1. Introduction to C Programming
ET2560 Introduction to C Programming
Introduction to C Programming
Unit 7 Modular Programming Using Functions
Unit 1 Presentations

2. Top-Down Design
Unit 7: Modular Programming

3. Top-Down Design Break Problem down into sub-problems
Use available solutions for any sub-problems already solved
Create modules that solve recurring sub-problems
C library functions provide pre-written solutions
Large real-world problems are large and complex
Problem must be broken down to manage complexity
Software reuse saves time and increases reliability
C language provides the function as unit of modularity
Black-box concept, defined inputs and outputs

4. C Library Functions Contain pre-written and pre-tested solutions
Basic functions applicable to wide variety of common problems
Input and Output
Mathematical Operations
Character Manipulation
String Manipulation
Sorting and Searching
Time and Date
Other Utility Functions

5. Functions
Unit 7: Modular Programming

6. C Language Functions - Header
Function Header
Return Type (or void if no return value)
Function Name - C identifier
Parameter List
int f1(void) …
double f2 (int p1, double p2) …
void f3 (int p3, char p4) …

7. C Language Functions - Body
Function Body
Compound Statement (delimited by brace characters)
Last statement: return statement - if value is returned
double f2 (int p1, double p2) {
/* Function body */
return (value); }

8. Types of Functions No inputs or outputs - Perform an action
Use keyword void for return and parameter list
No output, one or more inputs - Perform an action with data
Use keyword void for return
Parameter list specifies inputs
One output only - Retrieve information
Specify return type, use keyword void for parameter list
One output, one or more inputs - Typical, many uses
Specify return type, input parameter list
Multiple outputs - Special circumstances
scanf() is an example
The return statement can only represent one output
Multiple outputs require output parameters

9. Use of a Function - Syntax
Syntax: Name of function followed by argument list
Parenthesized list, arguments separated by commas
If function has no inputs, parenthesized list is empty
f1()
If function has inputs, appear in argument list
f1(a1, a2, a3)
Must match the parameter list in number and order
Argument values are copied to parameter list

10. Use of a Function - Function Call
Use of function is termed a "function call"
A function can be a statement by itself
f1(a1, a2);
A function with a return value can be part of an expression
x = y + f1(a1, a2*4) - 63;

11. Placement of Function Definitions
One or more function definitions in a single file
Functions must be placed one after another
Cannot have a function definition inside another function
A function must be declared before it is used
Function declarations accomplished with prototypes
Function prototypes should be placed at top of file
When creating a large multi-file program, use include file
Prototype is like function header, with semicolon instead of body

12. Function Call: Control of Execution
When a function call appears in code:
The arguments are evaluated and passed to function
Execution transfers to the first statement in function
The function executes until a return statement (or it finishes)
Execution transfers back to the point of the call
Note that the calling function may pause mid-statement
A function may call one or more additional functions
The program stack is used to manage nesting function calls

13. Variable Scope
Unit 7: Modular Programming

14. Variable Scope Scope determines lifetime of variable
How long variable uses memory
How long the data in the variable is valid
A variable declared at the top of a function body is local scope
Lifetime only while function is active
Static keyword modifies this behavior
A variable declared outside of a function is file scope
File scope is also called "global" scope
Lifetime is for entire time program is executing
A variable declared inside a compound statement is block scope
Lifetime is only while execution is inside the block

15. Function Output Parameters
Unit 7: Modular Programming

16. Pointers Every variable in memory has a memory address
A pointer is a variable that holds a memory address
Pointers are used in C for multiple reasons
This unit introduces pointers to explain output parameters
The pointer can be used to change memory indirectly

17. Pointers Pointers are declared using the "*" character
int * ip;
The address of a variable is obtained with the "&" character
int x, y;
ip = &x;
To modify a variable using the pointer, the "*" is used
*ip = 5; /* Stores 5 in "x" */
ip = &y; /* Change to point to y */
*ip = 12; /* Stores 12 in "y" */

18. Output Parameters A function with output parameters uses pointers
The output parameter is declared as a pointer using "*"
The argument must evaluate to a variable's address
Internally, the function changes the variable via the pointer
int f1(int * p1, int * p2) {
*p1 = 0;
*p2 = 27;
return (55); }