CSC 012

Introduction to Computer Science

Summary of 3/4 Lecture

The Selection Structure

We then introduced decision making into the computer's vocabulary.   That is, the computer's calculations and processing will be determined by the input to the program.  We began by reviewing a familiar problem.

NOTE:  All of the following require client classes.

Example 1.  Write a program that takes the user's income as input and then calculates and outputs the resulting tax.

//File:    Taxes.java
import java.text.DecimalFormat;
import CSLib.*;
public class Taxes
{
    public static void compute()
    {
        //Input dialog with user
        InputBox in = new InputBox ( "Salary Input" );
        in.setPrompt ( "How much did you make last year?" );
        double salary = in.readDouble ();
       
        //Calculate the tax due
        final double RATE = 0.2; //20% Tax Rate
       
        double tax = RATE * salary;
       
        //Output the results
        OutputBox out = new OutputBox ( "Your Taxes" );
        out.setSize ( 250, 75 );
        out.setLocation ( 300, 300 );
       
        DecimalFormat guy = new DecimalFormat ( "0.00" );
       
        out.println ( "Your income was: $ " + guy.format ( salary ) );
        out.println ( "Your tax due is: $ " + guy.format ( tax ) );
    }
}

Selection Structure

Example 2.  Modify the previous program to accommodate a 2-tiered tax structure.

//File:    Taxes2.java
import java.text.DecimalFormat;
import CSLib.*;
public class Taxes2
{
    public static void compute()
    {
        //Input dialog with user
        InputBox in = new InputBox ( "Salary Input" );
        in.setPrompt ( "How much did you make last year?" );
        double salary = in.readDouble ();
       
        //Calculate the tax due
        final double RATE1 = 0.15; //15% Tax Rate
        final double RATE2 = 0.25; //25% Tax Rate
        final double BRACKET = 50000.0;     //separates low income
                                                                     //from high income
       
        double tax;
       
        RATE1 = 0.2;
       
        //if salary is under 50000 then
        if (salary < BRACKET)
            tax = RATE1 * salary;
        //or else
        else
            tax = RATE1 * BRACKET
                + RATE2 * (salary - BRACKET);
       
        //Output the results
        OutputBox out = new OutputBox ( "Your Taxes" );
        out.setSize ( 250, 75 );
        out.setLocation ( 300, 300 );
       
        DecimalFormat guy = new DecimalFormat ( "0.00" );
       
        out.println ( "Your income was: $ " + guy.format ( salary ) );
        out.println ( "Your tax due is: $ " + guy.format ( tax ) );
    }
}

Note that the calculation section has grown from a one-liner to a if-else statement taking several lines. 

We then considered the problem of converting a score between 0 and 100 into a corresponding letter grade.

Example 3.

//File:    Grades.java
import CSLib.*;
public class Grades
{
    public static void compute()
    {
        //input score
        InputBox in = new InputBox ( "Score" );
        in.setPrompt ( "Enter score (between 0 and 100)" );
        int score = in.readInt ();
       
        //calculate letter grade
        char grade = ' '; //letter
       
        if ( score >= 90 && score <= 100 )
            grade = 'A';
        else if ( score >= 80 && score < 90 )
            grade = 'B';
        else if ( score >= 70 && score < 80 )
            grade = 'C';
        else if ( score >= 60 && score < 70 )
            grade = 'D';
        else if ( score >= 0 && score < 60 )
            grade = 'F';
       
        //output results
        OutputBox out = new OutputBox ( "Grade Conversion" );
        out.println ( "Your score was " + score );
        out.println ( "Your grade is " + grade );
    }
}

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