c programming a modern approach

C ProgrammingA Modern Approach

K. N. King, C Programming

A Modern Approach,W. W. Norton & Company, 1996.

Note About Coverage

This class has only part of the semester dedicated to C

You already know Java, which has similar syntax

You should be able to read the book quickly

Similarities of C to Java

/* Comments */

Variable declarations

If / else statements

For loops

While loops

Function definitions (like methods)

Main function starts program

Differences between C and Java

C does not have objects There are “struct”ures

But data are not tied to methods

C is a functional programming language

C allows pointer manipulation

Input / Output with C Output with printf function

Input with scanf function

C Strengths Efficiency

Limited amount of memory

Fast Portability

Compilers are small and easily written

C: UNIX and ANSI/ISO standard Power

Flexibility

Standard library

Input/output, string handling, storage allocation, etc. Integration with UNIX

C Weakness Can be error-prone

Flexibility C compiler doesn’t detect many programming

mistakes Pitfalls

Can be difficult to understand

Some features are easier to be misused Flexibility

Can be difficult to modify

No modules

Compiling and Linking

Variables and Assignments C is case-sensitive

Compiler remembers only first 31 characters

Type

Should be declared

int height, length, width;

Declarations must precede the statements.

The value should be assigned before using the variable in computations:

height = 8;

length = 12;

width = 5;

int volume = height * length * width;

Constants

Macro definition:

#define SCALE_FACTOR (5.0/9.0)

No semicolon at the end!

Formatted Output: printf

printf(string, expr1, expr2,…)printf(string, expr1, expr2,…)

Format string contains both ordinary characters and conversion specifications

Conversion specification is a placeholder representing a value to be filled in during printing.

The information after % specifies how the value is converted form its internal form(binary) to printed form (characters)

Pitfalls

Formatted Input: scanf scanf(string, expr1, expr2,…)scanf(string, expr1, expr2,…)

Reads input according to a particular format.

Format string contains both ordinary characters and conversion specifications

scanf(“%d%d%f%f”, &&i, &j, &x, &y);

Number of conversion specification should match number of variables.

Each conversion should be appropriate for type of the variable.

while (scanf (“%d”, &i)==1) {

… }

How scanf Works For each conversion specification, tries to locate an

item of appropriate type, skipping blank spaces if necessary.

Reads it, stopping when it encounters the symbol that can’t belong o item.

Ignores white-space characters.

scanf(“%d%d%f%f”, &&i, &j, &x, &y);

Assignment Operators Simple assignment(right associative): ==

int i=5, j=3;

int k = 5*3;

i = 72.99;

float f;

f = 136;

i = j = k = 7;

f = i = 33.3;

k = 1 + (j = i);

Compound assignments(right associative): +=, -=, *=, /=, %=+=, -=, *=, /=, %=

i += j += k;

i += k; vs i =+ k;

Increment and Decrement Operators

Prefix operators: ++i, --i++i, --i

Postfix operators: i++, i—i++, i—

Precedence and Associativity

Sub expression Evaluation

a = 5;

c = (b = a + 2) – (a = 1);

i = 2;

j = i * i ++;

Any expression can be used as a statement

i++;

i*j -1;

i + j ; /* i = j */

Relational Operators 0 (false) and 1 (true) Their precedence is lower than the

precedence of the arithmetic operators. Left associative

i < j < k

Equality Operators

0 (false) and 1 (true)

Their precedence is lower than the precedence of the relational operators.

i < j == j < k

Left associative

Logical Operators Logical negation: !!

!expr : 1 if expr has the value 0!expr : 1 if expr has the value 0

Right associative

The same precedence as unary plus and minus

Logical and: &&&&

expr1 && expr2 : 1 if both expr1 and expr2 has non-zero valuesexpr1 && expr2 : 1 if both expr1 and expr2 has non-zero values

Logical or: ||||

expr1 || expr2 : 1 if either expr1 or expr2 (or both) has non-zero expr1 || expr2 : 1 if either expr1 or expr2 (or both) has non-zero valuesvalues

