l3 - functions

7/26/2019 L3 - Functions

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CS1010E Programming Methodology

Functions

Get this job done for me!



Lecture Overview

Top Down Approach to Problem Solving

Functions

basic idea

execution flow

prototype and declaration

scoping rules

C Standard Library

Math Library

[ CS1010E AY1112S1 Lecture 3 ]2



Top Down Approach

When solving large problems, it is hard to

come up with a detailed solution right away

A common approach is:

Specify the major steps of the algorithm

Review each major step and break it down into

smaller steps (step-wise refinement)

Repeat this process if needed until the steps are

well defined

This is known as Top Down Approach to

problem solving

3[ CS1010E AY1112S1 Lecture 3 ]



Example 1

One way to calculate the PI π constant:

.........9

4

7

4

5

4

3

4

1

4−+−+−=π

When you compute the term using thesequence above:

1. PI starts as 4/1 (PI 4/1)

2. Subtract 4/3 from PI (PI PI – 4/3)

3. Add 4/5 to PI (PI PI + 4/5)

4. Subtract 4/7 from PI (PI PI – 4/7)

5. …

4



Example 1(cont’d)

A more general algorithm:

.........9

4

7

4

5

4

3

4

1

4−+−+−=π

even terms are

negative

odd terms are

positive

Denominator increases

by 2 every term

5

1. Initialize PI 4

2. Denom 3

3. nTerm 24. while nTerm is <= 1000

i. Calculate new term

ii. Update PI

iii. nTerm nTerm + 1

5. Show PI



PI Algorithm

6

a. Term 4 / Denom

1. Initialize PI 4

2. Denom 3

3. nTerm 2

4. while nTerm is <= 1000i. Calculate new term

ii. Update PI

iii. nTerm nTerm + 1

5. Show PI

a. If nTerm is even

PI PI – Term

b. If nTerm is odd

PI

PI + Term

First draft with only the major steps refinesMajor steps are further broken

down

[ CS1010E AY1112S1 Lecture 3 ]



Example 2: Anagrams

Two phrases are anagrams if one is formed

by rearranging the letters of the other,

disregarding spaces and punctuation marks.

“Debit card” = “Bad credit”

“The eyes” = “They see” “ Astronomer ” = “Moon starer ”

“ A telescope” = “To see place”

“ A decimal point” = “I’m a dot in place”

How do you determine if two words X and Y

are anagrams?7



Example: Anagram Algorithm

One possible solution using sorting

8

1. Read two words w1 and w2 from user

2. len1 = length of w1, len2 = length of w2

3. if len1 not equal len2

print "Not Anagrams" and stop

4. Sort w1 and w2 by alphabetical order

5. From position 0 to len1 - 1If w1position not equal w2position

print "Not Anagrams" and stop

6. print “They are Anagrams" and Stop

Steps to calculate the

length

Steps to sort a word

into alphabetical

order




Guidelines on Top Down Approach

As you learn more about C, you can see

clearer whether an algorithm step is

expressible in C with simple statement(s)

If it is not possible, break it down further to simpler

sub-problems

To learn a programming language is to know:

The expressive power of the language The limitations of the syntax

Such that high level algorithms can be translated

effectively




Functions

1. Using the top down approach:

A major step may correspond to many smaller

steps after refinement

If we place all the statements in one place, the

actual major step may no longer be obvious Large chunks of statements are hard to understand

2. Duplicated coding:

Some computations may be needed multiple times

in an algorithm

Error prone and wasteful to repeat the coding




Problem: A plate with 3 holes

Given the following plate, what is the area of

the shaded region after we drill 3 holes into

it?

11

a

b

c

Given:

R = radius of the plate Ra, Rb and Rc = radii

of the circles a, b and

c respectively[ CS1010E AY1112S1 Lecture 3 ]



Algorithm: A plate with 3 holes

A simple algorithm could be:

12

1. Read R, Ra, Rb and Rc from user

2. Calculate Area = πR2

3. Calculate Areaa = πRa2

4. Calculate Areab = πRb2

5. Calculate Areac = πRc2

6. Calculate Areashaded = Area – Areaa- Areab - Areac

7. Show Areashaded

Duplicate coding!

