№97 слайд
Содержание слайда: #include <stdio.h>
№98 слайд
Содержание слайда:
№99 слайд
Содержание слайда:
№100 слайд
Содержание слайда:
№101 слайд
Содержание слайда:
№102 слайд
Содержание слайда:
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