Develop Reference

Make program quit based on user input

I want my program to quit when I input "Q" or "q". However, the loop never finishes. Can you help me figure it out, please?
public static void main(String[] args) {
Scanner input= new Scanner(System.in);
char str ;
do {
System.out.println("Choose one of the following option : ");
System.out.println("U or u - to convert SAR amount to USD");
System.out.println("E or e - to convert SAR amount to EURO");
System.out.println("Q or q - to quit");
str = input.next().charAt(0);
if (str == 'U' || str == 'u' ) {
}
else if (str == 'E' || str == 'e' ){
}
} while( str != 'Q' || str != 'q' );
}

str != 'Q' || str != 'q' is always true. Any given string is not equal to one or the other of these (or both). You want && instead of ||.

A nice structure to use would be the structure similar to the one recommended for getopt_long() in C but for Java obviously. Can read the man page here https://linux.die.net/man/3/getopt_long
while(true)
{
str = input.next().charAt(0);
if(str.toUpperString.equals('Q'))
{
break;
}
// In general if you want upper and lower case to do the same thing
//use toUpperString
switch (str) {
case 'U':
case 'u':
// Do something
break;
case 'E':
case 'e':
// Do something
break;
default:
System.out.println("Wrong input");
break;
}
}
Another good solution to this problem, if you don't want to use the switch statement, is the loop-and-a-half.
https://codehs.gitbooks.io/introcs/content/Basic-JavaScript-and-Graphics/loop-and-a-half.html
The structure goes like this:
while(true)
{
String token = Character.toUpperCase( input.next().charAt(0) );
if(token.equals('A'))
{
// Do something
}
else if(token.equals('B'))
{
// Do something
}
else if(token.equals('Q'))
{
break;
}
else
{
System.out.println("Invalid Option");
}
}

If you want to stop the program when you write Q or q. you can convert the input to any of the lower or uppercase.
str.toUpperCase();
if (str == 'U'){
}
else if (str == 'E'){
}
} while( str != 'Q');
}
OR
change the condition for checking Q or q.
while( str != 'Q' && str != 'q' );
for the above code snippet, the while loop will execute if str is not Q or q.

public static void main(String [] args) {
Scanner input= new Scanner(System.in);
char str ;
do{
System.out.println("Choose one of the following option : ");
System.out.println("U or u - to convert SAR amount to USD");
System.out.println("E or e - to convert SAR amount to EURO");
System.out.println("Q or q - to quit");
str = input.next().charAt(0);
if (str == 'U' || str == 'u'){}
else if (str == 'E' || str == 'e'){}
} while(str != 'Q' && str != 'q'); // change the condition from || to &&
// Also close the input stream to avoided memery leakage
input.close();
}

Requiring user to enter only 1 char for a letter grade for a gpa program

I am trying to finish up this Java assignment and I am stuck on this last bit. I need to make it so when a user enters a letter grade in order to calculate their G.P.A. they can only enter one letter.
For example, they need to receive an error if they enter aaa instead of a.
I am stuck on how to go about doing this. Everything works except this one thing. I am very new to Java so any help would be great.
Here is my class:
public class Gpa {
private int sumCredits;
private int sumPoints;
public int getPointsForGrade(char letter) {
int gradepoints;
switch (letter)
{
case 'A':
case 'a':
gradepoints = 4;
break;
case 'B':
case 'b':
gradepoints = 3;
break;
case 'C':
case 'c':
gradepoints = 2;
break;
case 'D':
case 'd':
gradepoints = 1;
break;
case 'F':
case 'f':
gradepoints = 0;
break;
default:
gradepoints = -1;
break;
}
return gradepoints;
}
public void Constructor(){
sumCredits = 0;
sumPoints = 0;
return;
}
public void addGrade( char letter, int credits){
sumCredits = sumCredits + credits;
sumPoints = sumPoints + getPointsForGrade(letter) * credits;
}
public double calcGPA(){
double GPA;
if (sumCredits == 0){
return 0;
}
GPA = (double)sumPoints / sumCredits;
return GPA;
}
public int totalCredits(){
return sumCredits;
}
public int totalPoints(){
return sumPoints;
}
}
and here is my test class, or gpa calculator
import java.util.*;
import java.util.Scanner;
public class ComputeGpa {
public static void main (String [] args){
Scanner scan = new Scanner(System.in);
Gpa gpa = new Gpa();
int courses;
int credits;
char letter;
while (true){
System.out.println(" Enter number of courses: ");
courses = scan.nextInt();
if (courses > 0){
break;
}
else {
System.out.println(" Invalid number of courses - must be greater than 0");
}
}
for (int i = 1; i<= courses; i++)
{
while (true){
System.out.println(" Enter Grade ( one character )");
String grade = scan.next();
letter = grade.toUpperCase().charAt(0);
if (letter == 'A' || letter == 'B' || letter == 'C' || letter =='D' || letter == 'F' ){
break;
}
else {
System.out.println(" must enter A,B,C,D,F (Upper or Lower case)");
}
if (letter != 'A' || letter != 'B' || letter != 'C' || letter != 'D' || letter != 'F'){
System.out.println("must be 1 letter only");
}
}
while (true){
System.out.println(" Enter Credits: ");
credits = scan.nextInt();
if (credits < 0 || credits > 9){
System.out.println("Invalid credits = must be between 0 and 9, inclusively");
}
else {
break;
}
}
gpa.addGrade(letter, credits);
System.out.println(" Points: " + gpa.getPointsForGrade(letter) );
System.out.println(" Sum Points: " + gpa.totalPoints() + " Sum Credits: " + gpa.totalCredits());
}
System.out.printf(" GPA: %.2f%n", gpa.calcGPA());
}
}

