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CSU480CSU480

“Hello World" Program

Data Types & Variables

printf() Arithmetic & Logical

Operations Conditionals Loops Arrays & Strings

Outline PointersPointers FunctionsFunctions Command-Line ArgumentCommand-Line Argument Data Structure Data Structure Memory AllocationMemory Allocation Programming TipsProgramming Tips C C vvs. C++s. C++ Books recommendedBooks recommended

The source code

#include <stdio.h> int main()

{ printf("Hello World\n"); return(0);

}

Hello World Program

Hello World Program How to compile?

$ gcc hello.c –o hello

gccgcc compiling commandcompiling command

hello.chello.c source filesource file

hellohello compiler-generated executable filecompiler-generated executable file

Note: the default output filename is “Note: the default output filename is “a.outa.out””

How to execute? ./hello

“./ ” indicates the following file “hello” resides under the current directory.

Hello World Program

Q: why “.” is not included in $PATH Q: why “.” is not included in $PATH environment variable?environment variable?

Hello World ProgramA: security consideration.

CommandCommand LocationLocation CommentComment

lsls /bin/ls/bin/ls provided by the provided by the systemsystem

lsls current directorycurrent directory virusvirus

NameName DescriptionDescription Size*Size* Range*Range*

charchar Character or small Character or small integerinteger

1 byte1 byte signed: -128 to 127signed: -128 to 127unsigned: 0 to 255 unsigned: 0 to 255

short intshort int(short)(short)

Short integerShort integer 2 bytes2 bytes signed: -32768 to 32767signed: -32768 to 32767unsigned: 0 to 65535 unsigned: 0 to 65535

intint IntegerInteger 4 bytes4 bytes signed: -2147483648 to signed: -2147483648 to 21474836472147483647unsigned: 0 to 4294967295 unsigned: 0 to 4294967295

long intlong int(long)(long)

Long integerLong integer 4 bytes4 bytes signed: -2147483648 to signed: -2147483648 to 21474836472147483647unsigned: 0 to 4294967295unsigned: 0 to 4294967295

floatfloat Floating point Floating point numbernumber

4 bytes4 bytes 3.4e +/- 38 (7 digits) 3.4e +/- 38 (7 digits)

doubledouble Double precision Double precision floating point numberfloating point number


long long doubledouble

Long double Long double precision floating precision floating point numberpoint number


Data types

Variable Declarationint length = 100; char num = ‘9’; //The actual value is 57float deposit = 240.5;unsigned short ID = 0x5544;

Try the following statements, and see what happens

unsigned char value = -1;printf(“The value is %d \n”, value);

unsigned char value = 300;printf(“The value is %d \n”, value);

ResultDefinitionDefinition Memory layoutMemory layout DisplayDisplay commentcomment

unsigned char unsigned char value = -1value = -1

1111111111111111 255255

unsigned char unsigned char value = 300value = 300

0010110000101100 4444 overflowoverflow

Local variableLocal variables are declared within the body of a function, and can only be used within that function.

Static variableAnother class of local variable is the static type. It is specified by the keyword static in the variable declaration. The most striking difference from a non-static local variable is, a static variable is not destroyed on exit from the function.

Global variable A global variable declaration looks normal, but is located outside any of the program's functions. So it is accessible to all functions.

Variable types

An example

int global = 10; //global variable

int func (int x){static int stat_var; //static local variableint temp; //(normal) local variableint name[50]; //(normal) local variable……}

Variable Definition vs Declaration

DefinitionDefinition Tell the compiler about the variable: its type Tell the compiler about the variable: its type and name, as well as allocated a memory cell for and name, as well as allocated a memory cell for the variablethe variable

DeclarationDeclaration DDescribe information ``about'' the variableescribe information ``about'' the variable, , doesn’tdoesn’t allocate memory cell for the variable allocate memory cell for the variable

http://www-ee.eng.hawaii.edu/~tep/EE150/book/chap14/subsection2.1.1.4.html

The printf() function can be instructed to

print integers, floats and string properly. The general syntax is

printf( “format”, variables);

An exampleint stud_id = 5200;char * name = “Mike”;printf(“%s ‘s ID is %d \n”, name, stud_id);

printf()

Format Identifiers%d decimal integers%x hex integer%c character%f float and double number%s string%p pointer

How to specify display space for a variable?printf(“The student id is %5d \n”, stud_id); The value of stud_id will occupy 5 characters space in the print-out.

Why “\n”It introduces a new line on the terminal screen.