Short-circuit evaluation

Left associative

The precedence is lower that that of the relational and equality operators

if … else Statement if (expression) statementif (expression) statement

if (expression) statement else statementif (expression) statement else statement

Statement can be compound: { statements}{ statements}

if (i==0) vs if (i=0)

if (expression)if (expression)

statementstatement

else if (expression)else if (expression)

statementstatement

… …

elseelse

statementstatement

“Dangling else” Problem

if (y != 0)

if (x != 0)

result = x / y;

else

printf (“Error: y is equal to ) \n”);

Use compound statement {}

Compiler always match the closest unmatched if statement

Conditional Expression

expr1 ? expr2: expr3expr1 ? expr2: expr3

int i, j, k;

i = 1;

j = 2;

k = i > j ? i : j ;

k = (i > 0 ? i : 0) + j ;

return (i > j ? i : j);

printf (“%d\n”, i > j ? i : j);

switch Statement switch ( expression ){switch ( expression ){

case constant-expression: statementscase constant-expression: statements

… …

case constant-expression: statementscase constant-expression: statements

default: statementsdefault: statements

}}

Controlling expression should be an integer expression (characters)

Constant expression can’t contain variables or function calls.

Statements do not require {}. Usually, the last statement is breakbreak.

Example

while Loop while (expression) statementwhile (expression) statement

Statement can be compound: {}

while (i>0) printf (“%d\n”, i--);

while (i>0)

{

printf (“%d\n”, i);

i--;

}

Infinite loops: while(1)

Break, goto, returnBreak, goto, return

Example

scanf(“%d”, &n);

do Loop

do statement while (expression);do statement while (expression);

Statement can be compound: {}

do printf (“%d\n”, i--); while (i>0) ;

do

{

printf (“%d\n”, i);

i--;

} while (i>0);

for Loop for (expr1; expr2; expr3) statement

Statement can be compound: {}

expr1;

while (expr2) {

statement

expr3;

}

for (i=10; i>0; i--)

printf (“%d\n”, i);

Infinite loop: for (;;)

Comma operator:

for (i=1, j=2; i+j<10; i++, j++) printf (“%d\n”, i+j);

Example

Exiting From a Loop: break for (d=2; d<n; d++)

if (n%d==0) break;

if (d<n) printf (“%d is divisible by %d\n”, n,d);

else printf(“%d is prime \n”,n);

for (;;){

printf (“Enter a number(0 to stop): ”);

scanf(“%d”,&n);

if (n==0) break;

printf(“%d cubed is %d\n”,n,n*n*n);

}

break break escapes only one level of nesting.

Skipping the Rest of Iteration: continue

n = 10;

sum = 0;

while (n-->0){

scanf(“%d”, &i);

if (i%2==0) continue;

sum+=i;

}

Null statement ;;

for (d=2; d<n; d++)

if (n%d==0) break;

for (d=2; d<n && n%d !=0 ; d++);

Accidentally putting a semicolon after the parentheses in if, while or for statement ends the statement prematurely.

if (i==0);

printf (“Zero\n”);

while (i>0); printf (“%d\n”, i--);

Basic Types: Integers Signedness: signed (defaut), unsigned

Size: short, long

<limits.h> holds ranges for int types.

Integer Constants Decimal (base 10) literals

digits between 0-9, no leading zero15, 255, 32767

Octal (base 8) literals

digits between 0-7, must start with 0017, 0377, 077777

Hexadecimal (base 16) literals

digits between 0-9 and letters between A-F (a-f), must start with 0x (0X)

0xF, 0xFF, 0x7FFF

Basic Types: Floating Types <float.h>

Assume IEEE 754 standard

Scientific notation: sign, an exponent, a fraction

.57e2, 57, 5.7e+1, 570.0e-1

Basic Types: char Character set: Latin (7 bit), ASCII (8 bit)

Treats as integers

unsigned (0-255) and signed (-128-127) version

Some compilers use unsigned by default, the other compilers use signed by default.

char ch=65; /* it’s ‘A’ now */

int i = ‘a’; /* it’s 97 */

ch++; /* it’s ‘B’ now */

if (‘a’< =ch && ch <=‘z’)

ch = ch – ‘a’ + ‘A’; /* ch=toupper(ch);*/

for (ch=‘A’; ch<=‘Z’; ch++) …

ch=‘a’ * ‘b’ / ‘c’ …

Read and Write char: Alternative ch = getchar();

putchar(ch);

while ( ( ch = getchar() ) != ‘\n’ ) …

while ((ch=getchar())==‘ ’);

printf(“Enter an integer: ”);

scanf(“%d”, &i);

printf(“Enter a command: ”);

command = getchar();

sizeof Operator

sizeof (type-name)sizeof (type-name)

