PIC10A Lecture9

PIC10A Lecture 9

Reading

p117-125,p19

Scope Rules

Local Variables

Variables can be declared in function body(example from last lecture)

double finalScore(double midterm, double exam) {    
   double result1 = 0.5*midterm + 0.5*exam;    
   double result2 = 0.4*midterm + 0.6*exam;    
   // the final score is the max of result1 and result2    
   if(result1 >= result2) 	
       return result1;    
   else 	
       return result2;
}

The variables declared within a function are said to be "local to the function". They are also called local variables
Their scope(lifetime) is limited to the function
main is a (special) function. All variables declared here are "local to main".
Other parts of the program do not know local variables exist; you can repeat names

What happens if you have one variable in a function and one variable in main with the same name?

#include<iostream>
using namespace std;
void f();

int main() {
    int x;
    x = 5;
    f();
    cout << "In main, x is " << x << ".\n";
    return 0;
}

void f() {
    int x = 2;
    cout << "In f, x is " << x << ".\n";
}

Output

In f, x is 2.
In main, x is 5.

Although there are 2 x in the program, the x in main is different from the x in the function f.
variables in f (resp main) is local to f (resp main).

The scope of x in main

#include<iostream>
using namespace std;
void f();


int main() {
    int x;
    x = 5;
    f();
    cout << "In main, x is " << x << ".\n";
    return 0;
}

void f() {
    int x = 2;
    cout << "In f, x is " << x << ".\n";
}

The scope of x in f

#include<iostream>
using namespace std;
void f();

int main() {
    int x;
    x = 5;
    f();
    cout << "In main, x is " << x << ".\n";
    return 0;
}


void f() {
    int x = 2;
    cout << "In f, x is " << x << ".\n";
}

What is the advantage?
User of functions doesn't have to worry about variable names he choose.

Constants

#include<iostream>
using namespace std;

int main() {
    const double TAX_RATE = 0.25;
    double income;
    cout << "Please enter your income in year 2001: ";
    cin >> income;
    cout << "Your tax is " << income*TAX_RATE << ".\n";
   return 0;
}

Output

Please enter your income in year 2001: 33259
Your tax is 8314.75.

Syntax

const type variableName = value;

initialize variables so that it cannot be changed
If you programs try to change one of these variables, it produces a compiling error
This make sure that you won't change constants accidentally
Once a constant is declared, it can be used anywhere the number is allowed and it will have exactly the same meaning as the number it names.
Most people use all uppercase letters for constants.

An example with error. You can't compile the program.

#include<iostream>
using namespace std;

int main() {
	const double TAX_RATE = 0.25;
	TAX_RATE = 0.33; // error!!!! You can't reassign value to a constant
	return 0;
}

Global constants and global variables

#include<iostream>
#include<cmath>
using namespace std;

const double PI = 3.14159;

// returns the area of a circle with the specified radius
double area(double radius);

// returns the volume of a sphere with the specified radius
double volume(double radius);


int main() {
    double radius;

    cout << "Enter a radius (in inches): ";
    cin >> radius;

    cout << "Radius = " << radius << " inches\n"
         << "Area of circle = " << area(radius)
         << " square inches\n"
         << "Volume of sphere = " << volume(radius)
         << " cubic inches\n";

    return 0;
}

double area(double radius) {
    return PI*pow(radius, 2);
}

double volume(double radius) {
    return 4.0/3*PI*pow(radius,3);
}

Sample Output

Enter a radius (in inches): 2
Radius = 2 inches
Area of circle = 12.5664 square inches
Volume of sphere = 33.5103 cubic inches

variables declared inside a function body is a variable local to the function.
variables declared outside of any function body (and outside the functin body of main) are called global variables
Global variables can be used can be used anywhere in your program, i.e. the scope of global variables is the whole program.
although you can place declarations of global variables anywhere outside the function bodies, it is better the place them before the main.

Variables declared in a for loop

A variable can be declared in the heading of a for statements so that the variable is both declared and initialized at the start of the statments. example

for(int n = 0; n <= 10; n++)
   sum = sum + n;

The variable n is local to the body of loop.

Call-by-value parameter

What is the output of the following program?

#include<iostream>
using namespace std;

void increase(int);

int main() {
    int x = 1;
    cout << "Before calling the increase function, x is " 
         << x << ".\n";
    increase(x);
    cout << "After calling the increase function, x is "
         << x << ".\n";
    return 0;
}

void increase(int x) {
    x++;
}

Output

Before calling the increase function, x is 1.
After calling the increase function, x is 1.

The codes will be explained during the lectures.