2.io functions

8/11/2019 2.IO Functions

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Hello World

include

/* The simplest C Program */

int main(int argc, char **argv)

{

printf

Hello World

\

n);

return 0;

}

The main() function is always

where your program starts

running.

#include inserts another file. .h files are called header

files. They contain stuff needed to interface to libraries and

code in other .c files.

This is a comment. The compiler ignores this.

Blocks of code (lexical scopes)

are marked by -

Print out a message. \n means new line.Return 0 from this function

What do the < >

mean?

Can your program have

more than one .c file?


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Compilerinclude

/* The simplest C Program */


{

printfHello World\n);

return 0;

}

my_program

__extension__ typedef unsigned long long int __dev_t;

__extension__ typedef unsigned int __uid_t;

__extension__ typedef unsigned int __gid_t;

__extension__ typedef unsigned long int __ino_t;

__extension__ typedef unsigned long long int __ino64_t;

__extension__ typedef unsigned int __nlink_t;

__extension__ typedef long int __off_t;

__extension__ typedef long long int __off64_t;

extern void flockfile (FILE *__stream) ;

extern int ftrylockfile (FILE *__stream) ;

extern void funlockfile (FILE *__stream) ;


{

printf Hello World\n);

return 0;

}

Compilation occurs in two steps:Preprocessing and Compiling

In Preprocessing, source code is expanded into a

larger form that is simpler for the compiler to

understand. Any line that starts with # is a line that isinterpreted by the Preprocessor.

Include files are pasted in (#include)

Macros are expanded (#define)

Comments are stripped out ( /* */ , // )

Continued lines are joined ( \ )

Preprocess

Compile

The compiler then converts the resulting text into

binary code the CPU can run directly.

\ ?

Why ?


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Lexical ScopingEvery Variableis Definedwithin some scope. A

Variable cannot be referenced by name (a.k.a.Symbol) from outside of that scope.

The scope of Function Arguments is thecomplete body of the function.

void p(char x)

{

/* p,x*/

char y;

/* p,x,y*/

char z;

/* p,x,y,z*/

}

/* p*/

char z;

/* p,z*/

void q(char a)

{

char b;

/* p,z,q,a,b*/

{

char c;

/* p,z,q,a,b,c*/

}

char d;

/* p,z,q,a,b,d(not c)*/

}

/* p,z,q*/

(Returns nothing)

The scope of Variables defined inside a

function starts at the definition and ends at the

closing brace of the containing block

Lexical scopes are defined with curly braces { }.

The scope of Variables defined outside a

function starts at the definition and ends at the

end of the file. Called Global Vars.

legal?

char b?


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User Defined Data Declarationtypedef is used to define new data type names to make a program more readable to

the programmer.

typedef type identifier

Example: typedef int unites units batch1, batch2;typedef float marks marksname1[10];

#define preprocessor

The #define preprocessor allows us to define symbolic names and constants.

Example: #define PI3.14159 Ittranslate every occurrence of PI to 3.14159.


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Macros

Macros are built on the #define preprocessor.

#define SQUARE(x) x*x

Macros define an expression.

After preprocessing the code would become:

enumIt allows you to define a list of aliases which represent integer numbers.

enumis closely related to the #define. E.g., if you find yourself coding something like:

#define MON 1

#define TUE 2#define WED 3

You could use enumas:

enumweek{ Mon=1, Tue, Wed, Thu, Fri Sat, Sun} days;


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An advantage of enumover #define is that it has scope. This means that the variable (just like

any other) is only visible within the block it was declared within.

If a variable is defined with enum , it is considered by the compiler to be an integer, and can

have ANY integer value assigned to it, it is not restricted to the values in the enum.


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The const keyword

The constkeyword is used to create a read only variable.

Once initialized, the value of the variable cannot be changed but can be used just like any

other variable

The constkeyword is used as a qualifier to the following data types

Int , float, char , double


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C Storage ClassesC has a concept of 'Storage classes' which are used to define the scope (visibility) and life

time of variables and/or functions.

auto- storage class

auto is the default storage class for local variables.{

intCount;

autointMonth;

}

It defines two variables with the same storage class. auto

can only be used within functions, i.e. local variables

register- Storage Classregisteris used to define local variables that should be stored in a register instead of RAM.

This means that the variable has a maximum size equal to the register size (usually one

word) and can not have the unary '&' operator applied to it (as it does not have a memory

location).

{registerint Miles;

}

Register should only be used for variables that require quickaccess - such as counters.

Defining registerdoes not mean that the variable will be

stored in a register.

It means that it MIGHT be stored in a register - depending on

hardware and implementation restrictions.


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static - Storage Classstaticis the default storage class for global variables.

The two variables below (count and road) both have a static storage class.

staticintCount;

intRoad;

main()

{

printf("%d\n", Count);

printf("%d\n", Road);

}

staticcan also be defined within a function.

If this is done, the variable is initialized at compilation time and retains its value between calls.

