Week 5: Modular
Programming
BJ Furman
21SEP2009
The Plan for Today
„
Overview of modular programming
„
Functions
„
„
Top-down design
Role of functions
Function prototype
Return data type
Arguments
Function Example
Function Practice
Learning Objectives
„
„
„
„
Explain the concept of modular program
design
Explain the concept of a function in C
Explain why functions are important in
programming
Explain the structure of a function
„
Return data type
Parameters
Apply the concept of a function to a practical
problem
Modular Programming
„
Break a large problem into smaller pieces
Smaller pieces sometimes called ‘modules’ or
‘subroutines’ or ‘procedures’ or functions
Why?
„
Helps manage complexity
Smaller blocks of code
Easier to read
„
Encourages re-use of code
Within a particular program or across different programs
Allows independent development of code
„ Provides a layer of ‘abstraction’
a = sqrt(9.0);
„
Functions
„
The ‘building blocks’ of a C program
You’ve used predefined functions already:
main()
„ printf(), scanf(), pow()
„
User-defined functions
Your own code
„ In combination with predefined functions
„
Functions - Mathematical
Δ
f ( x) = x 2 + 2 x + 3
What is f(2)?
f (2) ⇒ (2) 2 + 2(2) + 3 ⇒ 4 + 4 + 3 ⇒ 11
∴ f (2) is 11
X
Function
Returned
value
2
f ( x)
11
Functions - Definition Structure
„
Function 'header'
Return data type
(if any)
Name
„
„
Descriptive
type function_name (type arg1, type arg1, )
{
statements;
}
Arguments (or
parameter list)
Statements
Variable declaration
Operations
Return value (if any)
double product(double x, double y)
{
double result;
result = x * y;
return result;
}
Functions - Example
„
Function prototype
Like a variable declaration
„
„
„
„
Function definition
„
See previous slide
Function call
„
Tells compiler that the function will
be defined later
Helps detect program errors
Note semicolon!!
main() is the 'calling function'
product() is the 'called function'
Control transferred to the function
code
Code in function definition is
executed
Function return
return statement terminates
execution of the current function
Control returns to the calling
function
if return expression;
„
„
then value of expression is
returned as the value of the
function call
Only one value can be returned this
way
#include <stdio.h>
/* function prototype */
double product(double x, double y);
int main()
{
double var1 = 3.0, var2 = 5.0;
double ans;
ans = product(var1, var2);
printf("var1 = %.2f\n"
"var2 = %.2f\n",var1,var2);
printf("var1*var2 = %g\n", ans);
}
/* function definition */
double product(double x, double y)
{
double result;
result = x * y;
return result;
}
Function - Practice 1
„
Write a function
named 'sum'
sums two
integers
returns the sum
2 min. on your
own
„ Share with
neighbor
„
Steps
1. Function header
• return data type
• function name
• argument list with data types
2. Statements in function definition
• variable declaration
• operations
• return value
Function - sum()
int sum(int x, int y)
{
int result;
result = x + y;
return result;
}
Function - Practice 2
„
Program to print out two
happy :) :) or sad faces
:( :(
Continuously prompts for
user input:
„
„
„
calls two functions
„
„
„
„
„
) for happy face
( for sad face
Quits if 'q' or 'Q' entered
happy_face()
sad_face()
Work in pairs
Pseudocode first!!
Divide tasks of writing
the two functions
Steps
1. Pseudocode for program logic
2. Function header
• return data type (if any)
• function name
• argument list with data types (if any)
3. Statements in function definition
• variable declaration (if any)
• operations
• return value
Review
References