Develop Reference

From String to Integer Functions

I am trying to write a program that will receive a function as a String and solve it. For ex. "5*5+2/2-8+5*5-2" should return 41
I wrote the code for multiplication and divisions and it works perfectly:
public class Solver
{
public static void operationS(String m)
{
ArrayList<String> z = new ArrayList<String>();
char e= ' ';
String x= " ";
for (int i =0; i<m.length();i++)
{
e= m.charAt(i);
x= Character.toString(e);
z.add(x);
}
for (int i =0; i<z.size();i++)
{
System.out.print(z.get(i));
}
other(z);
}
public static void other(ArrayList<String> j)
{
int n1=0;
int n2=0;
int f=0;
String n= " ";
for (int m=0; m<j.size();m++)
{
if ((j.get(m)).equals("*"))
{
n1 = Integer.parseInt(j.get(m-1));
n2 = Integer.parseInt(j.get(m+1));
f= n1*n2;
n = Integer.toString(f);
j.set(m,n);
j.remove(m+1);
j.remove(m-1);
m=0;
}
for (int e=0; e<j.size();e++)
{
if ((j.get(e)).equals("/"))
{
n1 = Integer.parseInt(j.get(e-1));
n2 = Integer.parseInt(j.get(e+1));
f= n1/n2;
n = Integer.toString(f);
j.set(e,n);
j.remove(e+1);
j.remove(e-1);
e=0;
}
}
}
System.out.println();
for (int i1 =0; i1<j.size();i1++)
{
System.out.print(j.get(i1)+",");
}
However, for adding and subtracting, since there isnt an order for adding and subtracting, just whichever comes first, I wrote the following:
int x1=0;
int x2=0;
int x3=0;
String z = " ";
for (int g=0; g<j.size();g++)
{
if ((j.get(g)).equals("+"))
{
x1= Integer.parseInt(j.get(g-1));
x2= Integer.parseInt(j.get(g+1));
x3= x1+x2;
z = Integer.toString(x3);
j.set(g,z);
j.remove(g+1);
j.remove(g-1);
g=0;
}
g=0;
if ((j.get(g)).equals("-"))
{
x1= Integer.parseInt(j.get(g-1));
x2= Integer.parseInt(j.get(g+1));
x3= x1-x2;
z = Integer.toString(x3);
j.set(g,z);
j.remove(g+1);
j.remove(g-1);
g=0;
}
g=0;
}
System.out.println();
for (int i1 =0; i1<j.size();i1++)
{
System.out.print(j.get(i1)+",");
}
After this, it prints:
25,+,1,-,8,+,25,–,2,
. What am I doing wrong? Multiplication and dividing seem to be working perfectly

You have 2 problems:
1) g=0; statements after if and else blocks will make you go into an infinite loop.
2) From the output you gave, the first minus (-) is Unicode character HYPHEN-MINUS (U+002D), while the second minus (–) is Unicode character EN DASH (U+2013), so (j.get(g)).equals("-") fails for the second minus as they are not equal.

Going for an answer that doesn't help with your exact specific problem, but that hopefully helps you much further than that.
On a first glance, there are various problems with your code:
Your are using super-short variable names all over the place. That saves you maybe 1 minute of typing overall; and costs you 5, 10, x minutes every time you read your code; or show it to other people. So: dont do that. Use names that say what the thing behind that name is about.
You are using a lot of low-level code. You use a "couting-for" loop to iterate a list (called j, that is really really horrible!) for example. Meaning: you make your code much more complicated to read than it ought to be.
In that way, it looks like nobody told you so far, but the idea of code is: it should be easy to read and understand. Probably you dont get grades for that, but believe me: in the long run, learning to write readable code is a super-important skill. If that got you curious, see if you can get a hand on "Clean code" by Robert Martin. And study that book. Then study it again. And again.
But the real problem is your approach to solve this problem. As I assume: this is some part of study assignment. And the next step will be that you don't have simple expressions such as "1+2*3"; but that you are asked to deal with something like "sqrt(2) + 3" and so on. Then you will be asked to add variables, etc. And then your whole approach breaks apart. Because your simple string operations won't do it any more.
In that sense: you should look into this question, and carefully study the 2nd answer by Boann to understand how to create a parser that dissects your input string into expressions that are then evaluated. Your code does both things "together"; thus making it super-hard to enhance the provided functionality.

