Develop Reference

How to iterate to find the lowest value

Struggling to understand where I went wrong with the iteration at the get best fare method
The array holds [5.77, 2.44, 2.35] and should return the second index, however it seems that it is stuck at the double lowestPrice = lowestPriceRide[0];
I thought that maybe I was putting the return out of scope, but it didn't work.
> import java.lang.*;
import java.util.Arrays;
public class TransitCalculator {
double numberOfDays = 0.0;
double numberOfRides = 0.0;
double pricePerRide = 2.75;
double pricePerWeek = 33.00;
double priceUnlimited = 127.00;
double perRide = 0.00;
public TransitCalculator(double days, double rides){
numberOfDays = days;
numberOfRides = rides;
}
public double unlimited7Price(){
double numOfWeeks = Math.ceil(numberOfDays/7) ; // Math.ceil will return the largest integer that is divisble without a remainder //
double totalPrice = numOfWeeks * pricePerWeek;
return totalPrice / numberOfRides;
}
public double[] getRidePrices(){ // 28/06/2020 Sunday. Math is verified.
double perRide = pricePerRide * numberOfRides / numberOfDays;
double perWeek = unlimited7Price();
double unlimited = priceUnlimited / numberOfRides;
double ridePrices[]; // Declared Array //
ridePrices = new double[] {perRide, perWeek, unlimited}; // New array, with added elements. Could be a mistake since I failed to declare elements//
return ridePrices;
}
public String getBestFare(){ // Error in the iteration and lowest value find! //
double lowestPriceRide[];
lowestPriceRide = getRidePrices();
double lowestPrice = lowestPriceRide[0];
for(int i = 0; i< lowestPriceRide.length; i++) {
if (lowestPrice < lowestPriceRide[i]) {
lowestPriceRide[i] = lowestPrice;
}
}
if(lowestPrice == lowestPriceRide[0]){
System.out.println("You should take the 'Pay per Ride' option in our NYC transit");
}
else if(lowestPrice == lowestPriceRide[1]){
System.out.println("You should take the 'Weekly Unlimited' plan in our NYC Transit");
}
else if(lowestPrice == lowestPriceRide[2]){
System.out.println("You should take the Unlimited ride plan in our NYC Transit");
}
return "at " + lowestPrice + "$ per Ride";
}
public static void main(String[] args){
TransitCalculator test = new TransitCalculator(26, 54);
System.out.println(test.getBestFare()); //
}
}

You are not setting the right value; currently, you set the element in the array to the lowest price instead of setting the lowest price to the element of the array. You also compare against the wrong value; you should check that the current array element is less than the best price, instead of the other way around.
Change
if(lowestPrice < lowestPriceRide[i])
lowestPriceRide[i] = lowestPrice;
To
if(lowestPriceRide[i] < lowestPrice)
lowestPrice = lowestPriceRide[i];
See the updated code in action here.
Note that it is unnecessary to import java.lang, as the package is implicitly imported.

The problem is in your if condition:
if (lowestPrice < lowestPriceRide[i]) {
lowestPriceRide[i] = lowestPrice;
}
You need to see if the current lowestPriceRide[i] is less than the already existing lowestPrice then update your existing lowestPrice. So the condition would be now:
if (lowestPriceRide[i] < lowestPrice) {
lowestPrice = lowestPriceRide[i];
}

This should be your comparison for lowest price :
double lowestPrice = lowestPriceRide[0];
for(int i = 0; i< lowestPriceRide.length; i++) {
if (lowestPriceRide[i] < lowestPrice) {
lowestPrice = lowestPriceRide[i];
}
}

