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Engineering Problem Solving With C++

An Object Based ApproachChapter 5

Functions

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Functions

• A program can be thought of as a collection of sub parts or sub tasks:

• input data• analyze data• output results

• In C++ these sub tasks are often organized into functions.

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Functions

• Complex problems can be broken down into sub tasks, each of which is easy to implement in C++.

• What are some other advantages to using functions, as opposed to writing the entire solution in main?

–Multiple programmers

–Testing/Debugging/Maintaining

–Reduce duplication of code

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Functions

• Pre-defined– standard libraries

• User defined

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Pre-defined Functions - Example

#include <iostream>#include <cmath>using namespace std;int main(){ double angle; cout << “input angle in radians: “; cin >> angle; cout << “\nthe sine of the angle is “ << sin(angle) << endl; return 0;}//end main

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Programmer Defined FunctionsTerminology

• Function Prototype– describes how a function is called

• Function Call• Function Arguments

– used in the function call• Function Definition

– function header– function body

• Formal Parameters– used in function definition

• Formal parameters must agree with arguments in order, number and data type, but the identifiers can be different.

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Programmer Defined Functions

• Can be defined to– return a single value to the calling function– perform a task – change the value of multiple variables

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Value Returning Functions

• A function returns a single value to the calling program

• The function header declares the type of value to be returned

• A return statement is required in the body of the function

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n! example

• n! = n*(n-1)*(n-2)*…*1

• n is a positive integer

• 0! is 1 by definition

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//function definition: n! = n*(n-1)*(n-2)*…*1, // 0! is 1 by definition//Function fact returns n! //Function fact assumes n is non-negative integerint fact(int n) //function header, NO SEMICOLON{ int nfact = 1; while(n>1) { nfact = nfact*n;

n--; }//end while block return(nfact);}//end fact

Example - factorial function

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Calling a function - a function prototype is required

int fact(int n); //function prototype, semicolon required// parameter identifier is optional

#include <iostream>using namespace std;int main(){ int n; cin >> n; if(n>=0)

cout<<n<<“! is ” <<fact(n)<<endl; //n is the argument else

cout <<“factorial not defined for negative numbers” <<endl;

return 0;}//end main

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Calling a function- second example

int fact(int); //function prototype, //parameter identifier is optional

#include <iostream>using namespace std;int main(){ int n, factorial; cin >> n; if(n>=0) { factorial = fact(n); //function call

cout << n <<“! is ” << factorial << endl; } else

cout << “factorial not defined for negative numbers” << endl;

return 0;}//end main

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2 Points of Style When Writing Value Returning Functions

• Formal parameters are used to pass information to the function. cin statements are usually not required.

• A return statement returns a value to the calling program. cout statements are usually not required.

• Use library functions as model (sin, log, etc)

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void Functions

• A void function may be called to• perform a particular task (clear the screen) • modify data• perform input and output

• A void function does not return a value to the calling program

• return statement is optional• if a return statement is used, it has the following

form– return;

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Example of void function//output formatted date//function definitionvoid print_date(int mo, int day, int year) //function header{ string month;

switch(mo){ case 1:

month = “January”; break;

case 2: month = “February”; break;

…case 12:

month = “December”;}//end switch

cout << month << ‘ ’ << day << “, << year << endl; return; //return is optional

} //end print date

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Parameter Passing - pass by value

• Pass by value– the default in C++ (except when passing

arrays as arguments to functions)– formal parameter receives the value of the

argument– changes to the formal parameter do not affect

the argument

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#include <iostream>using namespace std;int fact(int); //function prototypeint main(){ int n, factorial;

cin >> n;if(n>=0) { factorial = fact(n); //function call

cout << n <<“! is “ << factorial << endl;}//end if

return 0;}//end mainint fact(int n) //function header, NO SEMICOLON{ int nfact = 1; while(n>1) { nfact = nfact*n;

n--; }//end while block return(nfact);} //end fact

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Parameter Passing - pass by reference

• Pass by reference– append an & to the parameter data type in

both the function prototype and function header

void get_date(int& day, int& mo, int& year)

– formal parameter receives the address of the argument

– any changes to the formal parameter directly change the value of the argument

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Example - pass by reference

#include <iostream>using namespace std;

void swap(double&, double&); //function prototypeint main(){ double x=5, y=10; swap(x,y); //function call; x y are arguments cout >> “x = “ << x << ‘,’ << “ y= “ << y << endl; return 0;}//end main

Output is:x = 10, y = 5

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Example - pass by reference

//Function swap interchanges the values of two variables//function definitionvoid swap(double& x, double& y) //function header{

double temp; //local variable temptemp = x;x=y;y=temp;return; //optional return statement

}//end swap

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Practice! - What is the output?#include <iostream>using namespace std;void fun(int&, int&, int);int main(){ int c1=1, c2=2, c3=3;

cout << c1 << ‘,’ << c2 << ‘,’ << c3 << endl;fun(c1,c2,c3);cout << c1 << ‘,’ << c2 << ‘,’ << c3 << endl;fun(c3, c2, c1);cout << c1 << ‘,’ << c2 << ‘,’ << c3 << endl; return 0;

}void fun(int& a1, int& a2, int a3){ a1++;

a2++;a3--;

}

1,2,32,3,32,4,4

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Storage Class and Scope

• Scope refers to the portion of the program in which it is valid to reference a function or a variable

• Storage class refers to the lifetime of a variable

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Scope

• Local scope - a local variable is defined within a function or a block and can be accessed only within the function or block that defines it

• Global scope - a global variable is defined outside the main function and can be accessed by any function within the program file. Strongly discouraged!!