Cosc175/module.ppt1 Introduction to Modularity Function/procedures void/value-returning Arguments/parameters Formal arguments/actual arguments Pass by.

cosc175/module.ppt 1

Introduction to Modularity

• Function/procedures

• void/value-returning

• Arguments/parameters

• Formal arguments/actual arguments

• Pass by value/pass by reference

• Scope: global/local/name precedence


Modularity• Most programs so far have been simple -

less than one page in length

• In reality, most problems are not so simple

• top-down design - identify first the major tasks and further subtasks within them

• Modularity – breaking a program into subprograms


Module

• section of an algorithm which is dedicated to a single function

1. performs one single function

2. single entry, single exit

3. short enough to be easily read and modified

4. long enough to perform function


Modules• all tasks can be subdivided and further

subdivided into subtasks

• goal => can easily construct pseudocode for each subtask

• each module constructed and tested as unit

• Black Box

• Stubbing in (write dummy modules)– Always have something working


good names help

• Begin with Verb!

• Use several short words– PrintPageHeadings– CalcSalesTax– ValidateInputDate


Mainline or driver

• call subtasks

• should show the main processing functions, the order they should be performed

• easy to read

• manageable length

• Should include a loop!


Calling a subprogram/module

int main(){

… //line1DoProc1(); //line2…. //line3

} // end main //line4//***************************void DoProc1() { //line5 ….. //line6 ….. //line7} // end DoProc1 //line8

• line2 Invokes or calls the subprogram

• Control is transferred to the subprogram: line5

• The code in the subprogram is executed: line 6,7,8

• At line8, control returns to statement following call, line 3

• 1,2,5,6,7,8,3,4


Hierarchy Chart shows who calls whom

• illustrates structure of the solution to a problem– i.e organization chart for a company

• shows top-down design, communication flow• One block for each module• Program name in first block, begin with verb


int main() { . . DemoC(); DemoB(); DemoA(); return 0; } //*************** void DemoA() { . . } //**************** void DemoB() { . . } //**************** void DemoC() { . . }

DoStuff

DemoA DemoB DemoC


Writing larger, more complex Programs1. Define the problem

– Input/Output– Processing => list of activities to be performed

2. Plan1. Modularize => Hierarchy chart

• Group list of activities into modules

2. Construct code for main-line:Initial processingLoop

processing //Contains calls to major processing modules(stubs)END LoopFinal Processing

3. Construct code for each successive module in hierarchy chart 4. Desk-check each module in top-down fashion


advantages of well structured programs:

– can easily be changed, updated and maintained– division into tasks makes them easy to

understand– easy to construct– can test modules individually– easy to test and debug - easier to isolate errors

• can have each module print input and output

– more reliable - fewer bugs


void function Value-returning function

return Can return one or more values

GetVals(x,y,z)

returns one result

return x * x

name Begins with a verb

CalcSquare(..)

Noun

Square(..)

C++ void function

void CalcSquare(..)

Value returning function

float Square(..)

call stand-alone

CalcSquare(..)

Call is part of expression

x = Square(..)

misc Multipurpose Usually mathematical


void function example// prints lines of *’s where numLines specifies

// how many lines to print

void PrintLines( int numLines )

{

int count;

for (count = 1; count <=numlines; count++)

cout << "***************" << endl;

}


function as part of a programint main() { PrintLines(2);

cout << " Welcome Home!“ << endl;PrintLines(4);return 0;

}//***********************************************// prints lines of *’s// numLines specifies how many lines to printvoid PrintLines( int numLines ){ int count; for (count = 1; count <=numlines; count++) cout << "***************" << endl;} //End PrintLines


Value returning Function examples

//******************************//this function returns the cube of xint Cube (int x){ return x * x * x;}

//************************************************ //this function returns the maximum of 2 numbersint Max (int num1,int num2){ int max;

if (num1 > num2 ) max = num1;

else max = num2;

return max;}


Functions in a program void Show Funcs(){ int num1,num2;

cout << “Enter two numbers” cin >> num1 >> num2; cout << "The max of " << num1 << " and << " num2 " is " << Max(num1,num2); cube = Cube(num1) cout << "The cube of " << num1 << " is " << Cube(num1);}//**********************************************//this function returns the cube of xint Cube (int x){ return x * x * x;}

//************************************************ //this function returns the maximum of 2 numbers

int Max (int num1,int num2) {

int max; if (num1 > num2 )

max = num1; else

max = num2; return max;}


When to use void versus value-returning

• When returning more than one value - use void

• when I/O is required - use void• returning one Boolean value – value-

returning• returning one value to be used immediately

in an expression – value-returning• when in doubt - use void


arguments or parameters• In function definition (formal arguments):

void Name(type arg1, type arg2,…type argn)

• In call to function (actual arguments)Name(arg1,arg2,…argn)

• Arguments must match in number, order and data type but not name

• Can be 0,1, or many arguments


Formal and Actual Arguments

• Formal Argument• In the definition

– void PrintLines(int lines)

• Actual Argument• In the call

• PrintLines(2); // could be constant:• PrintLines(num); //could be variable:• PrintLines(num+2) //could be Expression

• Note: actual parameter and formal arguments may have different names


Multiple parameters:

• matched by position

• each param must be declared:void PrintLines(int numLines,char whichChar);

PrintLines(3,’$’);


Example

• Read three characters:

• Design a solution algorithm which will: prompt a terminal operator for three characters, accept those characters as input, sort them in ascending sequence and output them to the screen. The algorithm is to continue to accept characters until ‘XXX’ is entered.


