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Computer Science: A Structured Programming Approach Using C 1

Functions

All c programs have at least one functionthat is the main.

When the program grows into larger, mayhave many disadvantages.

1. Difficult to write a larger programs

2. Difficult to identify the logical errors and

correct.3. Difficult to read and understand.

4. Larger programs prone for more errors.


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A program is divided into a main module and its related

modules.

Each module is in turn divided into sub modules until the

resulting modules are smaller; that is, until they are not

further division.

The process of subdividing a problem into manageable partscalled top-down design.


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FIGURE 4-2 Structure Chart


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A large program divided into series ofindividual related programs called

modules. These modules are calledfunctions.

Functions are classified into two categories

1. Library functions ( built-in functions).

2. User-defined functions


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Library (built in) functions

C is a library it is a collection of various types of functionswhich perform some standard and predefined tasks.

This are part of the c compiler that have been written forgeneral purpose are called library functions.

exampleprintf()scanf()sqrt()

Advantage1.No need to write the code because the functions are

already available so it makes the programmers job mucheasier.

2. these functions are used anywhere in the program.


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User defined functions

The functions are written by theprogrammer to achieve some work


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Advantages of function

1. Readability of the program can beincreased. Problem can be easilyunderstandable.

2. Reuse- the function written in oneprogram can also used in other program.

(reduce the size).

1. Makes the program easy for testing,coding and debugging.

2. Saves time as well as space.


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FIGURE 4-3 Structure Chart for a C Program


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4-3 User-Defined Functions

LikeLike everyevery otherother variablevariable inin C,C, functionsfunctions mustmust bebe bothboth

declareddeclared andand defineddefined.. TheThe functionfunction declarationdeclaration givesgives

thethe returnreturn typetype andand functionfunction namename needsneeds toto bebe defineddefined

laterlater.. TheThe functionfunction definitiondefinition containscontains thethe codecode forfor aafunctionfunction..

Function Definition

Function Declaration

The Function Call

Topics discussed in this section:Topics discussed in this section:


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Definition

A function is a self- contained block of codethat performs a particular task.

It takes some data from the main programand may or may not returns a value.


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A function name is used three times: for declaration, in acall, and for definition.

NoteNote


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Function declaration

Funtion prototype(syntax)

Return_type function_name (argument list);


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Definition

Syntax for the function definitionReturntype function_name(argument list/parameters list)argument list declaration{local variable declarations:Statements;return(expression);}//function.

All the parts are not essential.

Argument list and declaration optional.Return statement is also optional.


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main(){-----------

----------

func1();

---------

--------

funct2();--------

---------

funct3();---------

-------

}//main

func1(){----------

-----------

}func1func2(){--------

--------

}//func2fucn3(){----------

---------

}//func3


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main()\\calling function

{

func1();

printf(we are in main\n);}//main

func1()\\ called funciton

{printf(we are in func1\n);

}//func1


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Points to remember

1. C program must have at least one function

2. If program have one function it must be main

3. If program have more than function, in which one mustbe main because program execution starts from thatonly.

4. We can use unlimited no of functions in one program.

5. Each function is called in the sequence we havespecified in the main function.

6. After each function task is over the control passed to thecalling function(main()).

7


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main(){printf(\n I am calling other functions);func1();

func2();func3();Printf(all functions are called\n);}//mainfunc1(){printf(in func1\n);}//func1

func2(){printf(in func2\n);}//func2func3(){printf(in func1\n);}//func3

Output:

I am calling other functionin func1in func2in func3all functions are called.


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7 One function can also call the other function which has alreadybeen called.main()

{printf(I am in main\n); outputfunc1(); I am in mainprintf(I back in main\n); in func1}//mainfunc1() In func2{ In func3printf(in func1\n); I back in func2func2(); I back in func1printf(I back in func1\n); I back in main}//func1func2(){printf(In func2\n);func3();printf(I back in func2\n);}//func2func3(){printf(in fac3\n);}//func3


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FIGURE 4-5 Declaring, Calling, and Defining Functions


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PROGRAM 4-2 voidFunction with a Parameter


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FIGURE 4-7 Non-void Function without Parameters


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FIGURE 4-8 Calling a Function That Returns a Value


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PROGRAM 4-3 Read a Number and Square It


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FIGURE 4-9 Function Definition


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Categories of functions

Based on the functions return type and itsarguments the functions are categorized into 4different types.

1. Functions with no arguments and no returnvalues

2.Functions with no arguments and return values

3. Functions with arguments and no return values4. Functions with arguments and return values


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2. FUNCTIONS WITH NO ARGUMENTS AND RETURN VALUES

There are two ways that a function terminatesexecution and returns to the caller.

