C Programming

FunctionsFunctionsMacrosMacros

Functions vs. Methods• Java classes include methods which can be called from any code Java classes include methods which can be called from any code

with appropriate access (recall with appropriate access (recall publicpublic methods) methods)

• C functions are like Java methods, but they don’t belong to any C functions are like Java methods, but they don’t belong to any class. Functions are defined in a file and may be either global to class. Functions are defined in a file and may be either global to your program or local to (“your program or local to (“privateprivate” in) the file in which they are ” in) the file in which they are defined*.defined*.

• Like Java methods, C functions Like Java methods, C functions – Have a name– Have a return type– May have parameters

• Unlike Java methods, a function in C is uniquely identified by its Unlike Java methods, a function in C is uniquely identified by its name. Therefore, there is no concept of method overloading in C name. Therefore, there is no concept of method overloading in C as there is in Java. There can be only one as there is in Java. There can be only one main( )main( ) function in a C function in a C application.application.

• See See the course coding standards for function naming conventions. for function naming conventions.

Arguments vs Parameters• A A parameterparameter is defined in the function definition. It is defined in the function definition. It

is a place holder for the argument during function is a place holder for the argument during function executionexecutionvoid printInt( int n ) // n is the parameter{

printf( “%20d\n”, n);}

• An An argumentargument is a value passed to a function when is a value passed to a function when the function is calledthe function is calledint age = 42;printInt( age ); // age is the argument

• These terms are often (incorrectly) used These terms are often (incorrectly) used interchangeably, but the context is usually clearinterchangeably, but the context is usually clear

Passing Arguments

• Primitive types (int, char, float, etc) and structs are Primitive types (int, char, float, etc) and structs are passed to function “by value”passed to function “by value”– A copy of the argument is passed to the function– Changing the parameter within the function has no effect on the

argument in the calling code

• Arrays are passed “by reference”Arrays are passed “by reference”– A copy of the address of the array is passed to the function

• The parameter is an “alias” for the argument

– References to array elements in the function refer to array elements in the calling code

– Array parameters which are not intended to be changed by a function should be qualified as const

Passing Arguments/* ages.c */#include <stdio.h>

void growOlder( int a[ ], int size){

int k;for (k = 0; k < size; k++)

++a[ k ];}

int avgAge( const int array[ ], int size){

int k, sum = 0;for (k = 0; k < size; k++)

sum += array[ k ];return sum / size;

}

int main( ){ int nrStudents = 6;

int ages[ 6 ] = {19, 18, 17, 22, 44, 55};growOlder( ages, nrStudents );int avg = avgAge( ages, nrStudents );printf(“The average age is %d\n”, avg);return 0;

}

Auto Type Conversion

• As we’ve seen, the compiler can and will As we’ve seen, the compiler can and will automatically convert a “small” data type to a automatically convert a “small” data type to a “larger” data type without problems.“larger” data type without problems.char initial = ‘B’;short age = 42;int intAge = 42, intInitial = initial;long longAge = age;

• The same type conversion occurs for function The same type conversion occurs for function arguments.arguments.short myAge = 42, yourAge = 33;long max( long a, long b);long older = max ( myAge, yourAge);

Passing 2-D Arrays

• Passing a 2-d array to a function is similar to passing a Passing a 2-d array to a function is similar to passing a 1-d array1-d array

• Basic function prototypeBasic function prototypevoid printChessBoard( char [ 8 ][ 8 ] theBoard);

• Calling the functionCalling the functionchar chessBoard[ 8 ] [ 8 ];printChessBoard( chessBoard );

• As we will see, the compiler needs to know the size of As we will see, the compiler needs to know the size of each row, but not the number of rows. This allows an each row, but not the number of rows. This allows an alternative prototypealternative prototypevoid printChessBoard( char[ ] [ 8 ] theBoard );

Recursion

• C functions may be called recursively.C functions may be called recursively.– Typically a function calls itself

• A properly written recursive function has the A properly written recursive function has the following propertiesfollowing properties– A “base case” - a condition which does NOT make a

recursive call because a simple solution exists– A recursive call with a condition (usually a parameter value)

that is closer to the base case than the condition (parameter value) of the current function call

• Each invocation of the function gets its own set Each invocation of the function gets its own set of arguments and local variablesof arguments and local variables

• We’ll see how recursion is implemented laterWe’ll see how recursion is implemented later

Recursion Example

/* print an integer in decimal** K & R page 87 (may fail on largest negative int) */

#include <stdio.h>void printd( int n ){

if ( n < 0 ){

printf( “-” );n = -n;

}if ( n / 10 ) /* (n / 10 != 0) -- more than 1 digit */

printd( n / 10 ); /* recursive call: n has 1 less digit */

printf( “%c”, n % 10 + ‘0’); /* base case --- 1 digit */}

Recursive Exercise

Complete the following recursive function that sums all Complete the following recursive function that sums all of the integers from 1 to Nof the integers from 1 to N

int sumToN( int N )

{

if (____________________) // base case

return N;

else // recursive call

return ______________________;

}

Inline Functions

• C99 onlyC99 only• Short functions may be defined as “Short functions may be defined as “inline”. This is ”. This is

a suggestion to the compiler that calls to the a suggestion to the compiler that calls to the function should be replaced by the body of the function should be replaced by the body of the function. function.

• inline functions provide code structure and functions provide code structure and readability and (may) increase performancereadability and (may) increase performanceinline bool isEven( int n )

{ return n % 2 == 0; }

inline max( int a, int b )

{ return a > b ? a : b; }

Macros• C provides macros as an alternative to small C provides macros as an alternative to small

functions.functions.• More common prior to C99 (inline functions are More common prior to C99 (inline functions are

better)better)• Handled by the preprocessorHandled by the preprocessor• Several “gotcha”sSeveral “gotcha”s• Parameters have no type.Parameters have no type.• OK if used carefullyOK if used carefully• General macro format. General macro format.

– #define NAME( params if any ) code here• Note: there is NO space between the name and the left parenthesis

• See macros.cSee macros.c

SQUARE( )

• A simple macro to square a variable A simple macro to square a variable – #define SQUARE( x ) x * x

• Like all Like all #defines#defines, the preprocessor performs text , the preprocessor performs text substitution. Each occurrence of the parameter is substitution. Each occurrence of the parameter is replaced by the argument text.replaced by the argument text.int y = 5;

int z = SQUARE( y );

• But now consider this statementBut now consider this statementint w = SQUARE( y + 1 );

A better SQUARE( )

• This version is betterThis version is better– #define SQUARE( x ) ( (x) * (x) )

int y = 5;

int z = SQUARE( y );

int w = SQUARE( y + 1 );

• But still doesn’t work in every caseBut still doesn’t work in every caseint k = SQUARE( ++y );