Because it is initialized at compilation time, the initialization value must be a constant.

void Func(void){

static Count=1;

}


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void func1(void);

static count=10; /* Global variable - static is the default */

main()

{

while (count--) func1();

}

void func1(void)

{

/* 'thingy' is local to 'func1' - it is only initialized at run time. Its value is NOT reset on

every invocation of 'func1 /

static thingy=5;

thingy++;

printf(" thingy is %d and count is %d\n", thingy, count);

}

/**************************************************************************

Program )/P looks like this:

thingy is 6 and count is 9


thingy is 8 and count is 7thingy is 9 and count is 6







**************************************************************************


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extern - Storage Class

externdefines a global variable that is visible to ALL object modules.

When you use externthe variable cannot be initialized, as all it does is point the variable

name at a storage location that has been previously defined

Source 1 Source 2

externint count; int count=5;

write() main()

{ {

printf("count is %d\n", count); write();

} }

Count in 'source 1' will have a value of 5. If source 1 changes the value of count - source 2

will see the new value.

void write_extern(void);

extern int count;

void write_extern(void)

{

printf("count is %i\n", count);

}

int count=5;

main()

{write_extern();

}

The compile command will look something like.

gcc source1.c source2.c -o program

staticand externvariables are automatically

initialized to zero.

autovariables contain undefined values

(garbage) unless initialized explicitly.


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Input/Output Operations

Input operation

an instruction that copies data from an input

device into memory

Output operation

an instruction that displays information stored in

memory to the output devices (such as themonitor screen)


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Input/Output Functions

A C function that performs an input or output

operation

A few functions that are pre-defined in the

header file stdio.h such as :

printf()

scanf()

getchar() & putchar()


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The printf function

Used to send data to the standard output (usuallythe monitor) to be printed according to specificformat.

General format: printf(string literal);

A sequence of any number of characters surrounded bydouble quotation marks.

printf(format string, variables); Format string is a combination of text, conversion

specifier and escape sequence.


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The printf function cont

Example:

printf(Thank you);

printf (Total sum is: %d\n, sum);

%d is a placeholder (conversion specifier)

marks the display position for a type integer variable

\n is an escape sequence

moves the cursor to the new line


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Escape Sequence

Escape Sequence Effect

\a Beep sound

\b Backspace

\f Formfeed (for printing)

\n New line\r Carriage return

\t Tab

\v Vertical tab

\\ Backslash

\ sign\o Octal decimal

\x Hexadecimal

\O NULL


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The scanf function

Read data from the standard input device(usually keyboard) and store it in a variable.

General format: scanf(Format string, &variable);

Notice ampersand (&) operator :

C address of operator

it passes the address of the variable instead of thevariable itself tells the scanf() where to find the variable to store

the new value


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The scanf function cont

Example :int age;

printf(Enter your age: );

scanf(%d, &age);

Common Conversion Identifier used in printf andscanf functions.

printf scanf

int %d %d

float %f %f

double %f %lf

char %c %c

string %s %s


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getchar() and putchar()

getchar() - read a character from standard

input

putchar() - write a character to standard

output

Example:

#include

void main(void)

{

char my_char;

printf(Please type a character: );

my_char = getchar();printf(\nYou have typed this character: );

putchar(my_char);

}


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getchar() and putchar() cont

Alternatively, you can write the previous code

using normal scanf and %c placeholder.

Example#include

void main(void)

{

char my_char;

printf(Please type a character: );

scanf(%c,&my_char);printf(\nYou have typed this character: %c , my_char);

}


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Few notes on C program cont

Punctuators (separators)

Symbols used to separate different parts of the C

program.

These punctuators include:

* + ( ) - , ; : #

Usage example:void main (void)

{

int num = 10;

printf (%d,num);

}


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Few notes on C program cont

Operators

Tokens that result in some kind of computation or

action when applied to variables or other

elements in an expression.

Example of operators:

* + = - /

Usage example: result = total1 + total2;


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Basic C Operators

Arithmetic operators

Unary operators

Binary operators

Assignment operators Equalities and relational operators

Logical operators

Conditional operator


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Arithmetic Operators I

In C, we have the following operators (notethat all these example are using 9as the value

of its first operand)


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Equality and Relational Operators

Equality Operators:Operator Example Meaning

== x == y x is equal to y

!= x != y x is not equal to y

Relational Operators:Operator Example Meaning

> x > y x is greater than y

< x < y x is less than y>= x >= y x is greater than or equal to y


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Selection Structure

In selection structure, the program is executed basedupon the given condition.

Only instructions that satisfy the given condition areexecuted.

There are 3 types of selection structure:

if

One alternative

ifelse

Two alternatives

nested if..else Multiple alternatives

switch

Multiple alternatives


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Selection structure: if

Syntax :if (condition)

Statement;

The statement is only executed if the condition is satisfied.

Example:

if (score >= 60)

printf(Pass!!\n);

In the example above, the word Pass!! will only be printed out ifscoreis larger than or equal to 60. If not, the word Pass!! will notbe printed out and the program will continue with the nextstatement.

A condition is an expression thatcan return trueor false(usually

involving the use of an operator).

Note that there is no semicolon (;) afterthe ifstatement. If there is one, that means

the ifstatement and the printf()statement

are 2 different statements and they will

both get executed sequentially.