Input Processing Output

char1 1. Prompt for characters

Sorted char1,char2,char3

char2 2. Accept three characters

char3 3. Sort three characters

4.Output three characters


void ProcessThreeChars(){ char char1,char2,char3; cout << “Enter three characters”; cin >> char1 >> char2 >>char3; while (!(char1 == ‘X’ && char2 == ‘X’ && char3 == ‘X’)) { if (char1 > char2 )

{ temp = char1 char1 = char2 char2 = temp } if (char2 > char3)

{ temp = char2 char2 = char3 char3 = temp } if (char1 > char2) { temp = char1 char1 = char2 char2 = temp } cout << char1 << char2 << char3; cout << “Enter three characters”; cin >> char1 >>char2>>char3; }}


void ProcessThreeChars(){ char char1,char2,char3; cout << “Enter three characters”; cin >> char1 >> char2 >>char3; while (!(char1 == ‘X’ && char2 == ‘X’ && char3 == ‘X’)) { SortThreeCharacters(char1,char2,char3)

cout << “Enter three characters”; cin >> char1 >>char2>>char3; }}


void SortThreeCharacters(char& c1, char& c2,char& c3){

char temp; if (char1 > char2 )

{ temp = char1 char1 = char2 char2 = temp } if (char2 > char3)

{ temp = char2 char2 = char3 char3 = temp } if (char1 > char2) { temp = char1 char1 = char2 char2 = temp }}


two modules -

• main module - ReadThreeCharacters

• submodule - SortThreeCharacters

• after processing is complete, control is returned to main

• naming the module - passes control to the module


• module invokes or calls subordinate modules– calling module– called module - return control to calling module

upon completion of its task

• module may only call modules that are at the same level and immediately below it

• exception - library or utility modules (later)• in general, no module should have more

than seven modules subordinate to it


Parameters

• in, in out, out• in - value parameters

– pass by value

– function receives a copy of the value

• in out, out - reference parameters– pass by reference

– attach ampersand to data type, int& param

– function receives the address of the actual parameter


Value Parameters• void PrintLines(int numLines)

– numLines is formal parameter• void PrintLines(lineCount);

– lineCount is actual parameter• formal parameter receives a copy of the value of

lineCount• PrintLines cannot change lineCount• # of actual params must match # of formal params• types of actual params should match types of formal

params– if not, implicit type coercion


Reference Parameters

• use & (C++ convention)

• function can change the value

• pass by reference

• location of parameter is passed

• only one copy of the value

• only variables can be passed as an actual parameter to a reference parameter


void SwapNums (){ int x,y; x = 5; y = 10; cout << "Originally x = “ << x << " and y = “ << y; Swap(x,y); cout << "Now x = " << x << " and y = " << y;}void Swap(int u,int v) int temp; temp = u; u = v; v = temp;}

Originally x = 5 and y = 10Now x = 5 and y = 10


void SwapNums (){ int x,y; x = 5; y = 10; cout << "Originally x = “ << x << " and y = “ << y; Swap(x,y); cout << "Now x = " << x << " and y = " << y;}void Swap(int& u,int& v) int temp; temp = u; u = v; v = temp;}

Originally x = 5 and y = 10Now x = 10 and y = 5


/// This program outputs an appropriate activity for a given temp./#include <iostream.h>void GetTemp( int& ); // Function prototypesvoid PrintActivity( int );int main(){ int temperature; // The outside temperature

GetTemp(temperature); // Function call PrintActivity(temperature); // Function call return 0;}//******************************************************// Prompt for, get, and echo current temperaturevoid GetTemp( int& temp ) // Reference parameter{ cout << "Enter the outside temperature:" << endl; cin >> temp; cout << "The current temperature is " << temp << endl;}


//******************************************************// Given temperature, print appropriate activityvoid PrintActivity( int temp ) // Value parameter{ cout << "The recommended activity is "; if (temp > 85) cout << "swimming." << endl; else if (temp > 70) cout << "tennis." << endl; else if (temp > 32) cout << "golf." << endl; else if (temp > 0) cout << "skiing." << endl; else cout << "dancing." << endl;}


Scope

• area of program where a variable is visible

• global - visible to all modules– declared in or above the main module

• local - visible only to the module in which it appears

• side effects - cross-communication of a module with other parts of a program


Local Variables

• each function is a block

• any function can declare variables within block

• accessible only within the block they are declared

• occupy space only when the function is executing

• when the function is invoked - local vars created

• when function is finished - local vars destroyed


global variables

• declared outside all functionsint gamma;

int main(){

}void SomeFunc(){ .}

can be accessed from main or SomeFunc


• It is possible to have identifiers with the same name both in the main program and the subprogram:

• name precedence - a local identifier in a module takes precedence over a global identifier with the same spelling in any reference the procedure makes to the identifier

Cosc175/module.ppt1 Introduction to Modularity Function/procedures void/value-returning Arguments/parameters Formal arguments/actual arguments Pass by.

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