You could just check to see if there are more entries in the input. I did not run this code, but this is a simple way to check.
System.out.println(" Enter Grade ( one character )");
String grade = scan.next();
letter = grade.toUpperCase().charAt(0);
if(grade.length() > 1){ // check the size of the input
System.out.println("You entered too many letters");
}
else if (letter == 'A' || letter == 'B' || letter == 'C' || letter =='D' || letter == 'F' ){
break;
}
else {
System.out.println(" must enter A,B,C,D,F (Upper or Lower case)");
}

Where you ran scan.next() and stored it into a string, you can check the length of the string.
if (grade.length() > 1) {
// show an error or throw an exception or smth
System.out.println("Error, too many chars"); // do this if you want to print an error
throw new Exception("TOO MANY CHARS"); // do this and don't catch it if you want to crash the program
}
EDIT: For OP's request of asking for the grade again if it's wrong
String grade = "x"; // set the base case that will never occur
while (!grade.equals("x")) { // check if it is x - if it's not x, continue
if ((grade = scan.next()).length() == 1) {
// check the length - if it's 1 letter
// check the letter here. If it's not a valid letter,
// set grade back to x
} else {
// if you're here, it wasn't 1 letter.
// print an error msg
System.out.println("too many chars");
// Therefore, set grade back to x so that we ask again
grade = "x";
}
}

Is there a more efficient way to check characters in a string?

For this assignment, I need to accept a lock combination and check if:
It is 9 chars long
At position 2,8 the char is R or r
At position 5 the char is l or L
All of the other positions are integers 0-9
I am wondering if there is a better way to do this than checking each position for a digit?
Scanner input = new Scanner(System.in);
String lockComb;
System.out.print("Please enter a lock combination ( ddRddLddR ): ");
lockComb = input.nextLine();
if((lockComb.length() == 9) && ((lockComb.charAt(2) == 'r') || (lockComb.charAt(2) == 'R')) &&
((lockComb.charAt(5) == 'l') || (lockComb.charAt(5) == 'L')) && ((lockComb.charAt(8) == 'r')
|| (lockComb.charAt(8) == 'R')))
{
if((Character.isDigit(lockComb.charAt(0))) && (Character.isDigit(lockComb.charAt(1))) &&
(Character.isDigit(lockComb.charAt(3)) && (Character.isDigit(lockComb.charAt(4))) &&
(Character.isDigit(lockComb.charAt(6))) && (Character.isDigit(lockComb.charAt(7)))))
{
System.out.println(lockComb + " is a valid lock combination!");
}
else
{
System.out.println(lockComb + " is not a valid lock combination!");
}
}
else
{
System.out.println(lockComb + " is not a valid lock combination!");
}

To simplify things, you can use a regular expression:
if (lockComb.matches("[0-9][0-9][rR][0-9][0-9][lL][0-9][0-9][rR]")
(That's lowercase-l and uppercase-L in the middle.)
(No need to check the length, which is implicitly defined by the regular expression.)

