shivani varshney/rcet/programming with c
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Functions
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Introduction
Divide and conquer Construct a program from smaller pieces or components
These smaller pieces are called modules Each piece more manageable than the original program
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Program Modules in C
Functions Modules in C Programs combine user-defined functions with library
functionsC standard library has a wide variety of functions
Function calls Invoking functions
Provide function name and arguments (data)Function performs operations or manipulations
Function returns results Function call analogy:
Boss asks worker to complete taskWorker gets information, does task, returns result Information hiding: boss does not know details
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Math Library Functions
Math library functions perform common mathematical calculations
#include
Format for calling functions FunctionName(argument); If multiple arguments, use comma-separated list
printf( "%.2f", sqrt( 900.0 ) );
Calls function sqrt, which returns the square root of its argumentAll math functions return data type double
Arguments may be constants, variables, or expressions
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Functions
Functions Modularize a program All variables declared inside functions are local variables
Known only in function defined ParametersCommunicate information between functionsLocal variables
Benefits of functions Divide and conquerManageable program development
Software reusabilityUse existing functions as building blocks for new programsAbstraction - hide internal details (library functions)
Avoid code repetition
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Function Definitions
Function definition formatreturn-value-type function-name( parameter-list ){
declarations and statements
} Function-name: any valid identifier
Return-value-type: data type of the result (default int)void indicates that the function returns nothing
Parameter-list: comma separated list, declares parametersA type must be listed explicitly for each parameter unless, the
parameter is of type int
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Function Definitions
Function definition format (continued)return-value-type function-name( parameter-list ){
declarations and statements}
Declarations and statements: function body (block) Variables can be declared inside blocks (can be nested)Functions can not be defined inside other functions
Returning control If nothing returned return; or, until reaches right brace
If something returned returnexpression;
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1. Functionprototype (3parameters)
2. Input values
2.1 Call function
3. Functiondefinition
Program Output
1 /* Fig. 5.4: fig05_04.c2 Finding the maximum of three integers */3 #include 45 int maximum( int, int, int ); /* function prototype */67 int main()
8 {9 int a, b, c;1011 printf( "Enter three integers: " );12 scanf( "%d%d%d", &a, &b, &c );13 printf( "Maximum is: %d\n", maximum( a, b, c ) );1415 return 0;
16 }1718 /* Function maximum definition */19 int maximum( int x, int y, int z )20 {21 int max = x;2223 if ( y > max )
24 max = y;2526 if ( z > max )27 max = z;2829 return max;30 }
Enter three integers: 22 85 17Maximum is: 85
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Function Prototypes
Function prototype Function name Parameters what the function takes in Return type data type function returns (default int)
Used to validate functions Prototype only needed if function definition comes after use in
program The function with the prototype
int maximum( int, int, int );
Takes in 3 intsReturns an int
Promotion rules and conversions Converting to lower types can lead to errors
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Header Files
Header files Contain function prototypes for library functions
,, etc Load with
#include #include
Custom header files Create file with functions
Save as filename.h Load in other files with #include "filename.h"
Reuse functions
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Calling Functions: Call by Value and Call byReference
Used when invoking functionsCall by value Copy of argument passed to function Changes in function do not effect original Use when function does not need to modify argumentAvoids accidental changes
Call by reference Passes original argument
Changes in function effect original Only used with trusted functions
For now, we focus on call by value
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Random Number Generation
randfunction Load Returns "random" number between 0 and RAND_MAX (at least32767)
i = rand();
Pseudorandom Preset sequence of "random" numbers Same sequence for every function call
Scaling To get a random number between 1 and n
1 + ( rand() % n ) rand() % n returns a number between 0 and n - 1Add 1 to make random number between 1 and n1 + ( rand() % 6) number between 1 and 6
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Random Number Generation
srandfunction