You can use the built-in Javascript engine
public static void main(String[] args) throws Exception{
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String code = "5*5+2/2-8+5*5-2";
System.out.println(engine.eval(code));
}

Primarily Don't Repeat Yourself (the DRY principle). And use abstractions (full names, extracting methods when sensible). Static methods are a bit cumbersome, when using several methods. Here it is handy to use separate methods.
Maybe you want something like:
Solver solver = new Solver();
List<String> expr = solver.expression("5*5+2/2-8+5*5-2");
String result = solver.solve(expr);
A more abstract Solver class would do:
class Solver {
List<String> expression(String expr) {
String[] args = expr.split("\\b");
List<String> result = new ArrayList<>();
Collections.addAll(result, args);
return result;
}
String solve(List<String> args) {
solveBinaryOps(args, "[*/]");
solveBinaryOps(args, "[-+]");
return args.stream().collect(Collectors.joining(""));
}
The above solveBinaryOps receives a regular expression pattern or alternatively simply in some form the operators you want to tackle.
It takes care of operator precedence.
private void solveBinaryOps(List<String> args, String opPattern) {
for (int i = 1; i + 1 < args.length; ++i) {
if (args.get(i).matches(opPattern)) {
String value = evalBinaryOp(args.get(i - 1), args.get(i), args.get(i + 1));
args.set(i, value);
args.remove(i + 1);
args.remove(i - 1);
--i; // Continue from here.
}
}
}
private String evalBinaryOp(String lhs, String op, String rhs) {
int x = Integer.parseInt(lhs);
int y = Integer.parseInt(rhs);
int z = 0;
switch (op) {
case "*":
z = x * y;
break;
case "/":
z = x / y;
break;
case "+":
z = x + y;
break;
case "-":
z = x - y;
break;
}
return Integer.toString(z);
}
}
The above can be improved at several points. But it is readable, and rewritable.

public class Solver {
public static void main(String args[]) {
operation("5+2*5-6/2+1+5*12/3");
}
public static void operation(String m) {
ArrayList<Object> expressions = new ArrayList<Object>();
String e;
String x = "";
for (int i = 0; i < m.length(); i++) {
e = m.substring(i, i + 1);
if (!(e.equals("*") || e.equals("/") || e.equals("+") || e
.equals("-"))) {
x += e;
continue;
} else {
if (!x.equals("") && x.matches("[0-9]+")) {
int oper = Integer.parseInt(x);
expressions.add(oper);
expressions.add(m.charAt(i));
x = "";
}
}
}
if (!x.equals("") && x.matches("[0-9]+")) {
int oper = Integer.parseInt(x);
expressions.add(oper);
x = "";
}
for (int i = 0; i < expressions.size(); i++) {
System.out.println(expressions.get(i));
}
evaluateExpression(expressions);
}
public static void evaluateExpression(ArrayList<Object> exp) {
//Considering priorities we calculate * and / first and put them in a list mulDivList
ArrayList<Object> mulDivList=new ArrayList<Object>();
for (int i = 0; i < exp.size(); i++) {
if (exp.get(i) instanceof Character) {
if ((exp.get(i)).equals('*')) {
int tempRes = (int) exp.get(i - 1) * (int) exp.get(i + 1);
exp.set(i - 1, null);
exp.set(i, null);
exp.set(i + 1, tempRes);
}
else if ((exp.get(i)).equals('/')) {
int tempRes = (int) exp.get(i - 1) / (int) exp.get(i + 1);
exp.set(i - 1, null);
exp.set(i, null);
exp.set(i + 1, tempRes);
}
}
}
//Create new list with only + and - operations
for(int i=0;i<exp.size();i++)
{
if(exp.get(i)!=null)
mulDivList.add(exp.get(i));
}
//Calculate + and - .
for(int i=0;i<mulDivList.size();i++)
{
if ((mulDivList.get(i)).equals('+')) {
int tempRes = (int) mulDivList.get(i - 1) + (int) mulDivList.get(i + 1);
mulDivList.set(i - 1, null);
mulDivList.set(i, null);
mulDivList.set(i + 1, tempRes);
}
else if ((mulDivList.get(i)).equals('-')) {
int tempRes = (int) mulDivList.get(i - 1) - (int) mulDivList.get(i + 1);
mulDivList.set(i - 1, null);
mulDivList.set(i, null);
mulDivList.set(i + 1, tempRes);
}
}
System.out.println("Result is : " + mulDivList.get(mulDivList.size() - 1));
}
}