How about just for fun a solution that doesn't involve regular expressions. I'm not arguing at all for or against just merely a different solution. One thing you do lose if using a regular expression is the ability to tell exactly why this is not a valid lock combination. It's up to you to figure out what the best solution is given the scenario you are coding for.
public static boolean matcher (String lockComb) {
if(lockComb.length() != 9) {
System.out.println(lockComb + " is not a valid lock combination!");
return false;
}
boolean isValid = true;
char[] comb = lockComb.toUpperCase().toCharArray();
for (int i = 0; i < comb.length; i++) {
switch (i) {
case 2:
case 8:
isValid = (comb[i] == 'R');
break;
case 5:
isValid = (comb[i] == 'L');
break;
default:
isValid = Character.isDigit(comb[i]);
break;
}
if(isValid == false) break;
}
if(isValid) {
System.out.println(lockComb + " is a valid lock combination!");
} else {
System.out.println(lockComb + " is not a valid lock combination!");
}
return isValid;
}

Java issue with scanner

I have a method that should scan for one of 3 letters in caps or lowercase and return the lower case version of the letter. If an improper letter is entered the user is warned and reasked for a letter. I have two issues, 1: as soon as the method is run I get the outputted line with the error message telling the user invalid entry without waiting for an entry! (so the second the method is run I see High, low or sevens (H/L/S):Invalid entry. Please try again using H/L/S! before entering anything then the method is recalled again and all works fine form there except for my next issue) 2: the entry that is gotten from the scanner never passes any of my if statements even though it should.
my code:
private static char getHighLow(Scanner inScanner) {
System.out.print("High, low or sevens (H/L/S):");
String entered = inScanner.nextLine();
System.out.print(entered);
if(entered.equals("H") || entered.equals("h")){
return 'h';
}
else if (entered.equals("L") || entered.equals("l")){
return 'l';
}
else if(entered.equals("S") || entered.equals("s")){
return 's';
}
char result = 0;
while(result != 'l' || result != 'h' || result != 's'){
System.out.println("Invalid entry. Please try again using H/L/S!");
result=getHighLow(inScanner);
}
return result;
}

Instead of using while(), you can use 'else' like this-
private static char getHighLow(Scanner inScanner) {
System.out.print("High, low or sevens (H/L/S):");
String entered = inScanner.nextLine();
System.out.print(entered);
if(entered.equals("H") || entered.equals("h")){
return 'h';
}
else if (entered.equals("L") || entered.equals("l")){
return 'l';
}
else if(entered.equals("S") || entered.equals("s")){
return 's';
}
else {
System.out.println("Invalid entry. Please try again using H/L/S!");
return getHighLow(inScanner);
}
}

You can simply use equalsIgnoreCase and trim the entered string. And add a while loop util your condition is satisfied.
Scanner scanner = new Scanner(System.in);
boolean loop = true;
String choice = null;
while (loop) {
System.out.print("High, low or sevens (H/L/S):");
choice = scanner.nextLine();
if ("H".equalsIgnoreCase(choice.trim())
|| "L".equalsIgnoreCase(choice.trim())
|| "S".equalsIgnoreCase(choice.trim())) {
System.out.println("Correct Choice");
loop = false;
}
else
{
System.out.println("Wrong Choice");
}
}
System.out.print(choice);

char result;
while(true){
System.out.print("High, low or sevens (H/L/S):");
String entered = inScanner.nextLine();
System.out.print(entered);
if(entered.equals("H") || entered.equals("h")){
result = 'h';break;
}
else if (entered.equals("L") || entered.equals("l")){
result = 'l';break;
}
else if(entered.equals("S") || entered.equals("s")){
result = 's';break;
}else{
System.out.println("Invalid entry. Please try again using H/L/S!");
}
}

Hey you are not breaking out of the while loop at all. Did you see that ?

This is what you want. Here is the program to iterate over characters in a String. And convert them in lower case letter if they are H,L OR S.
package testproj;
import java.util.Scanner;
public class TestProj {
public static void main(String[] args) {
Scanner scanner = new Scanner("HLs");
String result = getHighLow(scanner);
System.out.println("Result :"+result);
}
private static String getHighLow(Scanner inScanner) {
System.out.println("High, low or sevens (H/L/S):");
String entered;
String result = "";
boolean isCharFound = false;
String temp = "";
while (inScanner.hasNext()) {
temp = inScanner.next();
System.out.println(temp);
for (int index = 0; index < temp.length(); index++) {
entered =new Character(temp.charAt(index)).toString() ;
if (entered.equals("H") || entered.equals("h")) {
result = result + 'h';
isCharFound = true;
} else if (entered.equals("L") || entered.equals("l")) {
result = result + 'l';
isCharFound = true;
} else if (entered.equals("S") || entered.equals("s")) {
result = result + 's';
isCharFound = true;
}
if (!isCharFound) {
System.out.println("Invalid entry. Please try again using H/L/S!");
}
isCharFound = false;
}
}
return result;
}
}

