Lecture 5: Modular

Programming (functions

– part 1

BJ Furman

27FEB2012

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

� Explain how larger C programs should be structured using .h and .c files

Observations so far

� Starting to get the hang of it!� Don’t give up if you are still fuzzy on some of the

concepts! See Prof. Furman or lab instructor.

� Don’t slack off on the steps in the process of developing a program:� Define the problem� Determine requirements/specifications

� Define inputs and outputs

� Develop the ‘logic’ of the program using pseudocode and/or a flowchart

� Develop the C code (including comments!!!)

� Test the program (consider full range of possible inputs)

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 View

32)( 2++=

∆

xxxf

11 is )2(

113443)2(2)2()2( 2

f

f

∴

⇒++⇒++⇒

f(2)? isWhat

)(xf2 11

XFunction

Returnedvalue

Functions - Definition Structure

� Function 'header'� Return data type

(if any)

� Name� Descriptive

� Arguments (or parameter list)� Notice: data type and

name

� Statements� Variable declaration

� Operations

� Return value (if any)

type function_name (type arg1, type arg2 )

{statements;

}

double product(double x, double y)

{double result;

result = x * y;return result;

}

A function that calculates the product of two numbers

Functions - Example� Function prototype

� Like a variable declaration� Tells compiler that the function will

be defined later

� Helps detect program errors� Note semicolon!!

� Function definition� See previous slide� Note, NO semicolon

� 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

� Function call� main() is the 'calling function'

� product() is the 'called function'

� Control transferred to the function code

� Code in function definition is executed

#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

Steps1. 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(int x, int y)

{int result;

result = x + y;return result;

}

Functions that do not return a value

� Use the return type of void

� void my_fun( arg_list,F)

� Practice

� Write two functions, the first prints out first

name, and the second prints out last name

Function - Practice 2

� Program to print out two happy :) :) or sad faces :( :(� Continuously prompts for

user input:

� ) for happy face

� ( for sad face

� Quits if 'q' or 'Q' entered

� calls two functions

� happy_face()

� sad_face()

� Work in pairs

� Pseudocode first!!

� Divide tasks of writing the two functions

Steps1. 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

Program - Faces logic

� Pseudocode

1. Declare and initialize variables

2. WHILE user input not equal to q AND not equal to Q

1) Switch on user input to

2) Case ')‘:

call happy_face();

break;

5) Case '(‘:

call sad_face();

break;

6) Case ‘q’:

7) Case ‘Q’:break;

8) Case ‘0’:

9) Default:

re-prompt for user input

Program - Faces code

Structuring C Programs

� Modularization� Breaking a program up into smaller pieces:

� Instead of: � one_big_program.c

� break into groupings of header files (.h) and source code (.c) files:

� module_1.h� module_1.c� etc.

� Rationale� separates the user-interface description (.h) from the nitty-

gritty details of implementation (.c)� The Application Programming Interface (API), the .h file, is

distinct from the implementation, the .c file (which may already be compiled and not readily viewed)

� Example: math.h from Ch

� can construct and test modules independently

� promotes re-use of code

Example: math.h used in Ch

� See C:/ Ch / include / math.h

� Declaration of constants

� #define M_PI 3.14159265358979323846

� Declaration of macro subsitutions

� #define isgreater(x, y) ((x)>(y))

� Declaration of global variables (caution!)

� Function prototypes

� extern double sin(double x);

� Pertinent comments

Review

Structured Programming

� All programs can be written using these control structures:

� Sequence

� Decision (three structures)� IF

� IF-ELSE

� SWITCH

� Repetition (three structures)� WHILE

� DO-WHILE

� FOR

Structure of a C program

� Ex. free_fall_d_vs_time.c

C Code for D&D 3.15cProgrammer’s block

Pre-processor directive

Declare and initialize variables

While loop(repetition structure)

Main function (statements go between { } )

return statement

Arithmetic with Integers and Mixed Data Types

� Arithmetic with integers� Result is an integer

� 1+1 --> 2

� 4/2 --> 2

� 2/4 --> ? BE CAREFUL!!!

� Arithmetic with mixed data types� Automatic conversion of operand so that data types match

� Conversion is ‘upward’ in the sizeof() sense

� Example in Chchar a = 7;

sizeof(a);

double b=3;

sizeof(b);

printf("a+b == %lf and needs %d bytes\n ", a+b, sizeof(a+b));

References

� Modular Programming in C

http://www.icosaedro.it/c-modules.html

� math.h

http://www.opengroup.org/onlinepubs/007

908799/xsh/math.h.html