1. When the last statement in the function has

executed and conceptually the functions ending} is encountered.

2. Whenever it faces return statement.

there is no data transfer from the calling functionto the called function. But, there is data transferfrom called function to the calling function.


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example// C program to find sum of two numbers using functions with no arguments and return values

#include int sum ();

void main () { int c; clrscr (); c=sum (); /*calling function */ printf (\n The sum = %d, c); getch (); } int sum () { int x, y;

printf (\n Enter the values of x and y: ); scanf (%d%d, &x, &y); return (x+y); }


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3. FUNCTIONS WITH ARGUMENTS AND NO RETURN VALUES

In this category there is data transfer from the calling function to thecalled function using parameters. But, there is no data transfer fromcalled function to the calling function.

Function arguments The arguments that are supplied in to two categories 1. actual arguments/parameters 2. formal arguments/parameters

Actual arguments/parameters Actual parameters are the expressions in the calling functions.

These are the parameters present in the calling statement (functioncall).

formal arguments/parameters formal parameters are the variables that are declared in the header

of the function definition. These list defines and declares that willcontain the data received by the function. These are the valueparameters, copies of the values being passed are stored in thecalled functions memory area.

Note:Actual and Formal parameters must match exactly in type,order, and number. Their names however, do not need to match.


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->the values of actual arguments are assigned to the formal arguments.func1(x1, x2, x3,xn);

| | | |

func1(y1, y2, y3,yn)

->In case, the actual arguments are more than the formal arguments , extraactual arguments are discarded.

-> on other hand, the formal arguments are more, the extra formal argumentsare initialized to some garbage values.

-> if any mismatch found in their data type may also takes some value. No

error is reported.


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example//C program to find sum of two numbers using functions with arguments and no return values

#include void sum (int ,int ); void main () { int a, b; clrscr (); printf (\n Enter the values of a and b: );

scanf (%d%d, &a, &b); sum (a, b); /*calling function */ getch (); } void sum (int x, int y) { int z; z=x+y; printf (\n The Sum =%d, z); }


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4. FUNCTIONS WITH ARGUMENTS AND RETURN

VALUES

In this category there is data transferbetween the calling function and calledfunction.


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example

//C program to find sum of two numbers using functions with arguments and return values

#include

int sum (int ,int ); void main () { int a, b; clrscr (); printf (\n Enter the values of a and b: ); scanf (%d%d, &a, &b); c=sum (a, b); /*calling function */ printf (\n The Sum =%d, c); getch (); }

int sum (int x, int y) { int z; return x+y; }


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Note: generally we are using functions with argumentsand return values (category 4) in our applications. Whybecause the job of a function is to perform a well-

defined task, it carries inputs and sends result afterexecuted. In real world the programming teams codesthe modules (functions) according to the input

(arguments) given by the team coordinators.


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Storage classes in c

The variables are stored in computer.

- Basically variables are stored in computer in two locations.

- 1. memory 2. registers.

- Location of the variable to store is determined by the storage

classes.- It also tells us the following

1. where the variable would be stored.

2. what is initial value of the variable, if variable is not initializedwhile declaring.

3. what is the scope of the variable; i.e in which functions thevalue of the variable would be available.

4. what is the life of a variable; i.e how long variable would beexist.


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Auto (default and most common)

-Memory for automatic variables is allocated when a block or function isentered.

- They are defined and are local to the block.

- When the block is exited, the system releases the memory that was

allocated to the auto variables, and their values are lost.

- For Example, a variable in the body of a loop is created and destroyed ineach iteration.

declaration- auto type variable_name;

int a;

By default a is treated as auto.


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Features of auto variable when they declared

Storage Memory

Default initial value garbage value

Scope local to the block

in which variabledefined

Life remains within theblock in whichvariable defined.


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Example- auto

main(){auto int x=10;{auto int x=20;

{auto int x=30;printf(%d,x);// 30

}printf(%d,x);//20

}Printf(%d,x);//10}//main


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Static storage classes

syntax-

static type identifier;

A variable with a static extent is created andinitialized to its value when the function

executed for first time and destroyed when it is

exit from the function. This is true no matter

how many times the declaration is encountered

during the execution.


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Features of static variable

- Storage -------- Memory

- Default initial value ----- zero

- Scope ------------ Local to the block inwhich the variable is defined.

- Life --------- value of the variable persistsbetween different function calls.

- Once memory is allocated to the staticvariable is persists its value until he end ofthe program


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exampleint i;main()

{

printf(\n%d=,i);

demo1();

demo1();

demo2();

demo2();

}//main

demo1()

{i++;

printf(\n%d,i);

}//demo1()

demo2()

{

i--;printf(\n%d,i);

}//demo2()

Output

0

12

1

0


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storage classes 4

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