Generate Data Points for Graph from an equation

I don't want to solve an equation and my question is not about Graphs and Trees Data Structures. I am trying to generate Data Points for graph from an equation given by user. I want efficient algorithm, easy to use and easy to maintain data structures. I have two solutions in mind
1: This is trivial and I have seen in many Applications.
String expr = "2*x+3*x";
Evaluator evaluator = new Evaluator();//I have this class
for (int i = start; i < end; i += step)
{
evaluator.setConstant("x", i);
double ans = evaluator.evaluate(expr);
}
This is very slow because each time every step is repeated like tokenzing, verifying, conversion to RPN, preparing stacks and queues and at last result calculation. The possible solution to this problem is somehow caching all stacks and queues but after that a comparison would be required between current expression and previous expression to use last stored state.
2: Currently I am developing second solution. The purpose of this is efficiency and would be used in Symbolic calculation in future.
So far my implementation
Variable.java
import java.text.DecimalFormat;
public class Variable
{
private final double pow;
private final double coefficient;
private final String symbol;
public Variable(String symbol)
{
this.symbol = symbol;
this.pow = 1.0;
this.coefficient = 1.0;
}
public Variable(String symbol, double coefficient, double pow)throws IllegalArgumentException
{
if (coefficient == 0.0)throw new IllegalArgumentException("trying to create variable with coefficient 0");
if (pow == 0.0)throw new IllegalArgumentException("trying to create variable with exponent 0");
this.symbol = symbol;
this.pow = pow;
this.coefficient = coefficient;
}
public final String getSymbol()
{
return this.symbol;
}
public final double getPow()
{
return this.pow;
}
public final double getCoefficient()
{
return this.coefficient;
}
#Override
public String toString()
{
StringBuilder builder = new StringBuilder();
DecimalFormat decimalFormat = new DecimalFormat("#.############");
if (coefficient != 1.0)builder.append(decimalFormat.format(this.coefficient));
builder.append(this.symbol);
if (this.pow != 1.0)builder.append("^").append(decimalFormat.format(this.pow));
return builder.toString();
}
/*
* Stub Method
* Generate some unique hash code
* such that chances of key collision
* become less and easy to identify
* variables with same power and same
* symbol*/
#Override
public int hashCode()
{
return 0;
}
}
Equation.java
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
public class Equation
{
private final ArrayList<Boolean> operations;
private final HashMap<String, Variable> variableHashMap;
private int typesOfVariables;
public Equation(Variable variable)
{
this.variableHashMap = new HashMap<>();
this.operations = new ArrayList<>();
this.typesOfVariables = 1;
this.variableHashMap.put(variable.getSymbol(), variable);
}
/*Stub Method*/
public void addVariable(Variable variable, boolean multiply)
{
/*
* Currently not covering many cases
* 1: Add two variables which have same name
* and same pow.
* 2: variable which are wrapped inside functions e.g sin(x)
* and many other.*/
if (multiply && variableHashMap.containsKey(variable.getSymbol()))
{
Variable var = variableHashMap.get(variable.getSymbol());
Variable newVar = new Variable(var.getSymbol(), var.getCoefficient() * variable.getCoefficient(), var.getPow() + variable.getPow());
/*
* Collision chances for variables with same name but
* with different powers*/
this.variableHashMap.replace(var.getSymbol(), newVar);
}
else
{
++this.typesOfVariables;
this.variableHashMap.put(variable.getSymbol(), variable);
}
this.operations.add(multiply);
}
/*Stub Method
*Value for every variable at any point will be different*/
public double solveFor(double x)
{
if (typesOfVariables > 1)throw new IllegalArgumentException("provide values for all variables");
Iterator<HashMap.Entry<String, Variable>> entryIterator = this.variableHashMap.entrySet().iterator();
Variable var;
double ans = 0.0;
if (entryIterator.hasNext())
{
var = entryIterator.next().getValue();
ans = var.getCoefficient() * Math.pow(x, var.getPow());
}
for (int i = 0; entryIterator.hasNext(); i++)
{
var = entryIterator.next().getValue();
if (this.operations.get(i))ans *= var.getCoefficient() * Math.pow(x, var.getPow());
else ans += var.getCoefficient() * Math.pow(x, var.getPow());
}
return ans;
}
#Override
public String toString()
{
StringBuilder builder = new StringBuilder();
Iterator<HashMap.Entry<String, Variable>> entryIterator = this.variableHashMap.entrySet().iterator();
if (entryIterator.hasNext())builder.append(entryIterator.next().getValue().toString());
Variable var;
for (int i = 0; entryIterator.hasNext(); i++)
{
var = entryIterator.next().getValue();
if (this.operations.get(i))builder.append("*").append(var.toString());
else builder.append(var.toString());
}
return builder.toString();
}
}
Main.java
class Main
{
public static void main(String[] args)
{
try
{
long t1 = System.nanoTime();
Variable variable = new Variable("x");
Variable variable1 = new Variable("x", -2.0, 1.0);
Variable variable2 = new Variable("x", 3.0, 4.0);
Equation equation = new Equation(variable);
equation.addVariable(variable1, true);//2x+x
equation.addVariable(variable2, true);
for (int i = 0; i < 1000000; i++)equation.solveFor(i);//Calculate Million Data Points
long t2 = System.nanoTime();
System.out.println((t2-t1)/1000/1000);
System.out.println(equation.toString());
}
catch (Exception e)
{
System.out.println("Error: " + e.getMessage());
}
}
}
Am I going in right direction?
Is there any commonly used Algorithm for this problem?
My main goal is efficiency, code cleanness and code maintainability.
Note: I am not native English speaker so please ignore any grammatical mistake.
Thanks.