Unsigned integer representing the number of bytes required to store a value of type-nametype-name

sizeof(char) is always 1

Can be applied to constants, variables, expressions

int i, j;

int k= sizeof(i); /* k is assigned 2*/

k = sizeof (i + j );

Implicit Type Conversion Convert operands to the “narrowest” type that will

safely accommodate both values.

If the type of either operand is a floating point:

float -> double - > long double

Otherwise:

if there are short and char operands, convert them to int, then

int -> unsigned int -> long int -> unsigned long int int i= -10;

unsigned int u=10;

Conversion During Assignment char c = ‘A’;

int ind;

float f;

double d;

i = c; /* will get 65 */

f = i; /* will get 65.0 */

d = f;

i = 824.97; /* 824 */

c= 100000000;

f = 1.0e1000;

Explicit Type Conversion: cast

(type-name) expression(type-name) expression

Unary operatorUnary operator

float f = 3.45, frac;

frac = f – (int) f;

int num1=5, num2 =3;

float quotient = (float) num1/ num2;

int i=1000;

long int i = (long int) j * j;

long int i = (long int) (j * j)

Celsius vs Fahrenheit table (in steps of 20F)

C = (5/9)*(F-32);

#include <stdio.h> int main() { int fahr, celsius, lower, upper, step; lower = 0; upper = 300; step = 20; fahr = lower; while (fahr <= upper) { celsius = 5 * (fahr - 32) / 9; printf("%d\t%d\n",fahr, celsius); fahr += step; } return 1; }

5/9 = 0

Integer arithmetic: 0F = 17C instead of 17.8C

%d, %3d, %6d etc for formatting integers

New Version Using Float #include <stdio.h> int main() { float fahr, celsius; int lower, upper, step; lower = 0; upper = 300; step = 20; fahr = lower; while (fahr <= upper) { celsius = (5.0 / 9.0) * (fahr - 32.0); printf("%3.0f %6.2f \n", fahr, celsius); fahr += step; } return 1; }

%6.2f 6 wide; 2 after decimal

5.0/9.0 = 0.555556

Float has 32 bits

Double has 64 bits

Long Double has 80 to 128 bits

Depends on computer

Version 3 with “for” loop#include <stdio.h>

int main() { int fahr;

for (fahr=0; fahr <= 300; fahr += 20) printf("%3d %6.1f \n", fahr, (5.0 / 9.0) * (fahr – 32.0));

return 1;}

Version 4 with Symbolic Constants#include <stdio.h>

#define LOWER 0#define UPPER 300#define STEP 20

int main() { int fahr;

for (fahr=LOWER; fahr <= UPPER; fahr += STEP) printf("%3d %6.1f \n", fahr, (5.0 / 9.0) * (fahr - 32.0));

return 1;}

One –Dimensional Array Data structure containing a number of values of the same

type.

Declare array: type, name and number of elements

int a[10] Subscripting:

for (i=0; i<N; i++) a[i]=0;

for (i=0; i<N; i++) scanf (“%d”, &a[i]);

i=0;

while (i<N)

a[i++] = 0;

const int month[]={31,28,31,30,31,30,31,31,30,31,30,31};

Array Initialization int a[10] = { 1,2,3,4,5,6,7};

int a[]= { 1,2,3,4,5,6,7};

sizeof Operator for Arrays

sizeof(varName)sizeof(varName)

Determines the size of variable in bytes.

for ( i = 0; i< sizeof(a) / sizeof(a[0]); i++)

a[i]=0;

Multidimensional Array int m[i][j]