Step 1 is also another source of duplicated

coding[ CS1010E AY1112S1 Lecture 3 ]



Functions: The basic idea

A function in C: A program unit that performs a well defined task

May take input and produces output

Visualization:

13

Functioninput output

sum

Example:

3

912




Function: Syntax

14

S Y N T A X

result_datatype function_name( [input parameters] )

{

[0 or more declaration statements]

[0 or more other statements]

[return statement]

}

function header

function body




Function Header Function header indicates:

Input (if any) Data type of output result (if any)

15

int sum ( int x, int y )

sum3

912

sum gives

integer result

sum takes two

integer inputs

Function Name is an identifier

The usual identifier naming rule applies




Function Header: Input Parameters

The syntax for input parameters is similar to

declaration statements: Note that the data type is required before every

identifier

16

S Y N T A

X

datatype id1, datatype id2, ...

If the function does not take in any input

You can leave the input parameters blank

OR, you can use void

void is a special data type that means "Nothing"




Function Header: Output Data Type

You need to specify the type of values that a

function will produce

If the function does not return anything, you mustuse a void to indicate that

A function can return at most 1 result directly

Examples of function headers:

17

double areaOfCircle( double radius )

void print_info ( )

void print_info ( void )

void something ( int x, double y, int z )




Function Body: Statements

The statements in a function body:

Follow the rules we have learned so far

Additional variables can be declared

Computation can be expressed by statements

The input parameters:

Work as variables declared in this function

Only difference is that their values are initializedat the point of function call (more on this later)




Some Functions:

19


{int result;

result = x + y;

return result;

}

additional variable

x and y will have well defined values

return the calculated result

void print_info ( )

{ printf("Take a 4-digit number X\n");

printf("Rearrange the digits to get Y\n");

printf("Subtract and get R\n");

return;

}no need to return anything

Print instructions for theuser to follow




Functions in a Program

20

//Preprocessor directive not shown

int sum( int x, int y ){

int result;

result = x + y;

return result;

}

int main( )

{

int input1, input2;

//read input1 and input2 from user

//use sum() to add the two inputs

}

Any number of functions, complete

with header and body can beplaced before the mai n( )

Only restriction: You must

have the function declared

before its point of usage




Function Calls and Execution Flow

21

int sum( int x, int y )

{int result;

result = x + y;

return result;

}

int main( )

{

int input1, input2, output;

//read inputs

output = sum( input1, input2 );

printf("Sum is %d\n", output);

}

3.

4.

5.

2.

1.

6.




Function Calls: Parameters and Arguments

Actual arguments in a function call have a

1-to-1 correspondence with the formalparameters declared in function header

The effect is just like a variable declaration with

initialization in that function

22

int sum( int x, int y ){

............

}

sum( 3, 9 );//a function call

Effectively:int x = 3;

int y = 9;

actual arguments

formal parameter




Function Calls: Returned Result The result returned by a function: is a single value essentially replaces the function call and can be

used in normal arithmetic operations andassignment

23

result = sum( 3, 9 ) + sum( 5, 2 );

result = 12 + sum( 5, 2 );

result = 12 + 7;

result = 19;




Problem: A plate with 3 holes

24

#include <stdio.h>

#define PI 3.14159

double circleArea( double radius ){

double area = PI * radius * radius;

return area;

}

int main( ){

double rPlate, rA, rB, rC;

double areaPlate;

//read rPlate, rA, rB, rC – not shown

//calculate area of plate

areaPlate = circleArea( rPlate ) – circleArea( rA )– circleArea( rB ) – circleArea( rC );

//print areaPlate – not shown

return 0;

}




Functions: Scoping Rules

It is important to realise:

Variables declared in a function are only visible

within the function

25

int function( int x )

{int y;

... ...

}

int main( )

{

int x, y;

}

These variables are totallyindependent of each other.