I do not see any problem with your first code. Yes may be at every step your code "repeat like tokenzing, verifying, conversion to RPN, preparing stacks and queues and at last result calculation", but in the end all of this is just linear number of steps. So I fail to see how it can make it really slow.
One of the biggest screens I have seen was 2560x1440 pixels, which means that most of the time you would need less than 2500 points to draw your graph there.
If you point is code cleanness and code maintainability, then most probably a code consisting of 5 lines is better than the code consisting of 200.

Converting Units using if statements?

I have to write a program to convert between linear units in, ft, mi, mm, cm, m, km. I know there are easier and better ways to do this. I think we'ere just trying to fully understand if else if statements. But this is what I have so far. I'm just trying to figure out if I am on the right track. I've tried to write out some pseudocode but it just seems like a lot going on so I find it a bit overwhelming. Next I'm going to add a method to convert form in or mm to whatever is selected by the user.
When I test the program i get this: UnitConversion#76c5a2f7 (EDIT: THIS ISSUE WAS FIXED)
Ok I made the suggested changes and that allowed the first part of the program to run properly. I have now added my second method to convert from in/mm to the other measurements.. I was having issues but I figured it out.
Here is my main method;
public class LinearConversion
{
public static void main(String[] args)
{
UnitConversion newConvert = new UnitConversion("km", "m", 100);
System.out.println(newConvert);
}
}
Any suggestions? What am I missing or not understanding about doing this sort of program?
public class UnitConversion
{
private String input;
private String output;
private double value;
private double temp;
private double in, ft, mi, mm, cm, m, km;
private final double inch_feet = 12;
private final double inch_miles = 63360;
private final double inch_millimeters = 25.4;
private final double inch_centimeters = 2.54;
private final double inch_meters = 0.0254;
private final double inch_kilometers = 0.0000254;
private final double millimeters_inch = 0.0393701;
private final double millimeters_feet = 0.00328084;
private final double millimeters_miles = 0.000000622;
private final double millimeter_centimeters = 10;
private final double millimeter_meters = 1000;
private final double millimeter_kilometers = 1000000;
public UnitConversion(String in, String out, double val)
{
input = in;
output = out;
value = val;
}
public String toString()
{
if (input.equals("mi"))
{
in = value * inch_miles;
input = "in";
}
else if (input.equals("ft"))
{
in = value * inch_feet;
input = "in";
}
else
{
in = value;
input = "in";
}
if (input.equals("km"))
{
mm = value * millimeter_kilometers;
input = "mm";
}
else if (input.equals("m"))
{
mm = value * millimeter_meters;
input = "mm";
}
else if (input.equals("cm"))
{
mm = value * millimeter_centimeters;
input = "mm";
}
else
{
mm = value;
input = "mm";
}
return value + input + " " + output;
}
public double getUnit()
{
if (input.equals("in"))
{
if (output.equals("ft"))
{
ft = in * inch_feet;
System.out.println(ft + "ft");
}
else if (output.equals("mi"))
{
mi = in * inch_miles;
System.out.println(mi + "mi");
}
else if (output.equals("mm"))
{
mm = in * inch_millimeters;
System.out.println(mm + "mm");
}
else if (output.equals("cm"))
{
cm = in * inch_centimeters;
System.out.println(cm + "cm");
}
else if (output.equals("m"))
{
m = in * inch_meters;
System.out.println(m + "m");
}
else if (output.equals("km"))
{
km = in * inch_kilometers;
System.out.println(km + "km");
}
else
{
System.out.println(in + "in");
}
}
else
{
if (output.equals("cm"))
{
cm = mm * millimeter_centimeters;
System.out.println(cm + "cm");
}
else if (output.equals("m"))
{
m = mm * millimeter_meters;
System.out.println(m + "m");
}
else if (output.equals("km"))
{
km = mm * millimeter_kilometers;
System.out.println(km + "km");
}
else if (output.equals("in"))
{
in = mm * millimeters_inch;
System.out.println(in + "in");
}
else if (output.equals("ft"))
{
ft = mm * millimeters_feet;
System.out.println(ft + "ft");
}
else if (output.equals("mi"))
{
mi = mm * millimeters_miles;
System.out.println(mi + "mi");
}
else
{
System.out.println(mm + "mm");
}
}
}

Basically, you need/want to give a String argument to System.out.println in order to display it.
Thus, when you use System.out.println with an Object (that is not a String) as the argument, Java actually outputs the result of the toString method on that object.
If you haven't overridden it, the Object class' implementation of toString is used: this is what gives you your current output: UnitConversion#76c5a2f7.
To learn more about how is this default toString implementation generating that String, you can refer to the javadoc entry for Object#toString.