Takes an integer seed and jumps to that location in its"random" sequence
srand( seed);
srand( time( NULL ) ); //load time( NULL ) Returns the time at which the program was compiled in seconds
Randomizes" the seed
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1. Initialize seed
2. Input value forseed
2.1 Use srandtochange randomsequence
2.2 Define Loop
3. Generate andoutput randomnumbers
1 /* Fig. 5.9: fig05_09.c
2 Randomizing die-rolling program */
3 #include
4 #include
5
6 int main()7 {
8 int i;
9 unsignedseed;
10
11 printf( "Enter seed: " );
12 scanf( "%u", &seed );
13 srand( seed );
14
15 for ( i = 1; i
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Program OutputEnter seed: 867
2 4 6 1 6
1 1 3 6 2
Enter seed: 676 1 4 6 21 6 1 6 4
Enter seed: 676 1 4 6 21 6 1 6 4
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Storage Classes
Storage class specifiers Storage duration how long an object exists in memory Scope where object can be referenced in program Linkage specifies the files in which an identifier is known
(more in Chapter 14)Automatic storage Object created and destroyed within its block auto: default for local variables
auto double x, y; register: tries to put variable into high-speed registersCan only be used for automatic variables
register int counter = 1;
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Storage Classes
Static storage Variables exist for entire program execution
Default value of zero
static: local variables defined in functions.Keep value after function ends
Only known in their own function
extern: default for global variables and functionsKnown in any function
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Scope Rules
File scope Identifier defined outside function, known in all functions
Used for global variables, function definitions, functionprototypes
Function scope Can only be referenced inside a function body
Used only for labels (start:, case: , etc.)
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Scope Rules
Block scope Identifier declared inside a blockBlock scope begins at declaration, ends at right brace
Used for variables, function parameters (local variables of
function) Outer blocks "hidden" from inner blocks if there is a variable
with the same name in the inner block
Function prototype scope Used for identifiers in parameter list
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1 /* Fig 5 12: fig05 12 c
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1. Functionprototypes
1.1 Initializeglobal variable
1.2 Initialize localvariable
1.3 Initialize localvariable in block
2. Call functions
3. Output results
1 /* Fig. 5.12: fig05_12.c2 A scoping example */3 #include 45 voida(void); /* function prototype */6 voidb(void); /* function prototype */7 voidc(void); /* function prototype */89 int x = 1; /* global variable */1011 int main()12 {13 int x = 5; /* local variable to main */14
15 printf("local x in outer scope of main is %d\n", x );1617 { /* start new scope */18 int x = 7;1920 printf( "local x in inner scope of main is %d\n",x );21 } /* end new scope */
2223 printf( "local x in outer scope of main is %d\n", x );2425 a(); /* a has automatic local x */26 b(); /* b has static local x */27 c(); /* c uses global x */28 a(); /* a reinitializes automatic local x */
29 b(); /* static local x retains its previousvalue */30 c(); /* global x also retains its value */
31
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3.1 Functiondefinitions
31
32 printf( "local x in main is %d\n", x );
33 return 0;
34 }
35
36voida(void)
37 {38 int x = 25; /* initialized each time a is called */
39
40 printf( "\nlocal x in a is %d after entering a\n", x );
41 ++x;
42 printf( "local x in a is %d before exiting a\n", x );
43 }
4445voidb(void)
46 {
47 staticint x = 50; /* static initialization only */
48 /* first time b is called */
49 printf( "\nlocal static x is %d on entering b\n", x );
50 ++x;
51 printf( "local static x is %d on exiting b\n", x );
52 }
53
54voidc(void)
55 {
56 printf( "\nglobal x is %d on entering c\n", x );
57 x *= 10;
58 printf( "global x is %d on exiting c\n", x );
59 }
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Program Output
local x in outer scope of main is 5local x in inner scope of main is 7local x in outer scope of main is 5local x in a is 25 after entering alocal x in a is 26 before exiting alocal static x is 50 on entering blocal static x is 51 on exiting bglobal x is 1 on entering cglobal x is 10 on exiting c
local x in a is 25 after entering alocal x in a is 26 before exiting alocal static x is 51 on entering blocal static x is 52 on exiting bglobal x is 10 on entering cglobal x is 100 on exiting clocal x in main is 5
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Pointers as function arguments
We can pass the address of a variable as an argument to afunction.
When we pass addresses to a function, the parametersreceiving the addresses should be pointers.
The process of calling a function using pointers to passthe addresses of variables is known as call by reference.
The function which is called by reference can change thevalue of the variableused in the call.