\a\a alert (bell) character alert (bell) character \\\\ backslash backslash \b\b backspace backspace \?\? question mark question mark \f\f formfeedformfeed \’\’ single quote single quote \n\n newlinenewline \”\” double quote double quote \r\r carriage returncarriage return \000\000 octal number octal number \t\t horizontal tab horizontal tab \xhh\xhh hexadecimal number hexadecimal number \v\v vertical tab vertical tab

escape sequence

Arithmetic Operations

Arithmetic Assignment Operators

Increment and Decrement Operators

awkwardawkward easyeasy easiesteasiest

x = x+1;x = x+1; x += 1x += 1 x++x++

x = x-1;x = x-1; x -= 1x -= 1 x--x--

Arithmetic operatorsint i = 10;int j = 15;int add = i + j; //25int diff = j – i; //5int product = i * j; // 150int quotient = j / i; // 1int residual = j % i; // 5i++; //Increase by 1i--; //Decrease by 1

Example

Comparing themint i = 10;int j = 15;float k = 15.0;

j / i = ?j % i = ?k / i = ?k % i = ?

The Answerj / i = 1; j % i = 5;k / i = 1.5;k % i It is illegal.

Note: For %, the operands can only be integers.

What is “true” and “false” in C

In C, there is no specific data type to represent “true” and “false”. C uses value “0” to represent “false”, and uses non-zero value to stand for “true”.

Logical OperatorsA && B=> A and BA || B => A or BA == B => Is A equal to B?A != B => Is A not equal to B?

Logical Operations

A > B => Is A greater than B?A >= B => Is A greater than or equal to B?

A < B => Is A less than B?A <= B => Is A less than or equal to B?

Don’t be confused&& and || have different meanings from & and |.& and | are bitwise operators.

Short circuiting means that we don't evaluate

the second part of an AND or OR unless we really need to.

Some practicesPlease compute the value of the following logical expressions?

Short circuiting

int i = 10; int j = 15; int k = 15; int m = 0; if( i < j && j < k) =>if( i != j || k < j) =>if( j<= k || i > k) =>if( j == k && m) =>if(i) =>if(m || j && i ) =>

int i = 10; int j = 15; int k = 15; int m = 0; if( i < j && j < k) => falseif( i != j || k < j) => trueif( j<= k || i > k) => true if( j == k && m) => falseif(i) => trueif(m || j && i ) => true

Did you get the correct answers?

if statement

Three basic formats,if (expression){ statement …}if (expression) {statement …

}else{ statement …

}

Conditionals

if (expression) {

statement…} else if (expression) {

statement… } else{

statement… }

An exampleif(score >= 90){

a_cnt ++;}else if(score >= 80){

b_cnt++;}else if(score >= 70){

c_cnt++;}else if (score>= 60){

d_cnt++}else{

f_cnt++}

The switch statementswitch (expression) {

case item1: statement; break;

case item2: statement; break;

default: statement; break;

}

for statement

for (expression1; expression2; expression3){statement…

} expression1 initializes; expression2 is the terminate test; expression3 is the modifier;

Loops

An exampleint x;for (x=0; x<3; x++) {

printf("x=%d\n",x); }

First time: x = 0;Second time: x = 1;Third time: x = 2;Fourth time: x = 3; (don’t execute the body)

The while statementwhile (expression) {statement …}while loop exits only when the expression is false.

An exampleint x = 3; while (x>0) { printf("x=%d n",x); x--; }

for (expression1; expression2; expression3){

statement…}

for <==> whileexpression1;expression1;while (expression2)while (expression2){{statementstatement……;;expression3;expression3;}}

equals

Arrays

int ids[50];char name[100];int table_of_num[30][40];

Accessing an arrayids[0] = 40;i = ids[1] + j;table_of_num[3][4] = 100;Note: In C Array subscripts start at 0 and end one less than the array size. [0 .. n-1]

Arrays & Strings

StringsStrings are defined as arrays of characters.The only difference from a character array is, a symbol “\0” is used to indicate the end of a string.

For example, suppose we have a character array, char name[8], and we store into it a string “Dave”.Note: the length of this string 4, but it occupies 5 bytes.DD aa vv ee \0\0

Functions are easy to use; they allow complicated programs

to be broken into small blocks, each of which is easier to write, read, and maintain. This is called modulation.

How does a function look like?returntype function_name(parameters…) { local variables declaration; function code;return result; }

Functions

Sample function int addition(int x, int y)

{int add;add = x + y;return add;

} How to call a function?

int result;int i = 5, j = 6; result = addition(i, j);

Pointer is the most beautiful (ugliest) part of C, but

also brings most trouble to C programmers. Over 90% bugs in the C programs come from pointers.