nearest neighbor algorithm copy element (city) to output array java

So I have a program written so far that reads in a csv file of cities and distances in the following format:
Alaska Mileage Chart,Anchorage,Anderson,Cantwell,
Anchorage,0,284,210,
Anderson,284,0,74,
Cantwell,210,74,0,
So the algorithm works and outputs the cities in the order they should be visited following the shortest path using the nearest neighbor algorithm always starting with Anchorage as the city of origin or starting city.
Using this data, the example output for the algorithm is: 1,3,2. I have ran this with a 27 element chart and had good results as well. I am using this small one for writing and debugging purposes.
Ideally the output I am looking for is the Name of the City and a cumulative milage.
Right now I am having working on trying to get the cities into an array that I can print out. Help with both parts would be appreciated or help keeping in mind that is the end goal is appreciated as well.
My thought was that ultimately I may want to create an array of {string, int}
so my output would look something like this..
Anchorage 0
Cantwell 210
Anderson 284
I am able to set the first element of the array to 1, but can not get the 2nd and 3rd element of the new output array to correct
This is the code I am having a problem with:
public class TSPNearestNeighbor {
private int numberOfNodes;
private Stack<Integer> stack;
public TSPNearestNeighbor()
{
stack = new Stack<>();
}
public void tsp(int adjacencyMatrix[][])
{
numberOfNodes = adjacencyMatrix[1].length;
// System.out.print(numberOfNodes);
// System.out.print(Arrays.deepToString(adjacencyMatrix));
int[] visited = new int[numberOfNodes];
// System.out.print(Arrays.toString(visited));
visited[1] = 1;
// System.out.print(Arrays.toString(visited));
stack.push(1);
int element, dst = 0, i;
int min = Integer.MAX_VALUE;
boolean minFlag = false;
System.out.print(1 + "\n");
//System.arraycopy(arr_cities, 0, arr_final, 0, 1); // Copies Anchorage to Pos 1 always
//System.out.print(Arrays.deepToString(arr_final)+ "\n");
while (!stack.isEmpty())
{
element = stack.peek();
i = 1;
min = Integer.MAX_VALUE;
while (i <= numberOfNodes-1)
{
if (adjacencyMatrix[element][i] > 1 && visited[i] == 0)
{
if (min > adjacencyMatrix[element][i])
{
min = adjacencyMatrix[element][i];
dst = i;
minFlag = true;
}
}
i++;
}
if (minFlag)
{
visited[dst] = 1;
stack.push(dst);
System.out.print(dst + "\n");
minFlag = false;
continue;
}
stack.pop();
}
}

Given the existing structure you are using, you can output the cities in the path using:
public void printCities(Stack<Integer> path, int[][] distances, List<String> names) {
int cumulativeDistance = 0;
int previous = -1;
for (int city: path) {
if (previous != -1)
cumulativeDistance += distances[previous][city];
System.out.println(names.get(city) + " " + cumulativeDistance);
previous = city;
}
}

I'd like to answer your question slightly indirectly. You are making life hard for yourself by using arrays of objects. They make the code difficult to read and are hard to access. Things would become easier if you create a City class with appropriate methods to help you with the output.
For example:
class City {
private final String name;
private final Map<City,Integer> connections = new HashMap<>();
public static addConnection(City from, City to, int distance) {
from.connections.put(to, distance);
to.connections.put(from, distance);
}
public int getDistanceTo(City other) {
if (connections.containsKey(other))
return connections.get(other);
else
throw new IllegalArgumentException("Non connection error");
}
}
I've left out constructor, getters, setters for clarity.
Now outputting your path becomes quite a bit simpler:
public void outputPath(List<City> cities) {
int cumulativeDistance = 0;
City previous = null;
for (City current: cities) {
if (previous != null)
cumulativeDistance += previous.getDistanceTo(current);
System.out.println(current.getName + " " + cumulativeDistance);
previous = current;
}
}

How do I implement HashMap in my code, I am trying to use it as a memory?