Stack Calculator: Trouble evaluating postfix expression because of casting issue

I'm working on a homework assignment that asks me to create a calculator that changes the expression given to it from infix to postfix to then evaluate. I must do so using stacks but may choose any stack implementation I want as long as I don't use the java.util.Stack from the JCF. I chose a referenced based stack.
The problem I'm having is in my evaluatePostfix method. In order to evaluate the expression I had to cast my operand variables as Integers but eclipse doesn't seem to like that. I keep getting a "java.lang.Character cannot be cast to java.lang.Integer" error. I'm not sure how to fix this issue. Does anyone have any insight?
Here is my code:
public class InfixToPostfixAndEvaluateCalculator {
private String infix;
private String postfix;
private int result;
public InfixToPostfixAndEvaluateCalculator() {
infix=null;
postfix=null;
result=0;
}
public InfixToPostfixAndEvaluateCalculator(String infix) {
this.infix=infix;
postfix=null;
result=0;
}
public String getInfix() {
return infix;
}
public String getPostfix() {
return postfix;
}
public int getresult() {
return result;
}
public void setInfix(String infix) {
this.infix=infix;
}
public void setPostfix(String postfix) {
this.postfix=postfix;
}
public String toString() {
return " Infix: "+infix+"\n Postfix: "+postfix+"\n Result: "+result+"\n";
}
public String infixToPostfix() { //Carrano 2nd ed. p.354
//opStack is a stack of Character objects, such as '+','-','*','/', and ')'
StackInterface opStack=new StackReferenceBased();
String postfixExp=""; //the expression to be built in this method
//for each character ch in the string infix
for (int i=0; i<infix.length(); i++) {
char ch=infix.charAt(i);
switch (ch) {
//if ch is an operator
case '+': case '-': case '*': case '/':
while ( (!opStack.isEmpty())
&& (!opStack.peek().equals('('))
&& (precedence(ch) <= precedence((Character)opStack.peek()))){
postfixExp = postfixExp + opStack.pop();
}
opStack.push(ch);
break;
case '(': //add to stack
opStack.push(ch);
break;
case ')': //start popping things off the stack until you find opening parenthesis, use peak
while (!((Character)opStack.peek()).equals('(')){
postfixExp = postfixExp + opStack.pop();
}//end while
opStack.pop();
break;
default: //ch is an operand
postfixExp = postfixExp + ch;
break;
}//end of switch
}//end of for
System.out.println("End of for loop.");
//append to postfixExp the operators remaining in the stack
while (! opStack.isEmpty()) {
postfixExp=postfixExp+((Character) opStack.pop()).charValue();
}//end of while
postfix=postfixExp; //update the instance variable
return postfixExp;
}//end of infixToPostfix()
//little helper function to determine precedence value of an operator
// *,/ have value of, say 20
// +,- have value of, say 10
private int precedence(char ch) {
int prec = 20;
int prec2 = 10;
if (ch == '*' || ch == '/'){
return prec;
}
if (ch == '+' || ch == '-'){
return prec2;
}
return -1;
}
public int evaluatePostfix() { //Carrano 2nd ed. pp.350-351
//valueStack is a stack of Integer objects:
StackInterface valueStack=new StackReferenceBased();
//variables for the operands:
int operand1, operand2;
//for each character ch in the string postfix
for (int i=0; i<postfix.length(); i++) {
char ch=postfix.charAt(i);
switch (ch) {
//if ch is an operator
case '+':
operand2 = (Integer)valueStack.pop();
operand1 = (Integer)valueStack.pop();
result = operand1 + operand2;
valueStack.push(result);
break;
case '-':
operand2 = (Integer)valueStack.pop();
operand1 = (Integer)valueStack.pop();
result = operand1 - operand2;
valueStack.push(result);
break;
case '*':
operand2 = (Integer)valueStack.pop();
operand1 = (Integer)valueStack.pop();
result = operand1 * operand2;
valueStack.push(result);
break;
case '/':
operand2 = (Integer)valueStack.pop();
operand1 = (Integer)valueStack.pop();
result = operand1 / operand2;
valueStack.push(result);
break;
default: //ch is an operand
valueStack.push(ch);
break;
}//end of switch
}//end of for
//at the end, the value of the expression will be on the top of the stack
result=((Integer) valueStack.pop()).intValue();
return result;
}//end of evaluatePostfix()
} // end StackTest

Yes, you cannot cast Character to Integer.
To do that you can use,
Integer.parseInt(String.valueOf(valueStack.pop()));
parseInt doesn't take Character as argument so, you have to convert first into String and then to Integer.

there is a function to get the numeric int value of a Unicode character
Character.getNumericValue( ch );
hopefully the StackInterface supports type-information
this would prevent dozens of (Integer)-casts
StackInterface<Integer> valueStack = new StackReferenceBased<Integer>();