EPSII

59:006

Spring 2004

Call-by-value example#include <stdio.h>void increment(int); //prototype for increment function

int main(void) { int a=1; printf("Value of a before function call is %d\n", a); increment(a); printf("Value of a after function call is %d\n", a); return 0; }

void increment(int x) { //parameter passed by value printf("Starting value of parameter x in function is %d\n",x); x=x+1; printf("Ending value of parameter x in function is %d\n",x); }

Value of a before function call is 1Starting value of parameter x in function is 1Ending value of parameter x in function is 2Value of a after function call is 1

Note: incrementing the parameter xdoes not affect the value of the argument a

Functions – Quick Review Functions

Modularize a program All variables declared inside functions are local variables

Known only in function defined Parameters

Communicate information to/from functions Act like local variables within the function

Benefits of functions Divide and conquer

Manageable program development Software reusability

Use existing functions as building blocks for new programs Abstraction - hide internal details (library functions)

Avoid code repetition

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 parameters

A type must be listed explicitly for each parameter unless the parameter is of type int (However you should explicitly specify the type, even for ints)

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 execution reaches right brace (end of function)

If something returned: return expression;

Function prototype (3 parameters)

Input values

Call function

Function definition

Program Output

1 /* Fig. 5.4: fig05_04.c2 Finding the maximum of three integers */3 #include <stdio.h>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

Function Prototypes Function prototype declares:

Function name Parameter types – what the function takes in Return type – data type function returns (default int) Used by compiler to determine typing information

about functions. Prototype required only if function definition comes

after use in program (But it is a good idea to always use them.)

The function with the prototypeint maximum( int, int, int );

Takes in 3 ints Returns an int

Header Files Header files

Contain function prototypes for library functions <stdlib.h> , <math.h> , etc Load with #include <filename>

#include <math.h> Custom header files:

You can create your own header files---e.g. containing prototypes, function definitions, etc

Save as filename.h Load into other files with #include "filename.h" The quotes around the file name tell the C

preprocessor to look for the named file in the current working directory

Calling Functions: Call by Value and Call by Reference

Used when invoking functions Call by value

Argument’s current value is passed to function Changes to the parameter in the function do not affect the value

of the argument Use when function does not need to modify argument

Avoids accidental changes Call by reference

Passes the address (location) of the argument to the function Changes to the parameter in function modify the argument Only used with trusted functions

For now, we focus on call by value

Random Number Generation rand function

#include <stdlib.h> Returns "random" number between 0 and RAND_MAX (at least 32767)

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 - 1 Add 1 to make random number between 1 and n

1 + ( rand() % 6) number between 1 and 6

Random Number Generation

srand function <stdlib.h> Takes an integer “seed” to initialize the "random" sequence

srand( seed ); srand( time( NULL ) ); //need #include <time.h>

time( NULL ) Returns the current time (in seconds) since some starting

time which is system-dependent For Unix this starting time is 00:00:00 GMT Jan 1, 1970

(the ‘Epoch’ – the birth of Unix)

Used this way, time() “randomizes" the seed

Initialize seed

Input value for seed

Use srand to change random sequence

Define Loop

Generate and output random numbers

1 /* Fig. 5.9: fig05_09.c

2 Randomizing die-rolling program */

3 #include <stdlib.h>

4 #include <stdio.h>

5

6 int main()

7 {

8 int i;

9 unsigned seed;

10

11 printf( "Enter seed: " );

12 scanf( "%u", &seed );

13 srand( seed );

14

15 for ( i = 1; i <= 10; i++ ) {

16 printf( "%10d", 1 + ( rand() % 6 ) );

17

18 if ( i % 5 == 0 )

19 printf( "\n" );

20 }

21

22 return 0;

23 }

This exampleshows that a givenseed alwaysgenerates thesame randomnumber sequence

Program Output

Enter seed: 867 2 4 6 1 6 1 1 3 6 2

 Enter seed: 67 6 1 4 6 2 1 6 1 6 4

Enter seed: 67 6 1 4 6 2 1 6 1 6 4

Example of time() functionint main(void) { int i; int secs_per_year=31536000; i = time(NULL); printf("The current time returned by time() is %d\n", i); printf("This represents the year %d\n", 1970 + (i/secs_per_year)); }

Note that time()returns a different valueeach time the programruns

l-ecn003% gcc timeTest.c l-ecn003% a.outThe current time returned by time() is 1076704538This represents the year 2004l-ecn003% a.outThe current time returned by time() is 1076704544This represents the year 2004

Example: A Game of Chance

Craps simulator Rules

Roll two dice 7 or 11 on first throw, player wins 2, 3, or 12 on first throw, player loses 4, 5, 6, 8, 9, 10 - value becomes player's "point"

