04 0 revisit terms and concepts of functions

Terms and Concepts of Functions | SCP1103 Programming Technique C | Jumail, FSKSM, UTM, 2006 |Last Updated: August 2006 Slide 1

Revisit Terms andConcepts of Functions

byJumail Bin Taliba

Faculty of Computer Science & Information System


Page 1

AVRG (average, 10, 5, N)

Start

Stop

ReadN

Printaverage

AVRG ( result,n1, n2,n3)

Return

sum = n1+ n2+n3

result = sum/3

How a function works?Example: What is the output of the following flowchart when the input is N = 6

average

10

5

N=6

Sum = 10 + 5 + 6

average = 21/3

Output:Average: 7

This flowchart calculates the average of three numbers


Thinking in “function way”Example: Create a flowchart to calculate the average of three numbers

Page 1

AVRG (result, n1, n2,n3)

Start

Stop

Readn1, n2 , n3

Printresult

Page 2

AVRG ( result,n1, n2,n3)

Return

sum = n1+ n2+n3

result = sum/3

End terminal must be a “Return”

Start terminal for afunction is different.Do not use “Start”

The detail of how the function should work is put into another flowchart.

This flowchart is known as a function-definition

This part is known as a function-call

At this point, we only focus on whatwe are going to do. But, how it is going to be done, we focus later.

This part is called body of function. It is nothing much differentfrom main flowchart



Function Name and Parameters

Page 1


Start

Stop

Readn1, n2 , n3

Printresult

Page 2

AVRG ( R, a, b,c)

Return

sum = a+ b+c

R = sum/3

AVRG is the function name

Objects enclosed by ( ) – result, n1, n2, n3 - are called parameters

Parameters used in a function-callare called actual parameters

result, n1, n2, n3 are actual parameters

Parameters used in a function-definitionare called formal parameters

R, a, b, c are formal parameters

Each formal parameter represents an actual parameter according to its order:

R represents result,a represents n1,b represents n2,c represents n3

The name of an actual parameter may bedifferent from its formal parameter



Page 1


Start

Stop

Readn1, n2 , n3

PrintR

Page 2

AVRG ( R, a, b,c)

Return

sum = n1 + n2 + n3

R = sum/3

In a function-definition, you should only use formal parameters – R, a, b, c

You shouldn’t use actual parameters

This is wrong!n1, n2, n3 are actual parameters.Should use a, b, c instead.

This is wrong!R is an formal parameters.Should use result instead.


Page 1

AVRG (average1, n1, n2,n3)

Start

Stop

Readn1, n2 , n3

Printresult

Readn4, n5 , n6

AVRG (average2, n4, n5, n6)

result = (average1 +average2) / 2

Page 2

AVRG ( R, a, b,c)

Return

sum = a+ b+c

R = sum/3

A function may be called more than once

At this time:R represents average1,a represents n1,b represents n2,c represents n3

When comes to this:R represents average2,a represents n4,b represents n5,c represents n6



Input and Output Parameters

Each function parameter may act as:

• Inputdata to be processed by the function

• Outputthe result produced by the function

• Both


Page 1


Start

Stop

Readn1, n2 , n3

Printresult

Page 2

AVRG ( R, a, b,c)

Return

sum = a+ b+c

R = sum/3

Function call:

result is the output parameter. n1, n2, n3 are the input parameters.

Function definition:

R is the output parametera, b, c are input parameters



Page 1 Page 2

EXCHG( x, y)

Return

hold = x

x = y

y = hold

Start

Stop

Printp, q

EXCHG (p, q)

Readp, q

Function call:

p and q act as both input and output parameters.

Function definition:

x and y act as both input and output parameters

This flowchart exchanges or swaps the value of x and y each other


Page 2

AVRG ( R, a, b,c)

Return

sum = a+ b+c

R = sum/3

If there is only one output parameter, the flowchart may “RETURN” the result

Example: let’s take a look again at the function that calculates the average of three numbers.