Base on your output, and your provided code, yes! Rename String getInput() to String toString() and your current main() will work, or change your current main()
System.out.println(newConvert.getInput()); // <-- added .getInput()

Trying to calculate user input with Double vs. double

Right now I'm working on a GUI that will calculate kinematic values based on what the user enters into a text field. I created a private inner class with values of type Double (not double), and then created a method to get a value based on values given. For example, this returns initial velocity:
public Double getInitialVelocity(Double vf, Double a, Double ti, Double tf) {
deltaT = deltaT(tf, ti);
initialVelocity = vf - (a * deltaT);
df.format(initialVelocity);
return initialVelocity;
}
The problem appears when I tried to test this method. I set up new doubles, and use getInitialVelocity in my main class:
Kinematics test = new Kinematics(); // creates object from inner class
Double vf = 1.0, a = 2.0, ti = 0.5, tf = 1.5;
test.getInitialVelocity(vf, a, ti, tf);
When I run this to test, I get this error:
Static Error: No method in Kinematics with name 'getInitialVelocity' matches this invocation
Arguments: (Double, Double, Double, Double)
Candidate signatures: double getInitialVelocity()
Does anyone know how to properly do this? I need to use type Double because I am comparing values given to null and then using the appropriate formula based on which values are null. Also, when converting from a String, should I just use Double.parseDouble(textField.getText()); ?
Edit 1: Here are the relevant parts of my class:
Private inner class (Kinematics):
private class Kinematics {
private Double initialVelocity, finalVelocity, acceleration, timeFinal, timeInitial;
private Double deltaT;
// constructor
public Kinematics() {
}
public Double deltaT(Double tf, Double ti) {
if(!(tf == null && ti == null)){
deltaT = tf - ti;
} return deltaT;
}
public Double getInitialVelocity(Double vf, Double a, Double ti, Double tf) {
deltaT = deltaT(tf, ti);
initialVelocity = vf - (a * deltaT);
df.format(initialVelocity);
return initialVelocity;
}
In my main class (KinematicsPanel), I have:
Kinematics values = new Kinematics();
viLabel = new JLabel("Initial Velocity: ");
viText = new JTextField(1);
vfLabel = new JLabel("Final Velocity: ");
vfText = new JTextField(1);
aLabel = new JLabel("Acceleration: ");
aText = new JTextField(1);
tiLabel = new JLabel("Initial Time: ");
tiText = new JTextField(1);
tfLabel = new JLabel("Final Time: ");
tfText = new JTextField(1);
// compute button & result
compute = new JButton("Compute");
compute.addActionListener(this);
result = new JTextField(2);
result.setEditable(false); // can not be edited
public void actionPerformed(ActionEvent e) {
String action = e.getActionCommand();
// parse each string to a value
Double vf = 0.0, a = 0.0, ti = 0.0, tf = 0.0;
if(vfText != null) {vf = Double.parseDouble(vfText.getText());}
if(aText != null) {a = Double.parseDouble(aText.getText());}
if(tiText != null) {ti = Double.parseDouble(tiText.getText());}
if(tfText != null) {tf = Double.parseDouble(tfText.getText());}
if(action.equals("Compute")) {
if(viText == null) { // get initial velocity
// get values
values.getInitialVelocity(vf, a, ti, tf);
System.out.println(values.toString()); // to test
result.setText(values.toString());
}
}
As of right now, this does nothing which is why I tested the method in the interactions pane in Dr.Java.
Edit2: The format function being used is in the main class:
DecimalFormat df = new DecimalFormat("#.00");

Its all ok with your code, which compiler are you using?
The method getInitialVelocity are in the class Kinematics?

it looks , you are using jdk 1.4 or lower one, there autoboxing is not supported.
so it can not convert Double to double. but that should give you a compile time
error if you are using IDE like eclipse.
OR
may be the method you are calling has different signature.
try to check the jdk version and post whole class if above one does no solve your problem.