Stores in row-major order

int m[3][3]={{1,2,3},

{4,5,6},

{7,8,9}}

int m[3][3]={{1,2},{4,5,6}}

Arrays

Count #occurrences of each digit, blank, tab, newline, other characters

#include <stdio.h>int main() { int c, i, nwhite, nother; int ndigit[10];

nwhite = nother = 0; for (i=0; i<10; ++i) ndigit[i] = 0; while ((c = getchar()) != EOF) if (c > '0' && c < '9') ++ndigit[c-'0']; else if (c == ' ' || c == '\n' || c == '\t') ++nwhite; else ++nother;

printf("digits = "); for (i=0; i<10; ++i) printf("%d ",ndigit[i]); printf(", white space = %d, other = %d\n",nwhite, nother);}

Functions return type function-name (parameters)return type function-name (parameters)

{{

declarationsdeclarations

statementsstatements

}}

Function can not return array

If function doesn’t return value, use void.

List of parameters: type name, type name… (void)

ex.:float average(float a, float b){ return (a+b)/2; }

Function Calls A call of void function is a statement:

void f (int a){

printf(“%d\n”, a);

}

f(3+4*5); A call of non-void function is an expression:

float average(float a, float b){

return (a+b)/2;

}

float av = average (3+4*5, 2);

Function Declaration C doesn’t require to define function before its first use.

void main() {

int i =4, k=5;

float av = average (i,k);

}

float average(float a, float b){ return (a+b)/2; }

To ensure error message, declare function before its first call (function prototypefunction prototype):

return-type function-name(parameters);return-type function-name(parameters);

float average (float a, float b); float average(float, float);

Arguments ParametersParameters appear in function definition

Arguments Arguments are expressions in function calls.

In C, arguments are passed by valuepassed by value

int power (int x, int n);

void main {

int k=3, pow = power (2, k);

}

int power (int x, int n){

int result=1;

while (n-- >0) result*=x;

return result;

}

Example void decompose (float x, int integer, float fract){

integer = (int) x;

fract = x – integer;

}

void main(){

int i=5;

float f= 3.4;

decompose (3.14, i, f);

}

Array Arguments int f (int a[]) {…}

int sum(int[], int);

int sum(int a[], int n){

int i,sum=0;

for(i=0; i<n; i++) sum+=a[i];

return sum;

}

int a[10]={1,4,6,7,3,2,4};

int total = sum (a, 10);

int f(int a[][10]){…}

return and exit Statements return expression;return expression;

double Largest (double x, double y){

return x>y? x : y;

}

void print_int(int i){

if (i<0) return;

printf(“%d\n”,i);

}

The value returned by main is a status code.

In <stdlib.h>: exit(expression); exit(expression);

Normal success: EXIT_SUCCESS (0)

Abnormal termination: EXIT_FAILURE (1)

Recursive Function

int power (int x, int n){

return n==0? 1: x*power(x, n-1);

}

int factorial(int n){

return n<=1 ? 1 : n*factorial(n-1);

}

int sum_digits(int n){

return n==0 ? 0: n%10+sum_digits(n/10);

}

Functions#include <stdio.h>

int power(int, int); /* function prototype */

int main() { int i; for (i = 0; i < 10; i++) printf("%d %d %d\n", i, power(2,i), power(-3,i));}

int power(int base, int n) { int p; for (p = 1; n > 0; --n) p = p * base; return p;}

Functions

Call by value mechanism

Change in n's value not reflected in main

Using pointers, we can achieve Call by reference effect.

External (Global) Variables

All variables declared so far are local to the functions

External or Global variables are accessible to all functions

These are defined once outside of functions

Must be defined once more within each function which is going to use them

Scope Rules

Automatic/Local Variables Declared at the beginning of functions

Scope is the function body

External/Global Variables Declared outside functions

Scope is from the point where they are declared until end of file (unless prefixed by extern)

Scope Rules

Static Variables: use static prefix on functions and variable declarations to limit scope static prefix on external variables will limit scope to

the rest of the source file (not accessible in other files)

static prefix on functions will make them invisible to other files

static prefix on internal variables will create permanent private storage; retained even upon function exit

Exampleint i; //global variable

void f1 ()

{

int j=0; //call every time when f invoked

j++;

}

void f2 ()

{

static int j=0; //only done once

j++;

}

Scope Rules

Variables can be declared within blocks too scope is until end of the block

Data Structures (struct) Arrays require that all elements be of the

same data type.