Original function flowchart:

Page 2

AVRG ( a, b,c)

Return R

sum = a+ b+c

R = sum/3

Since it has only one output, the output may be “RETURN”

The output parameter (R) is removed from the formal parameter list

and the result is return


Since the function-definition has been modified, the way of the function to be called will also be changed

Original main flowchart: Modified main flowchart:

Page 1


Start

Stop

Readn1, n2 , n3

Printresult

Page 1

result =AVRG (n1, n2, n3)

Start

Stop

Readn1, n2 , n3

Printresult

Now, result is notanymore a parameterof the function-call


Writing a C Program is a systematic task

Problem

Flowchart

Intermediate code-prepare a rough code-

Complete code- add details -

Converting function flowcharts to C Code


Converting flowchart to C code is almost straight forward

Example: multiplying two numbers

Start

Stop

Read ARead B

Display theResult C

Calculate ResutC=A*B

void main (void){ scanf(&A);

scanf(&B);

C = A * B;

printf(C);

}

Intermediate C Code

But!, the program still cannot be executed.It is not completed yet. This code is calledan intermediate code.


You will get some errors

Error 1: Call to undefined function ‘scanf’The compiler doesn’t recognize ‘scanf’

Error 2: Undefined symbol ‘A’The program is trying to use a variable A but has never been registered. Compiler doesn’t recognize the variable


Fixing the errors and completing the program

This line will help the compiler to recognize words ‘scanf’ and ‘printf’. File stdio.h contains the information of those functions and some others.

This tells to register (declare) variables. Compiler only recognizes registered variables.

You may notice some extra things.These are called prompts. They used to let the user knows what is going on while the program is running


Converting function flowcharts

Problem: Finding the average of three numbers

Flowcharts:

Start

Stop

Readn1, n2 , n3

Av erage(av rg, n1, n2, n3)

Printav rg

Av erage(result, a, b, c)

Return

sum = a + b + c

result = sum / 3

Example:


Start

Stop

Readn1, n2 , n3

Av erage(av rg, n1, n2, n3)

Printav rg

void main (){ scanf(&n1,&n2,&n3);

Average(&avrg, n1, n2, n3);

printf(avrg);

}

Intermediate code of the main flowchartPreparing the rough code

The ampersand (&)indicates that avrg is the output or resultof the function-call.


Av erage(result, a, b, c)

Return

sum = a + b + c

result = sum / 3

void Average(*result, a, b, c){

sum = a + b + c;

*result = sum / 3.0;

return;}

Intermediate code of the function flowchartPreparing the rough code

The asterisk (*)indicates that result is an output parameter.


#include <stdio.h>

void Average(float *result, float a, float b, float c){ float sum;

sum = a + b + c; *result = sum/3.0; return;}

void main (){ float n1; float n2; float n3; float avrg;

printf ("Enter three numbers: “); scanf(“%f%f%f”, &n1, &n2, &n3);

Average(&avrg,n1,n2,n3);

prinf("The average is %f“, avrg);}

Complete source codeAdding details to the rough code. The details are shown by bold texts


Example: This example is the same as the previous one but this time the result is “return”

Problem: Finding the average of three numbers

Flowcharts:

START

STOP

READn1, n2 , n3

avrg = Average (n1, n2, n3)

Printavrg

Average (a, b, c)

Return result

sum = a + b + c

result = sum / 3


START

STOP

READn1, n2 , n3

avrg = Average (n1, n2, n3)

Printavrg

void main (){ scanf(&n1,&n2,&n3);

avrg = Average(n1, n2, n3);

printf(avrg);

}

Intermediate code of the main flowchartPreparing the rough code


Average (a, b, c)

Return result

sum = a + b + c

result = sum / 3

Average(a, b, c){

sum = a + b + c;

result = sum / 3.0;

return result;}

Intermediate code of the function flowchartPreparing the rough code

Notice that, • result is removed from parameter list• “void” is also removed


#include <stdio.h>

float Average(float a, float b, float c){ float sum; float result;

sum = a + b + c; result = sum/3.0; return result;}

void main (){ float n1; float n2; float n3; float avrg;

printf ("Enter three numbers: “); scanf(“%f%f%f”, &n1, &n2, &n3);

avrg = Average(n1,n2,n3);

prinf("The average is %f“, avrg);}

Complete source codeAdding details to the rough code. The details are shown by bold texts

04 0 revisit terms and concepts of functions

Technology

main flowchart preparing

function flowchart preparing

formal parameters

void main

void main

actual parameter

represents n3

represents n2