So I am working on a PostFix calculator that is used in command line for a class project, and I am having a little trouble on developing a memory for it. I have been told to create a hashMap and I have researched it and understand the basics of it. I have the calculating method working, but what I am having trouble trying to implement a way for the user to declare variables. For example this what the user should be able to do:
> a = 3 5 + 1 -
7
> bee = a 3 *
21
> a bee +
28
> bee 3 %
0
> a = 4
4
> 57
57
> 2 c +
c not found
> mem
a: 4
bee: 21
> exit
As you can see the user can declare variables in the format " ="
My problem is, that I am not really sure how to Implement the hashMap, I have tried doing it by setting the variable name for the hashmap by getting it from an array list, and getting the integer value from it by getting the return value from my compute method, but all I get is this error:
>Exception in thread "main" java.lang.NumberFormatException: For input string: "
Error"
at java.lang.NumberFormatException.forInputString(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at java.lang.Integer.parseInt(Unknown Source)
at Program6.main(Program6.java:42)
Here is my code currently:
import java.util.*;
import java.io.*;
public class Program6
{
private static HashMap<String,Integer> memory = new HashMap<>();
public static void main(String args[])
{
System.out.println("Servando Hernandez");
System.out.println("RPN command line calculator");
Scanner scan = new Scanner(System.in);
System.out.print(">");
while(scan.hasNextLine())
{
System.out.print("> ");
String a = scan.nextLine();
String b = "quit";
String c = "mem";
String d = "clear";
if(a.equals(b))
{
System.exit(0);
}
else
{
System.out.println(compute(a));
}
System.out.print(">");
List<String> list = new ArrayList<String>();
if(!a.isEmpty())
{
StringTokenizer var = new StringTokenizer(a);
while(var.hasMoreTokens())
{
list.add(var.nextToken());
}
}
int pos = 0;
if (compute(a) != null)
{
pos = Integer.parseInt(compute(a));
}
memory.put(list.get(list.size()-1),pos);
}
}
public static String compute(String input)
{
List<String> processedList = new ArrayList<String>();
if (!input.isEmpty())
{
String myRegex = "[^a-zA-Z]";
StringTokenizer st = new StringTokenizer(input);
while (st.hasMoreTokens())
{
processedList.add(st.nextToken());
processedList.remove(myRegex);
processedList.remove("=");
}
}
else
{
return "Error";
}
Stack<String> tempList = new Stack<String>();
Iterator<String> iter = processedList.iterator();
while (iter.hasNext())
{
String temp = iter.next();
if (temp.matches("[0-9]*"))
{
tempList.push(temp);
}
else if (temp.matches("[*-/+]"))
{
if (temp.equals("*"))
{
int rs = Integer.parseInt(tempList.pop());
int ls = Integer.parseInt(tempList.pop());
int result = ls * rs;
tempList.push("" + result);
}
else if (temp.equals("-"))
{
int rs = Integer.parseInt(tempList.pop());
int ls = Integer.parseInt(tempList.pop());
int result = ls - rs;
tempList.push("" + result);
}
else if (temp.equals("/"))
{
int rs = Integer.parseInt(tempList.pop());
int ls = Integer.parseInt(tempList.pop());
int result = ls / rs;
tempList.push("" + result);
}
else if (temp.equals("+"))
{
int rs = Integer.parseInt(tempList.pop());
int ls = Integer.parseInt(tempList.pop());
int result = ls + rs;
tempList.push("" + result);
}
}
else
{
return "Error";
}
}
return tempList.pop();
}
}
Does anyone know how i can make the hashMap memory on the post fix calculator work to where the user can assign variables and be able to call them back, or a better way to approach this?