C and C++ support data structures that can store combinations of character, integer floating point and enumerated type data. They are called a structs.

Structures (struct) A struct is a derived data type composed of members

that are each fundamental or derived data types.

A single struct would store the data for one object. An array of structs would store the data for several objects.

A struct can be defined in several ways as illustrated in the following examples:

Declaring Structures (struct)

Struct

{

int day;

char month[3];

char year[4];

} my_date ;

my_date date3 = {2,”Jul”,”2010”};

struct date

{

int day;

char month[3];

char year[4];

} ;

/* The name “date" is called a

structure tag */

Struct date date1;

Struct date date2 = {3,”Jan”,”1912”};

Lect 23 P. 75Winter Quarter

User Defined Data Types (typedef) The C language provides a facility called typedef for creating

synonyms for previously defined data type names. For example, the declaration:

typedef int Length;

makes the name Length a synonym (or alias) for the data type int.

The data “type” name Length can now be used in declarations in exactly the same way that the data type int can be used:

Length a, b, len ;

Length numbers[10] ;

Typedef & Struct Often, typedef is used in combination with struct to declare a synonym

(or an alias) for a structure:

typedef struct /* Define a structure */

{

int label ;

char letter;

char name[20] ;

} Some_name ; /* The "alias" is Some_name */

Some_name mystruct ; /* Create a struct variable */

Accessing Struct Members

Individual members of a struct variable may be accessed using the structure member operator (the dot, “.”):

  mystruct.letter ; 

Or , if a pointer to the struct has been declared and initialized

  Some_name *myptr = &mystruct ;

  by using the structure pointer operator (the “->“):

  myptr -> letter ;

  which could also be written as:

  (*myptr).letter ;

Sample Program With Structs int main ( )

{

struct identity js = {"Joe Smith"}, *ptr = &js ;

js.person.id = 123456789 ;

js.person.gpa = 3.4 ;

printf ("%s %ld %f\n", js.name, js.person.id,

js.person.gpa) ;

printf ("%s %ld %f\n", ptr->name, ptr->person.id,

ptr->person.gpa) ;

}

/* This program illustrates creating structs and then declaring and using struct variables. Note that struct personal is an included data type in struct "identity". */

#include <stdio.h>

struct personal //Create a struct { long id; float gpa; } ;

struct identity //Create a second struct that includes the first one.

{ char name[30]; struct personal person; } ;

Enumeration: enum enumenum is a type whose values are listed (enumerated) by the

programmer who creates a name (enumeration constantenumeration constant) for each of the values.

enum {yellow,red, green, blue} c1,c2;

enum colors {yellow,red, green, blue};

enum colors c1,c2;

typedef enum {yellow,red, green, blue} Colors;

Colors c1,c2;

typedef enum {FALSE, TRUE} Bool;

C treats enumeration variables and constants as integers.

#include <stdio.h>

int main( )

{

int apr[5][7]={{0,0,0,0,1,2,3},{4,5,6,7,8,9,10},

{11,12,1314,15,16,17},{18,19,20,21,22,23,24},

{25,26,27,28,29,30}};

enum days enum days {Sunday, Monday, Tuesday,

Wednesday, Thursday, Friday, Saturday};

enum week enum week {week_1, week_2, week_3,

week_4, week_5};

printf ("Monday at the third week of April is

April %d\n“, apr [week_3] [Monday] ); }

enum enum dept {CS=20, MATH=10, STAT=25};

enum colors {BLACK, GRAY=7, DK_GRAY, WHITE=15} c;

int i=GRAY; /*it’s 7 now*/

c=0; /*it’s BLACK now*/

c+=8; /*it’s DK_GRAY now*/

i= c+4 *2; /* it’s 16 now */

typedef struct {typedef struct {

enum {RANK, DEG} kind;enum {RANK, DEG} kind;

union statusunion status{

int rank ;

char deg[4]; }; }statusstatus;

Pointers Executable program consists of both code and data.

Each variable occupies one or more bytes of memory

The address of the first byte is said to be the address of the variable.