“The International Obfuscated C Code Contest ”(http://www.ioccc.org/)

What is a pointer?A pointer is a variable which contains the address

in memory of another variable.

In C we have a specific type for pointers.

Pointers

Declaring a pointer variableint * pointer;

char * name; How to obtain the address of a variable?

int x = 0x2233;pointer = &x; where & is called address of operator.

How to get the value of the variable indicated by the pointer?int y = *pointer;

3333 2222 0000 0000

0x5200 0x5203

0x5200

pointer

What happens in the memory?

Suppose the address of variable x is 0x5200 in the above example, so the value of the variable pointer is 0x5200.

X

swap the value of two variables

Why is the left one not working?

swap

main

x, y

a, b

call swap(a, b) in main

x, y, a, b are all local variables

Why is the right one working?

Pointers and ArraysPointers and arrays are very closely linked in C. Array elements arranged in consecutive memory locations

Accessing array elements using pointersint ids[50];int * p = &ids[0];p[i] <=> ids[i]

Pointers and StringsA string can be represented by a char * pointer.

Char name[50];name[0] = ‘D’;name[1] = ‘a’;name[2] = ‘v’;name[3] = ‘e’;name[4] = ‘\0’;char * p = &name[0];printf(“The name is %s \n”, p);Note: The p represents the string “Dave”, but not

the arrayname[50].

In C you can pass arguments to main() function. main() prototype

int main(int argc, char * argv[]);argc indicates the number of argumentsargv is an array of input string pointers.

How to pass your own arguments? ./hello 10

Command-Line Argument

What value is argc and argv?Let’s add two printf statement to get the value of argc and argv.#include <stdio.h>

int main(int argc, char * argv[]);) { int i=0;printf("Hello World\n"); printf(“The argc is %d \n”, argc);for(i=0; i < argc; i++){ printf(“The %dth element in argv is %s\n”, i, argv[i]); }return(0); }

The output The argc is 2 The 0th element in argv is ./hello The 1th element in argv is 10 The trick is the system always passes the name of the executable file as the first argument to the main() function.

How to use your argument? Be careful. Your arguments to main() are always in string

format. Taking the above program for example, the argv[1] is string “10”, not a number. You must convert it into a number before you can use it.

A data structure is a collection of one or more variables,

possibly of different types.

An example of student recordstruct stud_record{char name[50];int id;int age;int major;……};

Data Structure

A data structure is also a data typestruct stud_record my_record;struct stud_record * pointer;pointer = & my_record;

Accessing a field inside a data structuremy_record.id = 10; “.”

or pointer->id = 10; “->”

Stack memory allocation

Non-static local variable is an example of stack memory allocation. Such memory allocations are placed in a system memory area called the stack.

Static memory allocationStatic local variable and global variable require static memory allocation. Static memory allocation happens before the program starts, and persists through the entire life time of the program.

Memory Allocation

Dynamic memory allocationIt allows the program determine how much memory it needs at run time, and allocate exactly the right amount of storage.

The region of memory where dynamic allocation and deallocation of memory can take place is called the heap.

Note: the program has the responsibility to free the dynamic memory it allocated.

Memory arrangement

Functions for the dynamic memory allocationvoid *malloc(size_t number_of_bytes);

allocates dynamic memorysize_t sizeof(type);

returns the number of bytes of typevoid free(void * p)

releases dynamic memory allocation

An example of dynamic memory allocationint * ids; //id arraysint num_of_ids = 40;ids = malloc( sizeof(int) * num_of_ids);

…….. Processing …...free(ids);

Allocating a data structure instancestruct stud_record * pointer;pointer = malloc(sizeof(struct stud_record));pointer->id = 10;

Never calculate the size of data structure yourself. The reason is the size of data types is machine-dependent. Give it to sizeof() function.

size of intsize of int

32-bytes machines32-bytes machines 3232

64-bytes machines64-bytes machines 6464

Replacing numbers in your code with macros

- don’t use magic numbers directly#define MAX_NAME_LEN 50;char name[MAX_NAME_LEN];

Avoiding global variables - modulation is more important

Giving variables and functions a nice name- a meaning name

Don’t repeat your code - make a subroutine/function

Don’t let the function body to exceed one screen- hard to debug

Programming Tips

Indenting your code (clearance)if(expression){

if(expression){

……}

} Commenting your code Don’t rush into coding. Plan first. Printing out more debugging information Using debugger (gdb)

C++ is a superset of C C++ has all the characteristics of C Using g++ to compile your source code

C vs. C++

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