Develop Reference

Java Sum of numbers until string is entered

i've just started java programming and was wondering on how to approach or solve this problem i'm faced with.
I have to write a program that asks a user for a number and continually sums the numbers inputted and print the result.
This program stops when the user enters "END"
I just can't seem to think of a solution to this problem, any help or guidance throughout this problem would be much appreciated and would really help me understand problems like this. This is the best i could do
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
while (true) {
System.out.print("Enter a number: ");
int x = scan.nextInt();
System.out.print("Enter a number: ");
int y = scan.nextInt();
int sum = x + y;
System.out.println("Sum is now: " + sum);
}
}
}
The output is supposed to look like this:
Enter a number: 5
Sum is now: 5
Enter a number: 10
Sum is now: 15
Enter a number: END

One solution would be to not use the Scanner#nextInt() method at all but instead utilize the Scanner#nextLine() method and confirm the entry of the numerical entry with the String#matches() method along with a small Regular Expression (RegEx) of "\d+". This expression checks to see if the entire string contains nothing but numerical digits. If it does then the matches() method returns true otherwise it returns false.
Scanner scan = new Scanner(System.in);
int sum = 0;
String val = "";
while (val.equals("")) {
System.out.print("Enter a number (END to quit): ");
val = scan.nextLine();
// Was the word 'end' in any letter case supplied?
if (val.equalsIgnoreCase("end")) {
// Yes, so break out of loop.
break;
}
// Was a string representation of a
// integer numerical value supplied?
else if (val.matches("\\-?\\+?\\d+")) {
// Yes, convert the string to integer and sum it.
sum += Integer.parseInt(val);
System.out.println("Sum is now: " + sum); // Display Sum
}
// No, inform User of Invalid entry
else {
System.err.println("Invalid number supplied! Try again...");
}
val = ""; // Clear val to continue looping
}
// Broken out of loop with the entry of 'End"
System.out.println("Application ENDED");
EDIT: Based on Comment:
Since since an integer can be signed (ie: -20) or unsigned (ie: 20) and the fact that an Integer can be prefixed with a + (ie: +20) which is the same as unsigned 20, the code snippet above takes this into consideration.

Do it like this:
public static void main(String[] args) throws Exception {
int sum = 0;
Scanner scan = new Scanner(System.in);
while (scan.hasNext()) {
System.out.print("Enter a number: ");
if (scan.hasNextInt())
sum += scan.nextInt();
else
break;
System.out.println("Sum is now: " + sum);
}
System.out.print("END");
}
This will end if the input is not a number (int).
As pointed out in the comments, if you want the program to stop when the user specifically enters "END", change the else-statement to:
else if (scanner.next().equals("END"))
break;

Scanner loop doesn't cycle when hitting enter twice

boolean loop = false;
double numberOfStudents;
System.out.print("Enter a number: ");
if ((scnr.nextLine().trim().isEmpty()) ) {
loop = true;
}
while (loop) {
System.out.println("Enter a number");
if (scnr.hasNextDouble() ){
System.out.println("Loop has stopped");
numberOfStudents = scnr.nextDouble();
loop = false;
}
}
System.out.println("You're outside the loop!");
I'm trying to get the program to say "Enter a number" until the user has entered an actual number (no white spaces or letters or signs). When the user has entered a number, it sets numberOfStudents equal to that number and breaks out of the loop.
But if you hit enter twice, it doesn't iterate. It only displays "Enter a number" once.
What is wrong with the loop logic? Why isn't it looping until valid input is taken?

For the actual answer to your question of "Why doesn't 'Enter a number' display more than once?" see Tom's comment (update: Tom's answer).
I've rewritten your loop in a way which preserves your code, but also makes it a little easier to handle format exceptions (though at the risk of silently swallowing an exception -- should be acceptable for this use case).
Can be up to you to use this design, here is an SO post on why empty catch blocks can be a bad practice.
public static void main(String args[])
{
boolean loop = true;
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
while(loop){
System.out.print("Enter a number: ");
String input = scnr.nextLine();
try{
numberOfStudents = Double.parseDouble(input);
loop = false;
}catch(NumberFormatException e){
}
}
System.out.println("You're outside the loop!");
}
Output:
Enter a number:
Enter a number:
Enter a number:
Enter a number: 50
You're outside the loop!

First of all: Since you're reading from System.in a call to the input stream will block until the user entered a valid token.
So let's check first scan using your scnr variable:
scnr.nextLine()
nextLine() reads everything til the next line delimiter. So if you just press return, then it will successfully read it and will perform the next stuff.
The next call is:
scnr.hasNextDouble()
This call expects a "real" token and ignores white spaces, except as a delimiter between tokens. So if you just press return again it doesn't actually read that input. So it still waits for more (for the first token). That is why it stucks in your loop and you won't get another "Enter a number" output.
You can fix that by either enter a real token, like a number, or by changing the loop like trobbins said.
I hope you now understand your program flow a bit more :).