Your problem is you are adding "Error" from you else clause in the compute method and then trying to parse it as a int.
public static String compute(String input)
{
List<String> processedList = new ArrayList<String>();
if (!input.isEmpty())
{
String myRegex = "[^a-zA-Z]";
StringTokenizer st = new StringTokenizer(input);
while (st.hasMoreTokens())
{
processedList.add(st.nextToken());
processedList.remove(myRegex);
processedList.remove("=");
}
}
else
{
return "Error"; //-->> problem
}
Parsing it as an int. This point compute(a) will not be null as it has "Error".
Next step you are trying to parse it as an int.
if (compute(a) != null)
{
pos = Integer.parseInt(compute(a));
}
You can change your code as
if (compute(a) != null && !compute(a).equals("Error"))
{
pos = Integer.parseInt(compute(a));
}
Also you should try you put your Integer.parseInt() code in a try catch.

Your HashMap can be used to store the variable names and their values. The key for the map will be the variable name, and the value is the number assigned to it. You currently have Integer but you might want something that handles decimals if you're going to allow things like a = 10 3 /.
In your compute(..) method you expect the input to be of the form var = <calculation> so you should first parse out the variable name which will be the key used in the hashmap memory, and after computing the calculation, store that result with memory.put(var,result);.
When computing the <calculation> part, if a variable name is encountered, look it up to find its value using memory.get(var) and use that value in the computation. With a postfix calculator, you just have to get the 2 values, followed by the operation and perform the math. The result of that is the first value of the next pair to operate on, and so on until you run out of operations and finally assign the result to the variable.

NullPointerException when trying to read from a text file

I'm trying to read these names from a text file, but getting an error. Here's the relevant part of my main method, and the method in question:
public class Prog4
{
int mostVoteCount = 0;
int mostVotes = 0;
int mostVotesIndex = 0;
int fewestVoteCount = 0;
int fewestVotes = 0;
int fewestVotesIndex = 0;
String[] candidateArray;
int[] votesArray;
public static void main(String args[]) throws Exception
{
//Scanner stdin = new Scanner(System.in);
Scanner stdin = new Scanner(new File("test.txt"));
int num_votes, num_candidates, election_num = 1;
System.out.println("Welcome to Prog4!\n");
Prog4 p4 = new Prog4();
while (stdin.hasNextLine())
{
System.out.println("Results for election " + election_num);
p4.read_candidates(stdin, p4.candidateArray);
p4.read_votes(stdin, p4.votesArray);
System.out.println("Total votes: " + p4.votesArray.length);
}
System.out.println("Done. Normal termination of Prog4.");
}
and the method in question:
public void read_candidates(Scanner stdin, String candidates[])
{
int candidateCount = 0;
candidateCount = (stdin.nextInt());
for (int i = 0; i < candidateCount; i++)
candidates[i] = (stdin.next());
}
Here's my test data used in the "test" text file:
4
Owen
Jack
Scott
Robbie
15 0 1 1 2 3 1 0 0 0 0 1 2 2 1 1
Is this the stack trace?
java.lang.NullPointerException
at Prog4.read_candidates(Prog4.java:75)
at Prog4.main(Prog4.java:35)

You're never instantiating your candidateArray and votesArray, so they are null when you pass them to the read_candidates method. I'm guessing it is failing on the line:
candidates[i] = (stdin.next());
You're basically attempting to do something with a null object (the candidates object in this case). At some point in your code you'll have to do something like candidateArray= new String[], and same thing for the votesArray, before you attempt to use them.

You need to initialize your array "candidates" before you do the assignment
public void read_candidates(Scanner stdin, String candidates[]) {
int candidateCount = 0;
candidateCount = (stdin.nextInt());
candidates = new String[candidateCount]; // here initialize your array
for (int i = 0; i < candidateCount && stdin.hasNext(); i++){
String data = stdin.next();
System.out.print(data + " ");
candidates[i] = data;
}
}

it can be an outfobound exaception so please be careful with the line candidateCount = (stdin.nextInt());
you may get exception here so calculate length correctly candidateCount = candidates.length;

Check the location of test.txt. Based the trace, your Scanner object being passed to read_candidates is null. Be sure that you can read the file from the specified location.