Pointer variable is a variable storing address.

int i=1;

int *p = &i;

Declaration:

type *name;type *name;

int i, j, *p;

…

1

…

i2000p

The Address and Indirection operators

&& operator: returns address of a variable

int i, *p;

p = &i;

int i, *p=&i;

p is aliasalias for i.

i=1; *p=2;

** operator : returns an object that a pointer points to.

printf(“%d\n”, *p);

int j=*&I;

Pointer Assignment int x ,y, *p, *q;

p = &x;

q= &y;

q=p;

*p = 1;

*q= 2;

int x=1, y=2, *p, *q;

p = &x;

q= &y;

*p = *q;

Pointers as Argument void decompose (float x, int **integer, float **fract){

**integer = (int) x;

**fract = x – **integer; }

void main(){

int i=5; float f= 3.4;

decompose (3.14, &&i, &&f); }

scanf(“%d”, &i);

*p= &i;

scanf(“%d”, p);

const to Protect Arguments void f(const int *p){

int j;

p=&j;

*p=1; *p=1; }

void f(int * const p){

int j;

p=&j;p=&j;

*p=1; }

void f(const int * const p){

int j;

p=&j;p=&j;

*p=1; *p=1; }

Pointers as Return Values

double *double *Largest (double **x, double **y){

return **x>**y? x : y;

}

int *p, x,y;

p =max(&x, &y);

You can return pointer to one element of the array passed as an argument, to static local variable, to external variable.

Never return a pointer to an automatic local variable!

Pointer Arithmetic

int a[5], *p=&a[0];

*p=a;

int *q;

p=&a[2];

q=p+2;

p++;

p

a

p

a

q

p

a

q

Pointer Arithmetic int a[5], *q, *p=&a[4];

q=p-3;

p-=4;

p

a

p

a

q

p

a

q

Pointer Arithmetic int a[5];

int *q=&a[1], *p=&a[4];

int i=p-q;

i=q-p;

p

a

q

Meaningful, if pointers point to the element of the same array.

p<=q int sum=0;

for (p=&a[0];p<&a[N]; p++)

sum+=*p;

Combining * and ++ (--)

int sum=0, *p=&a[0];

while (p<&a[N])

sum+=*p++;

Array Name as a Pointer int a[10], *p=a;

*a =7;

*(a+1)=12;

*(a+i) is the same as a[i]

p[i] is the same as *(p+i)

for (p=a; p<a+N; p++) sum+=*p;

Array parameter is treated as pointer, so it’s not protected against change.

The time required to pass array to a function doesn’t depend on its size.

int f(int a[]){…} is the same as int f(int *a){…}

int max = f(&a[5],10);

Pointer Example$ cat ptr_example.c#include <stdio.h>

int main() { int x=1; int *p; /* p points to an integer */

p = &x; /* Set p to x's address */ printf(" x is %d\n", x);

*p = 0; /* Set p's value to 0 */ printf(" x now is %d\n", x);

return 0;}

$ gcc ptr_example.c -o ptr_example$ ./ptr_example x is 1 x now is 0

Using pointers to achieve Call-By-Reference effect

Pass the address of a variable

Alter the value void swap (int *px, int *py) { int temp; temp = *px; *px = *py; *py = temp; }

int a=10,b=20; swap(&a,&b);

Main difference between arrays and pointers

Even though both contain addresses:

Array name is not a variable; so

pa = a; pa++ OK

a = pa; a++; NOT OK

When an array name is passed as a parameter to a function: Actually the address of the first element is passed

Arrays are always passed as pointers

Strings Double quotes

C treats string literals as character arrays, that is, a pointer of type char *.

For string literal of length n, it stores n characters of the string literal and null character (‘\0’) to mask the end of the string.

char ch = “abc”[1];

char digit_to_hex(int digit){

return “0123456789ABCDEF”[digit];

}

char *p=“abc”;

*p=‘b’; /* not recommended*/

String Variables char str[length+1];

char date[8]=“June 14”;

char date[9]=“June 14”; /*2 null characters at the end*/

char date[]=“June 14”;

char *dt=“June 14”;

Characters of array date can be modified, dt points to string literal which characters should not be modified.

date is an array name, dt is a pointer and can point to the other string literal.

char str[length+1], *p;

p=str;

Writing Strings

printf(“%s\n”,str);

printf(“%.4s\n”,”C is a fun language”);

printf(“%10s\n”,”Hi”);

printf(“%10.4s\n”,”C is a fun language”);

puts(str);

Reading Strings

scanf(“%s”, str); / *never contain white spaces*/

scanf(“%10s”, str);

gets(str);

Reads until finds newline character

It replaces newline character with null character before storing to a variable

Doesn’t skip leading white spaces

C String Library char str1[10],str2[10];

str1=“abc”; /*wrong*/

str2=str1; /*wrong*/

str1==str2 compares pointers!

