Understanding Function Declarations and Modular Programming in C

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

2. Functions Support Decomposition • Functions 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

3. Program Modules in C • Functions • Modules in C • 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

4. Math Library Functions • 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

5. Functions • 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

6. 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 • returnexpression;

7. 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 ints • Returns an int • Promotion rules and conversions • Converting to lower types can lead to errors

8. 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 */ 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 ) Function prototype (3 parameters) 2. Input values 3. Call function Function definition 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

9. Examples 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)

10. Acknowledgements • Some examples were obtained from the course textbook.