This mechanism is also known as call by address or passby pointers.
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1. The function parameters are declared as pointers.
2. The dereferencing pointers are used in thefunction body.
3. When the function is called, the addresses arepassed as actual arguments.
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Passing array to function
One dimensional arrays : To pass a one-dimensional array to a called function, it is sufficient to list
the name of the array, without any subscripts and the size of the array asarguments.
For example :
Declaration : void largest(int [ ],int); Calling : largest(a,n)
Definition : void largest(int a[ ],int n)
{
}
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Three rules to pass an array to a function
The function must be called by passing only the name of the array.
for example : largest(a);
In the function definition, the formal parameter must be an arraytype, the size of the array does not need to be specified.
for example : void largest(int a[ ]){
} The function prototype must show that the argument is an array.
for example : void largest(int [ ]);
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Two Dimensional Array
The rules are :
The function must be called by passing only the array name.
for example : average(matrix,m,n);
In the function definition, we must indicate that the array has two
dimensions by including two sets of brackets.for example : void average(int matrix[ ][n],int m,int n)
{
}
The size of the second dimension must be specified.
The prototype declaration should be similar to function header.
for example : void average(int matrix[ ][n],int,int);
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Passing strings to functions
The string to be passed must be declared as a formal argument ofthe function when it is defined.
for example : void display(char str[ ])
{
}
The function prototype must show that the argument is a string
void display(char str[ ]);
A call to the function must have a string array name withoutsubscripts as its actual argument.
display(str);
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Using Library Functions
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Standard Library Function Prototypes
Any standard library function used by your program mustbe prototyped.
Appropriate header for each library function has to beused.
All necessary headers are provided by the C compiler.
The library headers are (usually) files that use the .hextension.
A header contains two main elements: any definitions used by the library functions and the prototypes for the libraryfunctions.
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For example, is included in almost all programsbecause it contains the prototype for printf( ).
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Macro Definitions
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The #define directive has another powerful feature:The macro name can have arguments. Each time themacro name is encountered, the arguments used inits definition are replaced by the actual argumentsfound in the program. This form of a macro is calledafunction-like macro.
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Example
#include
#define ABS(a) (a) < 0 ? -(a) : (a)
int main(void)
{
printf("abs of -1 and 1: %d %d", ABS(-1), ABS
(1));
return 0;
}
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When this program is compiled, a in the macrodefinition will be substituted with the values 1 and1. The parentheses that enclose a ensure propersubstitution in all cases.
ABS (10-20) would be converted to
10-20 < 0 ? -10-20 : 10-20
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Preprocessor Directive
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Preprocessor directives are lines included in the codeof our programs that are not program statements butdirectives for the preprocessor.
These lines are always preceded by a hash sign (#). The preprocessor is executed before the actual
compilation of code begins, therefore thepreprocessor digests all these directives before any
code is generated by the statements.
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The preprocessor directives are shown here:
#define #endif #ifdef #line
#elif #error #ifndef #pragma#else #if #include #undef
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There is no semicolon in this statement. There may beany number of spaces between the identifier and thecharacter sequence, but once the character sequence
begins, it is terminated only by a newline.
Example: If we wish to use the word LEFT for the value 1 and the word
RIGHT for the value 0, we could declare these two #definedirectives:
#define LEFT 1#define RIGHT 0
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This causes the compiler to substitute a 1 or a 0 eachtime LEFT or RIGHT is encountered in your
source file.
For example, the following prints 0 1 2 on the screen:printf("%d %d %d", RIGHT, LEFT, LEFT+1);
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#error
The #error directive forces the compiler to stopcompilation. It is used primarily for debugging. Thegeneral form of the #error directive is
#error error-messageThe error-message is not between double quotes.
When the #error directive is encountered, the
error message is displayed, possibly along with other
information defined by the compiler.
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#include
The #include directive tells the compiler to readanother source file in addition to the one thatcontains the #include directive.
The name of the source file must be enclosedbetween double quotes or angle brackets.
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Example,
#include "stdio.h"
#include
Both cause the compiler to read and compile theheader for the I/O system library functions.
Include files can have #include directives in them. Thisis referred to as nested includes.