<string.h><string.h>

char * strcpy(char *s1, const char *s2);char * strcpy(char *s1, const char *s2);

strcpy(str2, “abcd”);

strcpy(str1,strcpy(str2, “abcd”));

C String Library char * strcat(char *s1, const char *s2);char * strcat(char *s1, const char *s2);

strcat(str1, “abc”); strcat(str2, “def”);

strcat(str1, strcat(str2,”gi”));

int strcmp(const char *s1, const char *s2);int strcmp(const char *s1, const char *s2);

Lexicographic ordering

if (strcmp(str1,str2)<0) …

size_t strlen(const char *s);size_t strlen(const char *s);

int len = strlen(“abc”);

len = strlen(“”);

String Idiom

Search for the null character at the end of a string:

while(*s) s++; while(*s++);

size_t strlen(const char *s){

const char *p=s;

while (*s++);

return s-p;

}

Dynamic Storage Allocation It’s ability to allocate storage during program

execution

Available for all types of data

Mostly used for strings, arrays and structures.

Dynamically allocated structures can form lists, trees and other data structures.

<stdlib.h><stdlib.h>

mallocmalloc : allocates a block of memory, but doesn’t initialize it

calloccalloc: allocates a block of memory and clears it

freefree: releases the specified block of memory

NULL Pointer If memory allocation function can’t allocate a block of

the requested size, it returns a null pointer (NULL)null pointer (NULL).

<locale.h>,<stddef.h>,<stdio.h>,<stdlib.h>, <locale.h>,<stddef.h>,<stdio.h>,<stdlib.h>, <string.h>, <time.h><string.h>, <time.h>

It is responsibility of a programmer to test if It is responsibility of a programmer to test if received pointer is a null pointerreceived pointer is a null pointer.

p=malloc(1000);

if (p==NULL) { /* appropriate actions*/ }

if ((p=malloc(1000))==NULL) { /*actions*/ }

if (p) …. is the same as if (p!=NULL)

if (!p) …. is the same as if (p==NULL)

Using malloc: Strings void *malloc(size_t size);void *malloc(size_t size);

Allocates a block of sizesize bytes and returns a pointer to it; if fails, returns NULLNULL.

Since sizeof(char)sizeof(char) is1, to allocate space for a string of nn characters:

char *p=malloc(n+1);

Memory allocated using mallocmalloc isn’t cleared or initialized: strcpy(p, “abc”);strcpy(p, “abc”);

There are no checks no checks to detect usage of memory outside the bounds of the block

Dynamical allocation makes possible to write functions that return a pointer to a “new” string: char *p= char *p= concat(“abc”,”def”);concat(“abc”,”def”);

#include<string.h>

#include<stdlib.h>

#include<stdio.h>

char *concat(const char *s1, const char *s2 ){char *concat(const char *s1, const char *s2 ){

char *result=malloc(strlen(s1)+strlen(s2)+1);

if (result==NULL){

printf(“malloc failed\n”);

exit(EXIT_FAILURE);

}

strcpy(result,s1);

strcpy(result,s2);

return result;

}}

Dynamically Allocated Arrays Elements of an array can be longer than one byte => Elements of an array can be longer than one byte =>

sizeof should be used.sizeof should be used.

int *a=malloc(n * sizeof (int));

for(i=0;i<n;i++) a[i]=0;

void *calloc(size_t nmemb, size_t size);void *calloc(size_t nmemb, size_t size);

Allocates space for an array with nmembnmemb elements, each of which is sizesize bytes; sets all bits to 0 and returns a pointer to it.

If space is not available, returns NULL.

int *a=calloc(n, sizeof(int));

struct point {int x,y;} *p;

p=calloc(1, sizeof(struct point));

Deallocating Storage Memory block are obtained from a storage pool (heapheap).