Encoding codes in Java

Over the past couple of weeks I've read through the book Error Control Coding: Fundamentals and Applications in order to learn about BCH (Bose, Chaudhuri, Hocquenghem) Codes for an junior programming role at a telecoms company.
This book mostly covers the mathematics and theory behind the subject, but I'm struggling to implement some of the concepts; primarily getting the next n codewords.I have a GUI (implemented through NetBeans, so I won't post the code as the file is huge) that passes a code in order to get the next n numbers:
Generating these numbers is where I am having problems. If I could go through all of these within just the encoding method instead of looping through using the GUI my life would be ten times easier.
This has been driving me crazy for days now as it is easy enough to generate 0000000000 from the input, but I am lost as to where to go from there with my code. What do I then do to generate the next working number?
Any help with generating the above code would be appreciated.

(big edit...) Playing with the code a bit more this seems to work:
import java.util.ArrayList;
import java.util.List;
public class Main
{
public static void main(final String[] argv)
{
final int startValue;
final int iterations;
final List<String> list;
startValue = Integer.parseInt(argv[0]);
iterations = Integer.parseInt(argv[1]);
list = encodeAll(startValue, iterations);
System.out.println(list);
}
private static List<String> encodeAll(final int startValue, final int iterations)
{
final List<String> allEncodings;
allEncodings = new ArrayList<String>();
for(int i = 0; i < iterations; i++)
{
try
{
final int value;
final String str;
final String encoding;
value = i + startValue;
str = String.format("%06d", value);
encoding = encoding(str);
allEncodings.add(encoding);
}
catch(final BadNumberException ex)
{
// do nothing
}
}
return allEncodings;
}
public static String encoding(String str)
throws BadNumberException
{
final int[] digit;
final StringBuilder s;
digit = new int[10];
for(int i = 0; i < 6; i++)
{
digit[i] = Integer.parseInt(String.valueOf(str.charAt(i)));
}
digit[6] = ((4*digit[0])+(10*digit[1])+(9*digit[2])+(2*digit[3])+(digit[4])+(7*digit[5])) % 11;
digit[7] = ((7*digit[0])+(8*digit[1])+(7*digit[2])+(digit[3])+(9*digit[4])+(6*digit[5])) % 11;
digit[8] = ((9*digit[0])+(digit[1])+(7*digit[2])+(8*digit[3])+(7*digit[4])+(7*digit[5])) % 11;
digit[9] = ((digit[0])+(2*digit[1])+(9*digit[2])+(10*digit[3])+(4*digit[4])+(digit[5])) % 11;
// Insert Parity Checking method (Vandermonde Matrix)
s = new StringBuilder();
for(int i = 0; i < 9; i++)
{
s.append(Integer.toString(digit[i]));
}
if(digit[6] == 10 || digit[7] == 10 || digit[8] == 10 || digit[9] == 10)
{
throw new BadNumberException(str);
}
return (s.toString());
}
}
class BadNumberException
extends Exception
{
public BadNumberException(final String str)
{
super(str + " cannot be encoded");
}
}
I prefer throwing the exception rather than returning a special string. In this case I ignore the exception which normally I would say is bad practice, but for this case I think it is what you want.

Hard to tell, if I got your problem, but after reading your question several times, maybe that's what you're looking for:
public List<String> encodeAll() {
List<String> allEncodings = new ArrayList<String>();
for (int i = 0; i < 1000000 ; i++) {
String encoding = encoding(Integer.toString(i));
allEncodings.add(encoding);
}
return allEncodings;
}
There's one flaw in the solution, the toOctalString results are not 0-padded. If that's what you want, I suggest using String.format("<something>", i) in the encoding call.
Update
To use it in your current call, replace a call to encoding(String str) with call to this method. You'll receive an ordered List with all encodings.
I aasumed, you were only interested in octal values - my mistake, now I think you just forgot the encoding for value 000009 in you example and thus removed the irretating octal stuff.