Error trying to use command line arguments in Java

I am new to Java and am still getting used to the minor difference so please excuse any mistakes you may find ridiculous.
I am trying to write a program that stores temperature and can be used to call that temperature in Celsius or in Fahrenheit. My only issue comes with the command line arguments, after successfully compiling my program I enter the following:
java Driver 0.0C 32.0F
And then I get this:
Exception in thread "main" java.lang.NumberFormatException: For input string:
"0.0C"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1241)
at java.lang.Float.parseFloat(Float.java:452)
at Driver.main(Driver.java:47)
My program is still not completely polished up so I know that the getters can be written to be much for efficient and that the driver program doesn't even call the temperature class, but this is not my concern at the moment. My Driver is supposed to take in the input and determine from the 'C' or 'F' character whether the value is in Celsius or Fahrenheit. It then parses the string and truncates the C or F and stores the values contained in the strings as floats. I am using Eclipse and the program is object oriented, this is my code:
public class Temperature {
private float temperature;
private char scale;
// default constructor
Temperature() {
this.temperature = 0;
this.scale = 'C';
}
Temperature(float temperatureIn) {
this.temperature = temperatureIn;
this.scale = 'C';
}
Temperature(char scaleIn) {
this.temperature = 0;
this.scale = scaleIn;
}
Temperature(float temperatureIn, char scaleIn) {
this.temperature = temperatureIn;
this.scale = scaleIn;
}
float degreesC(float degreesF) {
float degreesC = (5 * (degreesF - 32)) / 9;
return degreesC;
}
float degreesF(float degreesC) {
float degreesF = (9*(degreesC / 5)) + 32;
return degreesF;
}
void setTemperature(float temperatureIn) {
temperature = temperatureIn;
}
void setScale(char scaleIn) {
scale = scaleIn;
}
void setBothValues(float temperatureIn, char scaleIn) {
temperature = temperatureIn;
scale = scaleIn;
}
int compareTemps(Temperature temp1, Temperature temp2) {
// both values will be compared in Farenheit
Temperature temp1temp = temp1;
if (temp1temp.scale == 'C') {
temp1temp.temperature = degreesF(temp1temp.temperature);
temp1temp.scale = 'F';
}
Temperature temp2temp = temp2;
if (temp2temp.scale == 'C') {
temp2temp.temperature = degreesF(temp2temp.temperature);
temp2temp.scale = 'F';
}
if (temp1temp.temperature == temp2temp.temperature) {
return 0;
}
if (temp1temp.temperature > temp2temp.temperature)
return 1;
if (temp1temp.temperature < temp2temp.temperature)
return -1;
return 0;
}
}
And the main driver program:
public class Driver {
public static void main(String[] args) {
// ints to hold the temperature values
float temp1Value = 0;
float temp2Value = 0;
// strings to hold the scale types
char temp1Scale = 'C';
char temp2Scale = 'C';
// declare objects of type temperature
Temperature firstTemp = null;
Temperature secondTemp = null;
// copy scale values of temperatures
int scaleIndex = 0;
int scaleIndex2 = 0;
if (args.length > 0) {
if (args[0].indexOf('C') != -1)
{
scaleIndex = args[0].indexOf('C');
temp1Scale = args[0].charAt(scaleIndex);
}
else if (args[0].indexOf('F') != -1)
{
scaleIndex = args[0].indexOf('F');
temp1Scale = args[0].charAt(scaleIndex);
}
if (args[1].indexOf('C') != -1)
{
scaleIndex = args[1].indexOf('C');
temp2Scale = args[1].charAt(scaleIndex2);
}
else if (args[1].indexOf('F') != -1)
{
scaleIndex = args[1].indexOf('F');
temp2Scale = args[1].charAt(scaleIndex2);
}
}
// parse the values to exclude scales and copy to strings holding temperature values
if (args.length > 0) {
temp1Value = Float.parseFloat(args[0].substring(0, scaleIndex));
temp2Value = Float.parseFloat(args[1].substring(0, scaleIndex2));
}
}
}

the exception you are getting is beacuse you passed '0.0C' to the float parser at:
tempValue = Float.parseFloat(args[1].substring(0, scaleIndex));
that is beacuse you do
scaleIndex = args[1].indexOf('F');
effectively overwriting the scaleIndex instead of setting scaleIndex2
please be open minded with my following recommendations:
object oriented means you create classes which will take up responsibility
your Temperature class stores temp in celsius and in fahrenheit too..which might be easier, but storing only for example Kelvins would mean you have a strong inner concept inside the class
when someone asks for C or F it calculates from the K
after that the Temperature class's constructor should be responsible for parsing '0.0C' and '42.0F'

It is better you take inputs as <temp1> <unit1> <temp2> <unit2>. This way you'll get all the parameter you need in the desired format. You can now parse args[0] and args[2] for tempValues and the other two parameter for the units. Even better, just take <temp1> <temp2> as you command line arguments and decide that <temp1> is in degC and <temp2> is in F.