1. Functions Declarations CSCI 230
Department of Computer and Information Science, School of Science, IUPUI
CSCI 230
Functions
Declarations
Dale Roberts, Lecturer
IUPUI

2. Functions Support Decomposition
Category Programmers Duration Size of Code
Trivial weeks < 500 lines (student homework)
Small Few Weeks lines (term projects)
Medium Few Months (research project)
Large years 10, ,000(current applications)
Very Large years 100, M (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 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

4. 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( ) );
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 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

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
return expression;

7. 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

8. 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 )
max = y; 25
26 if ( z > max )
max = z; 28
29 return max;
30 }
Program Output
Enter three integers:
Maximum is: 85

9. 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)

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