CS代写 COMP2017/COMP9017 Dr.

COMP2017/COMP9017 Dr.
FACULTY OF ENGINEERING

Simple variables

› C has a number of simple types
– float, int, char etc
– each implies an interpretation of the bit pattern stored in the memory.
› Declarations label and reserve memory: int counter;
› Initialisation or assignment specifies content:
int counter = 0;
counter = 0;
reserve memory for an integer and call it “counter”

0010110 0110010 0010100
0100010000101011000010010000111 0110010010101011101010110000100 1100110000101111001010010100011

0010110 0110010 0010100
0100010000101011000010010000111 0110010010001001001010110000100 1100110000101111001010010100011

› Arrays are indexed collections of the same type › Declaration of an array:
int counters[MAX];
char alphabet[26];
› Initialisation of an array:
for (i = 0; i < MAX; i++) counters[i] = i; 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 1100110000101111001010010100011 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 11001100000101100101100101010010001010100011 char ch[2]; 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 1100110000101111000110010001100100011 char ch[2]; printf(”%c\n”, ch[1]); 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 1100110000101111001010010100011 char ch[2]; printf(”%c\n”, ch[1]); Output of random data 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 11001100001001010110100100110010001100100011 char ch[2]; printf(”%c\n”, ch[1]); ch[0] = ’A’; ch[1] = ’B’; Output of random data 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 11001100001001010110100100110010001100100011 char ch[2]; printf(”%c\n”, ch[1]); ch[0] = ’A’; ch[1] = ’B’; printf(”%c%c\n”, ch[0], ch[1]); Output of initialised data Output of random data › Strings may be initialised at the time of declaration using an “array-like” notational convenience: char myHobby[] = ”rowing”; The compiler can determine the required size by counting characters, so the array size is optional. A larger size may be specified. › Strings resemble an array of characters. › However, in C, all strings are NULL-terminated. Note: NULL is the binary value 0 (denoted ‘\0’), not the ASCII representation of the character 0. char myHobby[] = ”rowing”; ’r’ ’o’ ’w’ ’i’ ’n’ ’g’ ’\0’ 0010110 0110010 0010100 0100010000101011000010010000111 0110010010101011101010110000100 110011000010010101101001001001000100100011 char str[] = ”A”; printf(”%s\n”, str); 0x100 0x101 0x102 0x103 0x104 0x105 0x106 0x107 0x100 0x101 0x102 0x103 0x104 0x105 0x106 0x107 Random values initially 0x100 0x101 0x102 0x103 0x104 0x105 0x106 0x107 › a pointer is essentially a memory address › we can find out the address of a variable using the & operator 0x100 0x101 0x102 0x103 0x104 0x105 0x106 0x107 char initial = ‘A’; char * initp = &initial &initial is the address of initial initp is a pointer to initial Somewhere in memory... Label: “ptr” 0001011001000100001010110000100100001110 001001000100010100100001001011000101100100100011000001001 1001010011001100001011110010100101000111 Somewhere else in memory... Label: “count” 0001011001000100001010110000100100001110 0011001001100100101010111010101100001001 1001010010010010100000100100101100010010000100100001101 int count; count = 2; ptr = &count; printf(”%d\n”, count); variable name: “count” address of count: 0x1000 = 4,096 Clearly, the value of a pointer can only be determined at run-time. printf(”%d\n”, *ptr); 2 printf(”%d\n”, &count); printf(”%d\n”, ptr); Pointers (notation) › Pointer operators: - address operator, ‘&’ - indirection operator, ‘*’ Note that these operators are “overloaded”, that is they have more than one meaning. - ‘&’ is also used in C as the bitwise ‘AND’ operator - ‘*’ is also used in C as the multiplication operator Pointers (notation) › The indirection operator, ‘*’, is used in a variable declaration to declare a “pointer to a variable of the specified type”: int * countp; /* pointer to an integer */ Variable name, “countp” Type is “a pointer to an integer” Pointers (notation) What do the following mean? float * amt; A pointer (labeled “amt”) to A pointer (labeled “tricky”) to a pointer to an int. int ** tricky; Pointers (notation) › The indirection operator, ‘*’, is used to “unravel” the indirection: countp points to an integer variable that contains the value 2. Then... printf(”%d”, *countp); ...prints ‘2’ to standard output. Unravel the indirection Pointers (notation) What is output in the following? printf(”%d”, count); printf(”%d”, *countp); 17 printf(”%d”, countp); Don’t know... but it will be the address of count. Why? Pointers (notation) › The address operator, ‘&’, is used to access the address of a variable. › This completes the picture! A pointer can be assigned the address of a variable simply: Declare “a pointer to an integer” called countp int * countp = &count; Assign countp the address of count. An example of the the address operator in action... Receiving an integer from standard input: int age; scanf(”%d”, &age); This argument is required by scanf() to be a pointer. Since we are using a simple integer, age, we pass it’s address. Pointers and arrays Use of pointer notation to manipulate arrays... char msg[] = ”Hello!”; char *str = &msg[0]; ’H’ ’e’ ’l’ ’l’ ’o’ ’!’ ’/0’ char *str = msg; ’H’ ’e’ ’l’ ’l’ ’o’ ’!’ ’/0’ Pointer notation leads to some (intimidating?) shortcuts as part of the C idiom. Moving through a string: while (*str != ’\0’) str++; ’H’ ’e’ ’l’ ’l’ ’o’ ’!’ ’/0’ The previous example may exploit the fact that C treats ‘0’ as FALSE: while (*str) str++; ’H’ ’e’ ’l’ ’l’ ’o’ ’!’ ’/0’ Why use pointers? › Some mathematical operations are more convenient using pointers - e.g., array operations › However, we have only looked at static data. Pointers are essential in dealing with dynamic data structures. › Imagine you are writing a text editor. - You could estimate the largest line-length and create arrays of that size (problematic). - Or you could dynamically allocate memory as needed, using pointers. Revision pointers › What is the value held by p? and how much memory is used by p (in bytes) using 64 bits for its addressable memory? › void foo( int *p ) › char *p; › char **p; Revision pointers › What is the value held by p? and how much memory is used by p (in bytes)? › void foo( int *p ) › char *p; › char **p; › int **p; › long *p; › void *p; › const unsigned long long int * const p; › bubblebobble ************p; Pointer interpretation - Address to a single char value - Address to a single char value that is the first in an array › char *argv[] - Array of “the type” with unknown length - Type is char * › char **argv - * Address to the first element to an array of type char * - Then, each element in * is an... - * address to the first element to an array of type char char argv[][3]; // oh no! Pointer interpretation › Interpretations of int **data; 1. Pointer to pointer to single int value 2. Array of addresses that point to a single int 3. Address that points to one array of int values 4. Array of addresses that point to arrays of int values Pointer interpretation › Interpretations of int **data; 1. 2. 3. 4. Pointer to pointer to single int value Array of addresses that point to a single int Address that points to one array of int values Array of addresses that point to arrays of int values Thinking about each * as an array: Array size ==1, Array size ==1 Array size >=1, Array size == 1 Array size ==1, Array size >= 1 Array size >=1, Array size >= 1

Pointer comments
› When you call a function in Java, compare passing a primitive type and Object type.
› You may have heard: – Pass by value
– Pass by reference
What is the meaning of this in C?
› void has no size, but sizeof(void*) is the size of an address › Pointers are unsigned numbers, why?