Player must roll his/her point again before rolling 7 to win

rollDice prototype

Initialize variables

Seed srand

Define switch statement for win/loss/continue

Loop

1 /* Fig. 5.10: fig05_10.c2 Craps */3 #include <stdio.h>4 #include <stdlib.h>5 #include <time.h>67 int rollDice( void );89 int main()10 {11 int gameStatus, sum, myPoint;1213 srand( time( NULL ) );14 sum = rollDice(); /* first roll of the dice */1516 switch ( sum ) {17 case 7: case 11: /* win on first roll */18 gameStatus = 1;19 break;20 case 2: case 3: case 12: /* lose on first roll */21 gameStatus = 2;22 break;23 default: /* remember point */24 gameStatus = 0;25 myPoint = sum;26 printf( "Point is %d\n", myPoint );27 break;28 }2930 while ( gameStatus == 0 ) { /* keep rolling */31 sum = rollDice();32

Print win/loss

Program Output

33 if ( sum == myPoint ) /* win by making point */

34 gameStatus = 1;

35 else

36 if ( sum == 7 ) /* lose by rolling 7 */

37 gameStatus = 2;

38 }

39

40 if ( gameStatus == 1 )

41 printf( "Player wins\n" );

42 else

43 printf( "Player loses\n" );

44

45 return 0;

46 }

47

48 int rollDice( void )

49 {

50 int die1, die2, workSum;

51

52 die1 = 1 + ( rand() % 6 );

53 die2 = 1 + ( rand() % 6 );

54 workSum = die1 + die2;

55 printf( "Player rolled %d + %d = %d\n", die1, die2, workSum );

56 return workSum;

57 }

Player rolled 6 + 5 = 11Player wins

Program Output

Player rolled 6 + 6 = 12Player loses

Player rolled 4 + 6 = 10Point is 10Player rolled 2 + 4 = 6Player rolled 6 + 5 = 11Player rolled 3 + 3 = 6Player rolled 6 + 4 = 10Player wins

Player rolled 1 + 3 = 4Point is 4Player rolled 1 + 4 = 5Player rolled 5 + 4 = 9Player rolled 4 + 6 = 10Player rolled 6 + 3 = 9Player rolled 1 + 2 = 3Player rolled 5 + 2 = 7Player loses

Storage Classes Storage class specifiers

Storage duration – how long a variable retains its value in memory

Scope – where a variable can be referenced in a program

Linkage – specifies the files in which an identifier (variable or function name) is known (more in Chapter 14)

C Storage Classes

auto (automatic) register static extern (external)

Automatic Storage Class Automatic storage

Variable created and destroyed within its block If an initializer is used, the variable is reinitialized each time its

block is entered.

Default for local variables of functions

auto double x, y; register: tries to put variable into high-speed registers

(not of much significance for modern compilers and computers)

Can only be used for automatic variables

register int counter = 1;

Static Storage Class Static storage

Variables exist for entire program executionDefault initial value of zeroCan specify an alternative initial value:

static int x = 1; Static variables are initialized once, at beginning of

program execution.Static local variables defined in functions:

Keep value after function ends Only known in their own function

Static Variable Example#include <stdio.h>int counter(void); //prototype

int main(void) { int value; value = counter(); printf("value = %d\n", value); value = counter(); printf("value = %d\n", value); value = counter(); printf("value = %d\n", value); return 0; }

int counter() { static int count=0; //would be initialized by default count = count +1; return count; }

value = 1value = 2value = 3

Try this example without thestatic designation and seewhat happens.

Extern Storage classextern: default for global variables and

functions declared outside of the scope of any function. Known in all functions global (extern) variable declarations should be

avoided unless absolutely necessary

Scope Rules File scope

Identifier defined outside of function body known in all functions defined in the file applies to:

global variables (variables defined outside of any function definition

function names function prototypes

Function scope Can only be referenced inside a function body Used only for labels (start:, case: , etc.) This is a language detail that you don’t need to worry

about.

Scope Rules Block scope

Identifier declared inside a block {…} Block scope begins at declaration, ends at right

brace Used for:

local variables function parameters

Can actually define local variables in any C block (not recommended)

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

Function prototypes

Initialize global variable

Initialize local variable

Initialize local variable in block

Call functions

1 /* Fig. 5.12: fig05_12.c2 A scoping example */3 #include <stdio.h>45 void a( void ); /* function prototype */6 void b( void ); /* function prototype */7 void c( void ); /* function prototype */89 int x = 1; /* global variable */1011 int main()12 {13 int x = 5; /* local variable to main */1415 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 previous value */30 c(); /* global x also retains its value */

Output Results

Function definitions

31

32 printf( "local x in main is %d\n", x );

33 return 0;

34 }

35

36 void a( 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 }

44

45 void b( void )

46 {

47 static int 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

54 void c( 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 }

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 5 local x in a is 25 after entering alocal x in a is 26 before exiting a local static x is 50 on entering blocal static x is 51 on exiting b global 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 a local static x is 51 on entering blocal static x is 52 on exiting b global x is 10 on entering cglobal x is 100 on exiting clocal x in main is 5

Scope Rules—Style Guidelines for this Class Do not use extern (global) variables unless there is an

absolutely compelling reason. If you think there is a compelling reason, you are probably wrong global variables can introduce subtle errors into programs

Declare local (block scope) variables only at the beginning of functions (including main() ) Don’t declare local variables inside the body of loops or other

control structures Don’t use the register designation

it’s probably meaningless anyway Declare variables as static only when they need to

retain their value between invocations of a function.