The number of levels of nesting allowed varies betweencompilers.
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Conditional Compilation Directives
There are several directives that allows to selectivelycompile portions of your program's source code. Thisprocess is called conditional compilation and is usedwidely by commercial software houses that provideand maintain many customized versions of oneprogram.
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#if, #else, #elif, and #endif
These directives allow you to conditionally includeportions of code based upon the outcome of aconstant expression.
The general form of#if is#ifconstant-expression
statement sequence
#endif
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If the constant expression following #if is true, the code that isbetween it and #endif is compiled.
Otherwise, the intervening code is skipped. The #endifdirective marks the end of an #if block.
For example:/* Simple #if example. */
#include
#define MAX 100
int main(void)
This program displays the message on the screen becauseMAX is greater than 99.
{
#if MAX>99
printf(''Compiled for array
greater than 99.\n");
#endifreturn 0;
}
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The #else directive works much like the else that ispart of the C language:
It establishes an alternative if #if fails. The previous
example can be expanded as shown here:
/* Simple #if/#else example. */
#include
#define MAX 10
int main(void)
{
#if MAX>99printf("Compiled for array greaterthan 99.\n");#else
printf("Compiled for smallarray.\n");#endifreturn 0;}
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Some pointer concepts
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Pointer I
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Pointer II
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Pointer III
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Pointer IV
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Pointer V
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Pointer VI
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Recursion
Recursive functions Functions that call themselves
Can only solve a base case
Divide a problem up into
What it can doWhat it cannot do
What it cannot do resembles original problem
The function launches a new copy of itself (recursion step) to
solve what it cannot do Eventually base case gets solvedGets plugged in, works its way up and solves whole problem
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Recursion
Example: factorials 5! = 5 * 4 * 3 * 2 * 1
Notice that5! = 5 * 4!
4! = 4 * 3! ... Can compute factorials recursively
Solve base case (1! = 0! = 1) then plug in2! = 2 * 1! = 2 * 1 = 2;
3! = 3 * 2! = 3 * 2 = 6;
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Example Using Recursion: The Fibonacci Series
Fibonacci series: 0, 1, 1, 2, 3, 5, 8... Each number is the sum of the previous two Can be solved recursively:fib( n ) = fib( n - 1 ) + fib( n 2 )
Code for the fibaonacci functionlong fibonacci( long n )
{
if (n == 0 || n == 1) // base case
return n;
elsereturn fibonacci( n - 1) +
fibonacci( n 2 );
}
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1 /* Fig. 5.15: fig05_15.c
2 Recursive fibonacci function */
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1. Functionprototype
1.1 Initializevariables
2. Input an integer
2.1 Call functionfibonacci
2.2 Output results.
3. Define
fibonaccirecursively
Program Output
3 #include 45 long fibonacci( long );67 int main()
8 {9 long result, number;1011 printf( "Enter an integer: " );12 scanf( "%ld", &number );13 result = fibonacci( number );14 printf( "Fibonacci( %ld ) = %ld\n", number, result );
15 return 0;16 }1718 /* Recursive definition of function fibonacci */19 long fibonacci( long n )20 {21 if ( n == 0 || n == 1 )
22 return n;23 else24 return fibonacci( n - 1 ) + fibonacci( n - 2 );25 }Enter an integer: 0Fibonacci(0) = 0Enter an integer: 1
Fibonacci(1) = 1
Enter an integer: 2Fibonacci(2) = 1
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Program Output
Enter an integer: 3Fibonacci(3) = 2Enter an integer: 4
Fibonacci(4) = 3Enter an integer: 5Fibonacci(5) = 5Enter an integer: 6Fibonacci(6) = 8
Enter an integer: 10Fibonacci(10) = 55Enter an integer: 20Fibonacci(20) = 6765Enter an integer: 30
Fibonacci(30) = 832040Enter an integer: 35Fibonacci(35) = 9227465
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Recursion vs. Iteration
Repetition Iteration: explicit loop
Recursion: repeated function calls
Termination Iteration: loop condition fails
Recursion: base case recognized
Both can have infinite loops
Balance Choice between performance (iteration) and good software
engineering (recursion)
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