CS 261 – Recitation 2 Fall 2013 Oregon State University School of Electrical Engineering and Computer Science

CS 261 – Recitation 2

Fall 2013

Oregon State University

School of Electrical Engineering and Computer Science

CS 261 – Data Structures

Outline

• Programming in Co Compiling reviewo Preprocessoro Headerso Random numberso Pointerso Dynamic memory allocationo Structures

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Compiling with GCC:

gcc <list of options> sourcefile.c

e.g.: gcc –Wall –std=c99 -o test test.c

Compiling with Makefile:default:main

main: main.cgcc –Wall –std=c99 main.c –o main

clean:rm main main.o

Executed when you type “make”

Executed when you type “make clean”

Compiling Review

3

The C Preprocessor

• C Preprocessor– preprocesses every source file before

performing the actual translation– replaces macros with their definitions

• “#define” directive: Used to define macros– Syntax:#define name [replacement_text]

– Example:#define BUF_SIZE 512#define MAX(a,b) ((a) > (b) ? (a) : (b))

CS 261 – Data Structures 4

The C Preprocessor (cont.)

• Consider the following code


struct myArray {

int data[100];

int count;

};

int myArrayTop(struct myArray *da);

int myArrayGet (struct myArray *v, int index);

struct myArray {

double data[100];

int count;

};

double myArrayTop(struct myArray *da);

double myArrayGet (struct myArray *v, int index);


• Solution in C


#define TYPE int

struct myArray {

TYPE data[100];

int count;

};

TYPE myArrayTop(struct myArray *da);

TYPE myArrayGet (struct myArray *v, int index);

#define TYPE double

struct myArray {

TYPE data[100];

int count;

};

TYPE myArrayTop(struct myArray *da);

TYPE myArrayGet (struct myArray *v, int index);


• Other directives• #if, #elif, #else, #endif• #ifdef and #ifndef

# ifndef TYPE# define TYPE double# endif

More about C Preprocessorhttp://en.wikipedia.org/wiki/C_preprocessor


# define TYPE int

Header files


• Purposes• To store standard library functions. E.g.

stdio.h• To store interfaces of C modules. E.g. dyArray.h

/* Prototypes */void dyArrayInit(struct dyArray *da, int

initialCapacity);void dyArrayFree (struct dyArray *v);

/* stack operations interface */void dyArrayPush(struct dyArray *da, TYPE e);void dyArrayPop(struct dyArray *da);TYPE dyArrayTop(struct dyArray *da);int dyArrayIsEmpty(struct dyArray *da);

Header files (cont.)


• To include a standard library in C, use “<>”.

E.g: #include <stdio.h>When using angle brackets, the

preprocessoronly searches for it in directories in yoursystem’s PATH variable.

• To include a header file, use quotation marks “”.

E.g: #include “sort.h”When using quotation marks, the

preprocessorfirst looks for the file in the current

workingdirectory.


Some useful headers are:

• stdlib.h: Contains the memory management tools like malloc(),calloc(), and free().

• stdio.h: Contains definitions for printf() and scanf(), useful functions for handling I/O.

• math.h: More advanced math functions, sqrt(), abs(), exp(), etc.

• ctype.h: Contains functions for working with characters

• time.h: Contains functions for accessing the system clock

Some particularly useful header files

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Pseudo-random numbers in C

Some of the functionality that will be useful for your homework is the creation of random numbers using rand(), which is defined in stdlib.h as:

int rand ( void );

This function returns a number between 0 – RAND_MAX

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In order to generate numbers in a range, you can use the modulo (%) operator.

int value = rand(); ( value % 100 ) is in the range 0 to 99( value % 100 + 1 ) is in the range 1 to 100( value % 30 + 1985 ) is in the range 1985 to 2014

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Without a seed value, rand() will usually return the same sequence of random numbers.

In order to properly seed the pseudo-random number generator first call srand(time(NULL)).

Which header file do you think the time function is declared in?

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/* srand example */ #include <stdio.h> #include <stdlib.h> #include <time.h> int main () {

printf ("First number: %d\n", rand() % 100); srand ( time(NULL) ); printf ("Random number: %d\n", rand() % 100); srand ( 1 ); printf ("Again the first number: %d\n", rand()

%100);return 0;

}

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First number: 41 Random number: 13 Again the first number: 41 (seed is 1)

*If seed is set to 1, the generator is reinitialized to its initial value and produces the same values as before any call to rand or srand.

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In order to generate a list of unique numbers between two values there a couple of techniques

•Generate an ordered list then permute the elements

•Generate random numbers then simply check to see if the generated number is in use yet.