GarbageGarbage is a block of memory that’s no longer accessible to a program.

A program that leaves garbage behind has a memory leakmemory leak.

int *p=malloc(100), *q=malloc(100);

p=q;

void free (void *ptr);void free (void *ptr);

int *p=malloc(100), *q=malloc(100);

free(p);

p=q;

Memory Leak: Examples int *int_ptr;

for (;;) {

int_ptr = malloc(100 * sizeof(int) );

if ( int_ptr == NULL ) {

fprintf(stderr, "...");

exit(1);

}

}

for ( i = 1; i <= 5; i++)

printf("%s\n", concat(“abc”, “012345”[digit]));

Memory Leak for ( i = 1; i <= 5; i++) {

char *str=concat(“abc”, “012345”[digit]);

printf("%s\n", str);

free(str);

}

for ( i = 1; i <= 5; i++) {

char *str=concat(“abc”, “012345”[digit]);

free(str);

printf("%s\n", str);

free(str); }

Dangling Pointers

char *p=malloc(4);

…

free(p);

…

strcpy(p,”abc”); /*wrong*/

Several pointers may point to the same block of memory. When the block is freed, all these pointers are left dangling.

Memory Allocation Rules

If memory is allocated it must be freed.If memory is allocated it must be freed.

Do not use memory after it is freed.Do not use memory after it is freed.

Do not free memory that will be used later.Do not free memory that will be used later. Do not free a block of memory more than Do not free a block of memory more than

once.once.

Do not free memory that was not allocated.Do not free memory that was not allocated.

Do not use memory outside the bounds of an Do not use memory outside the bounds of an allocated block.allocated block.

Linked Lists

A linked list linked list is a chain of structures (nodesnodes), with each node containing a pointer to the next node in the chain.

The last node in the list contains a null pointer.

Declaring a Node Type struct node{

int value;

struct node *next;

}

struct point {

double x,y;

};

struct vertex {

struct point element;

struct vertex *next;}

Building Linked List First, create an empty list

struct node *first=NULL;struct node *first=NULL;

Then create nodes one by one:

Allocate memory for the node

struct node *new_node;struct node *new_node;

new_node=malloc(sizeof (struct node));new_node=malloc(sizeof (struct node));

Store data into the node

(*new_node)..value=10;

new_node->->value=10; Insert the node into the list

10new_node

new_node

Inserting a Node at the Beginning of the List

If firstfirst points to the first node of the linked list:

new_node->next=first;new_node->next=first;

first=new_node;first=new_node;

struct node *first=NULL, *new_node;

new_nodefirst

new_node=malloc(sizeof (struct node));

new_nodefirst

Inserting a Node at the Beginning of the List

new_node->value=10;

first

new_node->next=first;

10new_node

first 10new_node

first=new_node;

first

10new_node

Inserting a Node at the Beginning of the List

new_node=malloc(sizeof (struct node));

new_nodefirst 10

new_node->value=20;

new_node->next=first;

20new_node

20new_node

first=new_node;

first 10

first 10

first

20

new_node

10

Searching a Linked List for (p=first; p!=NULL; p=p->next)

{… }

int value=20; struct node *p;

for (p=first; p!=NULL; p=p->next)

{ if (p->value== value) return p; }

struct node *find(struct node *list, int n){

while (list!=NULL and list->value!=n )

p=p->next;

return list; }

first

20

10

Inserting a Node at the Middle of the List

struct *node p=first;

struct node * cur= find(p, 10);

struct node *tmp =malloc(sizeof (struct node));

tmp->value= cur->value; tmp->next = cur->next_ptr;

first

20

10

cur

cur->value = 15;

cur->next = tmp;

10tmp

first 20

cur

15

first 20

cur

15

Deleting a Node from the List struct *node p=first;

struct node * cur= find(p, 20);

struct node *tmp;

tmp = cur->next; cur->value = tmp->value;

cur->next = tmp->next;

first

20

15

cur

tmp

10

first 15

cur

15 10

tmp

free(tmp);

first 15

cur

15 10

tmp

first 15 10

Review

C versus Java

C data types

Functions

Control Flow

Scope

Pointers, Arrays

Strings