Pointer arithmetic
› int *p = NULL; › int x[4];
x = 0x0100
0x0100 0x0101 0x0101 0x0102 0x0103 0x0104 0x0105 0x0106 0x0107 0x0108 0x0109 0x010A 0x010B 0x010C 0x010D 0x010E
*(p + 0) *(p + 1) *(p + 2) *(p + 3)
› Seeking to the nth byte from a starting address?
void *get_address( sometype *data , int n) {
unsigned char *ptr = (unsigned char*)data;
return (void*)(ptr + n); }

sizeof operator
› Not all h/w architectures are the same – different sizes for basic types
› C specification does not dictate exactly how many bytes an int will be
› sizeof operator returns the number of bytes used to represent the given type or expression
– sizeof( char )
– sizeof( int )
– sizeof( float * ) -sizeof ( 1 )
– sizeof( p )

sizeof operator
› Not all h/w architectures are the same – different sizes for basic types
› C specification does not dictate exactly how many bytes an int will be
› sizeof operator returns the number of bytes used to represent the given type or expression.
– sizeof( char )
– sizeof( int ), sizeof( double )
– sizeof( float * )
– sizeof ( 1 ), sizeof ( 1/2 ), sizeof (1.0 / 2.0) – sizeof( p ) ????

› Special case for p, what is it? – char p;
– char *p;
– char p[8];
– char msg[100];
– char *p = msg;
– char msg2[] = “hello message”; – char *p = msg2;
– char *p = “program has ended”;
› sizeof needs to be used carefully

Less familiar types
› The types char will support the value range from CHAR_MIN to CHAR_MAX as defined in file – #define UCHAR_MAX – #define CHAR_MAX – #define CHAR_MIN
255 /* max value for an unsigned char */ 127 /* max value for a char */ (-128) /* min value for a char */
› Most C implementations default types as signed values, but a warning that you should not assume this.
› unsigned and signed enforce the sign usage – char ch;
– signed char ch;
– unsigned char ch;
– unsigned int total;

Less familiar const
› const prevents the value being modified
– const char *fileheader = “P1”
– fileheader[1] = ‘3’; Illegal: change of char value
› It can be used to help avoid arbitrary changes to memory
› The value const protects depends where it appears
– char * const fileheader = “P1”
– fileheader = “P3”; Illegal: change of address value
› Reading right to left:
– Is an address, points to a char, that is constant – Is an address, that is constant

Less familiar const
› const prevents the value being modified
– const char *fileheader = “P1”
– fileheader[1] = ‘3’; Illegal: change of char value
› It can be used to help avoid arbitrary changes to memory
› The value const protects depends where it appears
– char * const fileheader = “P1”
– fileheader = “P3”; Illegal: change of address value
› You can cast if you know if the memory is writable
Non-writable
char fileheader[] = {‘P’, ‘1’};
const char *dataptr = (char*)fileheader;
char *p = (char*)dataptr;
p[1] = ‘3’;

P1 – fgets example reading from stdin
#include #include
#define BUFLEN (64)
int main(int argc, char **argv) { int len;
char buf[BUFLEN];
while (fgets(buf, BUFLEN, stdin) != NULL) {
len = strlen(buf);
printf(“%d\n”, len); }
return 0; }

Floating point types
› Exact bit representation unknown, usually IEEE 754
› Generally, floating point number x is defined as:
› b base of exponent (e.g. 2, 10, 16)
› e exponent
› p precision
› fk nonnegative integer less than b
+ve / 0 = +infinite
-ve / 0 = -infinite
NaN (not a number)
Zero exponents…

COMP2017/COMP9017
FACULTY OF ENGINEERING

The picture so far – simple types
› simple data types: – int, char, float…..
› pointers to simple data types: – int *, char *, float *

Enumerated types
› enums (enumerated types) are another simple type › enums map to int
› an enum associates a name with a value

Enumerated types
enum day_name {
Sun, Mon, Tue, Wed, Thu, Fri, Sat, day_undef
› Maps to integers, 0 .. 7
› Can do things like ‘Sun ++’ › very close to int

Enumerated types
enum month_name {
Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec, month_undef

Enumerated types
› we could always use integers to represent a set of elements
› but enums make your code much more readable › eg red instead of 0
– How many bytes for an array of enum?

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