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Nested if elsestatements

A nested ifelsestatement is an ifelsestatement with anotherifelsestatements inside it.

Example :if (score >= 90)

printf(A\n);

else if (score >= 80)printf(B\n);

else if (score >= 70)

printf(C\n);

else if (score >= 60)

printf(D\n)

elseprintf(F\n);

The else ifstatement means that if the above condition is not

satisfied, then try checking this condition.If any one of the condition is

already satisfied, then ignore the rest of the available conditions


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Plurality of Statements In the examples that we have seen so far, there is only one statement

to be executed after the ifstatement.

If we want to execute more than one statement after the condition issatisfied, we have to put curly braces { }around those statements totell the compiler that they are a part of the ifstatement, making it aCompound Statement

Example

if (score >= 90){

printf(You have done very well\n);printf(Ill give you a present\n);

}else if (score >= 60){

printf(You have passed the course\n);printf(Sorry No present from for you\n);printf(Go and celebrate on your own\n);

}

h l l


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Repetition : whileloop

Syntax :while (condition)

statement;

As long as the condition is met (the condition expression

returns true), the statement inside the whileloop will always

get executed.

When the condition is no longer met (the conditionexpression returns false), the program will continue on with

the next instruction (the one after the whileloop).

Example:

int total = 0;while (total < 5)

{

printf(Total = %d\n, total);

total++;

}

Similar as in the ifstatement, the condition

is an expression that can return trueor

false.


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Repetition : whileloop cont

In this example :

(total < 5)is known as loop repetition condition

(counter-controlled)

totalis the loop counter variable

In this case, this loop will keep on looping until the

counter variable is = 4. Once total = 5, the loop

will terminate


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Repetition : whileloop cont

Theprintf()statement will get executed as long as

the variable totalis less than 5. Since the variable

total is incremented each time the loop is

executed, the loop will stop after the 5th output.

Output:

Total = 0

Total = 1

Total = 2

Total = 3

Total = 4


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Infinite loop

If somehow the program never goes out of theloop, the program is said to be stuck in aninfinite loop.

The infinite loop error happens because thecondition expression of the while loop alwaysreturn a true.

If an infinite loop occurs, the program wouldnever terminate and the user would have toterminate the program by force.


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Repetition : do whileloop

Syntax

do {

statement;

}while(condition);

A dowhileloop is pretty much the same as the whileloop exceptthat the condition is checked after the first execution of thestatement has been made.

When there is a dowhileloop, the statement(s) inside it will beexecuted once no matter what. Only after that the condition will bechecked to decide whether the loop should be executed again orjust continue with the rest of the program.


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do whileloop cont

Let us consider the following program:

int total = 10;

while (total < 10)

{printf(Total = %d\n, total);total++;

}printf(Bye..);

What does this program do?

The program will only print the word Bye... The statementsinside the while loop will never be executed since the conditionis already not satisfied when it is time for the whileloop to get

executed.


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do whileloop cont

Now consider the following program:int total = 10;

do {printf(Total = %d\n, total);total++;

} while (total < 10)

printf(Bye..);

Compared to the previous one, what will the output be?

The program will get an output:

Total = 10

Bye..

because the condition is not checked at the beginning of the loop.

Therefore the statements inside the loop get executed once.


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Repetition :forloop

Syntax :

for (expression1; expression2; expression3)

statement;

Expression1: initialize the controlling variable Expression2: the loop condition

Expression3: changes that would be done to the

controlling variable at the end of each loop.

Note that each expression is separated by a

semicolon (;)


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forloop - example

Example:int total;

for (total = 0; total < 5; total++)printf(Total = %d\n, total);

Output:Total = 0

Total = 1

Total = 2Total = 3

Total = 4


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forloop cont

Notice that the output is the same as the one for the while

loop example. In fact, the two examples are exactly

equivalent. Using aforloop is just another way of writing a

whileloop that uses a controlling variable.

It is also possibleto omitone or more of theforloop

expressions. In such a case, we just put the semicolon without

the expression.

int total= 0;for (; total < 5; total++)

printf(Total = %d\n, total);


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continueand breakstatement

Both of these statements are used to modify the program flowwhen a selection structureor a repetition structureis used.

The breakstatement is used to break out of selection or repetitionstructure. For example:

for (a = 0; a < 5; a++){

if (a == 2) break;

printf(a = %d\n, a);

}

The output of this example would be:a = 0a = 1


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When a= 2, the program will break out of theforloop

due to the breakstatement. This will effectively

terminate the loop. It will not wait till the value of a

reaches 5 before terminating the loop.

The continuestatement is used to ignore the rest of the

statements in the loop and continue with the next loop.


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Example:

for (a = 0; a < 5; a++)

{

if (a == 2) continue;

printf(a = %d\n, a);}

Output:

a = 0

a = 1

a = 3

a = 4


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a = 2 is not printed out because the loop skips theprintf()

function when the continuestatement is encountered.

In a whileand dowhilestructure, the loop condition will

be checked as soon as the continuestatement isencountered to determine whether the loop will be

continued .

In aforloop, any modification to the controlling variable

will be done before the condition is checked.

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