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Pointers

• A pointer represents the address and type of a variable or a function.


a is a pointer whose value is the address of b a == &b and *a == b

Pointers

• Two fundamental operators:– &: address-of operator – to get a pointer to (address of) a

variable– *: dereference operator - get the thing the pointer points

to.

• * is also used to declare a pointer variableint i=5, int* p;p = &i;

• Note: – The name of an array is automatically converted to a

pointer to the array's first element.– The value of a null pointer is 0 or NULL.CS 261 – Data Structures 18

Pointers


var. name

value address

ptr ??? 0xA1

0xA3

var. name value address

a ??? 0x0A

b ??? 0x0C

int main { int *ptr; int a,b; a = 12; ptr = &a; b = *ptr;}


a 12 0x0A

b ??? 0x0C

var. name

value address

ptr 0x0A 0xA1

0xA3


a 12 0x0A

b 12 0x0C

Why pointers

•to simulate passing parameters by reference

•to pass a function big chunks memory

•to speed up the program

•to impress friends


Simulate passing parameter by reference

void swap(int x, int y) {

int temp; temp = x; x = y; y = temp;}


a = 12;b = 13;swap(a,b);

Is this code correct?

Call-by-value semantics

var. name

value

x ??

y ??

var. name

value

a 12

b 13

copied

main function’s code

var. name

value

x 12

y 13

copied

var. name

value

x 13

y 12

Simulate passing parameter by reference(cont.)


a = 12;b = 13;swap(&a,&b);

How about this code?

var. name

value

x ??

y ??

var. name

value

address

a 12 0x0A

b 13 0x0C

copied

main function’s code

copied

void swap(int *x, int *y) {

int temp; temp = *x; *x = *y; *y = temp;}

*x, *y are “aliases” of a and b

var. name

value

x 0x0A

y 0x0C

var. name

value

address

a 13 0x0A

b 12 0x0C

Pointer arithmetic

• Two arithmetic operations can be performed on pointers:– An integer can be added to or subtracted from a

pointer.– One pointer can be subtracted from another of

the same type.

• In arithmetic operations on pointers, the size of the objects pointed to is automatically taken into account. Which is great because you don’t have to multiply by sizeof(TYPE) all the time.


Pointer arithmetic (cont.)

An important thing to remember: *(ptr+num) == ptr[num]

int a[3] = { 0, 10, 20 }; /* An array with 3 elements */int *ptr_a = a; /* Let ptr_a point to a[0] */

/* pointers to the i-th element */&a[i] , a+i , ptr_a+i

/* the i-th array element */a[i] , *(a+i) , *(ptr_a+i) , ptr_a[i]

/* Let pa point to a[2] */ptr_a = a+2; or ptr_a = ptr_a + 2; int n = ptr_a – a; /* what is the value of n? */


Dynamic Memory Allocation

• Consider this code:

struct dyArray { /* dynamic array structure */

TYPE data[100]; int size; int capacity;};

What’ s wrong with this code?


Dynamic Memory Allocation (cont.)

• Corrected version:

struct dyArray { /* dynamic array structure */

TYPE *data; int size; int capacity;};

The memory needed to store data will be dynamically allocated.


Dynamic Memory Allocation (cont.)

struct dyArray { /* dynamic array structure */ TYPE *data; int size; int capacity;};

struct dyArray da;da.capacity = 100; da.size = 0;da.data = (TYPE *) malloc(da.capacity*sizeof(TYPE));assert(da.data != NULL);…free(da.data);


Structures in C

• The `struct` type:– A bit similar to class in Java with no methods.

• Declare a struct data type:struct item {/* item = name of this struct */char name[40];char color[20];double price; };


Structures in C (cont.)

• Declare variables with the structure type:struct item a1, a2;struct item *pointer_to_a1;struct item myItems[100];

• Initialize structure variables:struct item flower=

{ ”rose", ”green”, 2.49 };struct item bouquet[10] ;bouquet[0] = flower;


Structures in C (cont.)

• Access structure members:

– Using the dot operatorflower.name /* The array 'name' */flower.price /* The double variable 'price’

*/

– Using pointers to structsstruct item *pArticle;pArticle = &flower; /*Let pArticle point to flower*/

pArticle->color=”red”;/* Access members of flower */

pArticle->price=5.0; /* using the pointer pArticle*/



That’s all for today!

Documents

CS 261 – Recitation 2 Fall 2013 Oregon State University School of Electrical Engineering and Computer Science