local vs global variables

Local vs Global variables

• Local variable– Known only to the function in which it is

declared– Cannot be accessed outside the function

Local vs Global variables

• Global variable– Known to all functions in the same file– Can be known to other functions in other files– Cannot be accessed inside a function if that function

has a variable by the same name, unless the scope resolution operator is used

#include <iostream.h>

void Func(void);void main( ){ int Num = 1; // Num is a local variable cout << “in main, Num is” << Num << endl; Func( ); cout << “back in main, Num is still” << Num; }

void Func(void)

{ int Num = 20; // Num is a local variable

cout << “In Func, Num is” << Num << endl;}

#include <iostream.h>void Func(void);void main( ){ int Num = 1; // Num is a local variable cout << “in main, Num is” << Num << endl; Func( ); cout << “back in main, Num is still” << Num;}

void Func(void)

{ int a = 20; // Num is not known here

cout << “In Func, Num is” << Num << endl;}

Global variablesVariables that are defined outside a function

void Func( ); // function prototypeint Num =2; // global variablevoid main( ){ cout << “In main, Num is” << Num << endl; Func(); cout << “Back in main Num is ” << Num ; return 0; }

// the first cout displays 2 for Num

2

Num


void Func( ){ cout << “In Func, Num is” << Num << endl; Num = 50; cout << “But, it is now changed to” << Num; }

// the first cout displays 2// the second cout displays 50

2

Num

Global above main




50

Num


void Func( ); // function prototypeint Num =2; // global variablevoid main( ){ cout << “In main, Num is” << Num << endl; Func(); cout << “Back in main Num is ” << Num ; return 0; }

// the first cout displays 2 for Num// the second cout displays 50 for Num

50

Num2


void Func( ); // function prototypevoid main( ){ cout << “In main, Num is not visible”; Func(); cout << “Back in main Num is not visible” ; return 0; }

int Num =2; // global variable defined between // main and Func




int Num =2; // global variable defined between // main and Func

Scope resolution operator ::

int Val = 1; void main( ){ int Val = 100; cout << “The local variable is set to ” << Val << endl; // displays 100 cout << “The global variable is set to ” << ::Val ; // displays 1

Scope resolution operator ::

int Val = 1; void main( ){ int Val = 100; cout << “The local variable is set to ” << Val << endl; // displays 100 cout << “The global variable is set to ” << ::Val; // displays 1

Storage classification of variables

• auto default type, automatic variables are the ones we’ve have been using

• extern tells the computer that the variable is defined elsewhere in the program

• register tells the computer to use one of the CPU’s registers

• static persists (it’s value remains) even after leaving function

double Value = 3.5;

void main( )

{ extern double Value; // unnecessary in this use

cout << “The value is” << Value << endl;

register int Number; // use arithmetic/logic unit

for (Number =2; Number <=120; Number*=2)

cout << Number << endl; }

void main( ){ ShowLocal( ); ShowLocal( ); return 0;}void ShowLocal(void){ int LocalNum=5; cout << “localNum is “ << LocalNum << endl; LocalNum = 99; } // displays 5 always

void ShowLocal(void);

void main( ){ for (int Count=0; Count < 5; Count++) ShowStatic( ); // function is called 5 times}void ShowStatic(void){ static int StatNum=5; // initialized only once cout << “StatNum is “ << StatNum << endl; StatNum ++; } // value persists // 5 6 7 8 9 will display for StatNum value

void ShowStatic(void);

Default arguments

void DisplayStars(int = 10, int =1);void main( ){ DisplayStars( ); cout << endl; DisplayStars(5); cout << endl; DisplayStars(7,3); }

void DisplayStars(int Cols, int Rows){ for (int Down = 0; Down < Rows; Down++) { for (int Across=0; Across < Cols;Across++) cout << ‘*’; cout << endl; } }

Returning a value from a function

A function may return a value back to the part of the program that called the function by using a return statement

Argument

Argument

Argument

Argument

FunctionReturn Value

Returning a value from a function

int Square (int); // function prototypevoid main(void){ int Value, Result; cout << “Enter a number and I will square it:”; cin >> Value; Result = Square (Value);

int Square (int Number){ return Number * Number; }

cout << Value << “squared is” << Result;}

Write a program that asks the user to enter a number and calls a function which receives the number. The purpose of the function is to determine if the number is even or odd. The function will return a 0 or false if the number is odd or a 1 or true if the number is even.

#include <iostream.h>

bool IsEven(int);void main( ){ int Val; cout << “Enter an integer”; cin >> Val; if ( IsEven(Val)) cout << Val << “is even\n”; else cout << Val << “is odd\n”; }

0 or 1

bool IsEven(int Number){ if ( Number % 2 ) return 0; else return 1;}

Number has a copy of whatever is inside Val in main

5

5Val



5

5Val

5/2 is 2 with a remainder of 1



36

36Val

36/2 is 18 with a remainder of 0

local vs global variables

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