Functions Declarations CSCI 230 Department of Computer and Information Science, School of Science, IUPUI CSCI 230 Functions Declarations Dale Roberts, Lecturer IUPUI droberts@cs.iupui.edu

Functions Support Decomposition Category Programmers Duration Size of Code Trivial 1 1-2 weeks < 500 lines (student homework) Small 1 - 3 Few Weeks 500 -2000 lines (term projects) Medium 2 - 5 Few Months 2000 -10000 (research project) Large 5-25 1 - 3 years 10,000-100,000(current applications) Very Large 25-100 3 - 5 years 100,000 - 1M (real-time operations) Extremely Large > 100 > 5 years >1M (advanced military work) Divide and conquer Large programs cannot be monolithic Construct a program from smaller pieces or components. These smaller pieces are called modules Each piece is more manageable than the original program

Program Modules in C Functions Function calls Invoking functions Programs combine user-defined functions with library functions C standard library has a wide variety of functions Math function, I/O, string function, such as printf(), scanf() Function calls Invoking functions Provide function name and arguments (data) Function performs operations or manipulations Function returns results Write function once and call it many times Function call analogy: Boss asks worker to complete task Worker gets information, does task, returns result Information hiding: boss does not know details Also called Encapsulation

Math Library Functions perform common mathematical calculations #include <math.h> gcc compiler requires –lm parameter 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 argument All math functions return data type double Arguments may be constants, variables, or expressions

Functions Functions Benefits of functions Modularize a program All variables declared inside functions are local variables Known only in function defined Parameters Communicate information between functions Local variables 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 format return-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 An unspecified return-value-type is always assumed by the compiler to be int Parameter-list: comma separated list, declares parameters A type must be listed explicitly for each parameter, unless the parameter is of type int 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 return expression;

Function Prototypes Function prototype Promotion rules and conversions 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 ints Returns an int Promotion rules and conversions Converting to lower types can lead to errors

Function prototype (3 parameters) 2. Input values 3. Call function 1 /* Fig. 5.4: fig05_04.c 2 Finding the maximum of three integers */ 3 #include <stdio.h> 4 5 int maximum( int, int, int ); /* function prototype */ Function prototype (3 parameters) 2. Input values 3. Call function Function definition 6 7 int main() 8 { 9 int a, b, c; 10 11 printf( "Enter three integers: " ); 12 scanf( "%d%d%d", &a, &b, &c ); 13 printf( "Maximum is: %d\n", maximum( a, b, c ) ); 14 15 return 0; 16 } 17 18 /* Function maximum definition */ 19 int maximum( int x, int y, int z ) 20 { 21 int max = x; 22 23 if ( y > max ) 24 max = y; 25 26 if ( z > max ) 27 max = z; 28 29 return max; 30 } Program Output Enter three integers: 22 85 17 Maximum is: 85

Examples The minimal function, called a null function, is dummy(){} float plus1(float x,y) { float sum; sum = x + y; return sum; } plus2(int x,y) { int sum; sum = x + y; return sum; } The minimal function, called a null function, is dummy(){} Again, the return statement can be return expression; Example: return a*a; return (expression); Example: return ((a > 0)? a : -a)); return; In conclusion, to call a function from another function, you should: Declare the type of the called function, if not a int function Use format of function_name(actual parameters)

Acknowledgements Some examples were obtained from the course textbook.