A common misunderstanding is that by

changing a variable, it can directly affect

another variable of the same name. Thisis NOT true.

You can only pass information into a

function through the actual arguments

in a function call.




Functions: Prototype and Definition

User of a function only needs to know: Function name Number of input parameter and data type

Output data type

Description of function

Actual coding (the function body) is not essentialto the user

C Programming Language uses functionprototype to provide the essential information of a function




Function Prototype: Syntax and Usage

Note: Similar to function header

Semicolon at the end

Identifier of parameters optional

Usage: Ensure the prototype is declared before any actual

function call in the program27

S Y N T A X

rDatatype fname( [parameters datatype] );

E x a m p l e

int sum( int, int );




Function Definition Function definition is: the full coding of the function what we have seen so far

Separation of function prototype and definitionallows: Portability: You only need to show user the prototypes, not the full

coding

Ease of Maintenance: Function definition can be placed in a separate source

code file

Not covered in this course




Function Prototype: Example

29

int sum ( int, int );

// sum() takes two integer X, Y// and return the sum of X + Y

int main( )

{

.........

output = sum ( input1, input2 );.........

}


{int result;

result = x + y;

return result;

}

prototype

function call

definition




Example




Function Design: Guidelines

A good function should:

Rely only on its inputs to produce the output

Perform one task only

be reusable in your program and across programs

Use a function when:

You need to represent a complicated but well

defined step in the algorithm

Program Abstraction

You have found a generally useful task that

can/will be reused

Reusabil ity and Ease of Maintenance




Programming Style: Prototypes!

In this course, you need to:

Supply prototypes before the main() function

Supply definitions after the main() function

Additionally, you need to give a simplecomment to describe the function:

What are the inputs (if any)?

What is the purpose? What is the output (if any)?

A clear but simple description is needed




Programming Style: Modularity

You need to break your program into

reasonable modules (i.e. functions)

known as modularity

Use of functions will be evaluated: Is everything in mai n( ) ?

Is function useful in your program?

Is the design of function meaningful?




C Library

Useful predefined functions



C Standard Library Library:

A collection of functions Prototypes are organized into a header f ile (XXXX.h)

Provides great portability and reusability: User only needs to include XXXX.h in their program to use

those functions

C Standard Library: A set of libraries specified by C specification

All compliant compilers must support them

Provides commonly used functionalities: Input / Output

Mathematical Functions

others ( more later )




Learning to use C Standard Library

As a user, we only need to know information

specified by the function prototype to use it

Our usage of printf(), scanf() are good

examples

We highlight a few useful mathematical

functions here

Learn more by browsing the online C reference

http://www.acm.uiuc.edu/webmonkeys/book/c_guide/






Square Root Function

37

P r o t o

t y p e

double sqrt( double x);

//return the square root of x

H E A D E

R

#include <math.h>

E x

a m p l e double result;

result = sqrt( 4.0 ); //result = 2.0

C o m p i l a t i o n

gcc yourFile.c -lm




Power Function

38

P r o t o

t y p e

double pow( double base, double exp );

//return the baseexp

H E A D E

R

#include <math.h>

E x

a m p l e double result;

result = pow( 2.0, 4.0 ); //result = 16.0




A lot more……

Trigonometry functions: sine, cosine, etc….

Logarithm functions:

log10, loge, exponential, etc….

Ceiling, Floor, Absolution value etc….

Reminder: Learn to make use of standard library

Don’t reinvent the wheel




Some Math Library Functions

Some Useful Math Library Functions (compiled with –lm option)

function abs(x) from <stdlib.h>; the rest from <math.h>




Summary

C

E l e m e n t s

Function

- Header and Body

- Function call

- Prototype and Definition

Library

- Introduction to Math Library

P r o g r a m m i n g S t y l e

Separate Function Prototype and Definition

Give function description in comment

Modularity




Reference

Problem Solving and Program Design in C” –

Jeri R.Hanly & Elliot B.Koffman, 6th Edition,Pearson

Chapter 3

Online C reference:http://www.acm.uiuc.edu/webmonkeys/book/c_guide/



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