While trobbins code basically solves your problem, it's bad practice to use exceptions for flow control.
I used a small regexp to check if the value is a number. But this example is not complete, it will still crash it the user enters for example two decimal points. So you would need to create a proper number check or just use integers where the check is much easier.
Someone in the comments pointed out that people may want to enter scientific notation like 5e10, so this would also be another case to check for. If this is just some code you need as a proof of concept or something quick and dirty, you can go with the exception handling method but in production code you should avoid using exceptions this way.
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
while(true) {
System.out.print("Enter a number: ");
String input = scnr.nextLine().trim();
if(input.matches("^[0-9\\.]{1,}$")) {
System.out.println("Loop has stopped");
numberOfStudents = Double.parseDouble(input);
break;
}
}
System.out.println("You're outside the loop!");

The following code should help you:
double numberOfStudents = 0;
Scanner scnr = new Scanner(System.in);
boolean readValue = false; //Check if the valid input is received
boolean shouldAskForNumber = true; //Need to ask for number again? Case for Enter
do {
if (shouldAskForNumber) {
System.out.print("Enter a number:");
shouldAskForNumber = false;
}
if (scnr.hasNextDouble()) {
numberOfStudents = scnr.nextDouble();
readValue = true;
} else {
String token = scnr.next();
if (!"".equals(token.trim())) { //Check for Enter or space
shouldAskForNumber = true;
}
}
} while (!readValue);
System.out.printf("Value read is %.0f\n", numberOfStudents);
System.out.println("You're outside the loop!");
Update
Understood the following statement in question different way:
But if you hit enter twice, it doesn't loop back. It only displays
"Enter a number" once.
The code is set to print "Enter a number" only once if the user hits RETURN/ENTER or enters space character. You may remove the special check and use the code if needed.

import java.util.Scanner;
public class Testing {
public static boolean checkInt(String s)
{
try
{
Integer.parseInt(s);
return true;
} catch (NumberFormatException ex)
{
return false;
}
}
public static void main(String[] args) {
boolean loop = false;
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
String input = "";
while (!(checkInt(input))) {
System.out.println("Enter a number");
input = scnr.nextLine();
}
numberOfStudents = Integer.parseInt(input);
System.out.println("Number of students: " + numberOfStudents );
}
}
//this code is working fine, if you want you check it out.
//In your code your taking another input if the first is an int/double; if the first input is not a number then you have mentioned to take input again..

Use a debugger to see what the code is actually doing. Here's a guide on debugging in Eclipse. After you have finished debugging your code, you will probably know what the problem is.

Below code will help you
boolean loop = true;
double numberOfStudents;
Scanner scnr = new Scanner(System.in);
System.out.print("Enter a number: ");
String input = scnr.nextLine();
while(!scnr.hasNextDouble()){
System.out.print("Enter a number: ");
try{
numberOfStudents = Double.parseDouble(input);
break;
}catch(NumberFormatException e){
}
input = scnr.nextLine();
}
System.out.println("You're outside the loop!");

The following code is working,
boolean loop = true;
double numberOfStudents;
Scanner scnr=new Scanner(System.in);
while(loop) {
System.out.println("Enter a number");
if ((scnr.nextLine().trim().isEmpty()) ) {
loop = true;
}
if (scnr.hasNextDouble() ){
System.out.println("Loop has stopped");
numberOfStudents = scnr.nextDouble();
loop = false;
}
}
System.out.println("You're outside the loop!");
The output is,
run:
Enter a number
hj
po
Enter a number
lhf
Enter a number
o
Enter a number
p
Enter a number
a
Enter a number
34
Loop has stopped
You're outside the loop!

You have to scan the next line if you want to get more values form the scanner again. The code should be like:
while (loop) {
System.out.println("Enter a number");
if(!(scnr.nextLine().trim().isEmpty())){
if (scnr.hasNextDouble() ){
System.out.println("Loop has stopped");
numberOfStudents = scnr.nextDouble();
loop = false;
}
}
}

Do-While Qualifiers and Totalling in Java

I am trying my hand a few basic do-while codes, and am running into a couple of problems.
I want the code to ask the user to input 1 of 3 options (choosing which group they would like to add a number to, or to exit and total), give an error if they input an irrelevant option, and then total all ints at the end for each group.
public static void main(String[] args) {
String answer = "default";
int grp1 = 0;
int grp2 = 0;
int input1 = 0;
int input2 = 0;
do{
System.out.println("Make a selection:\n");
System.out.println("A: Enter a number for Group 1.");
System.out.println("B: Enter a number for Group 2.");
System.out.println("X: Exit and total the numbers for each group.\n");
System.out.println("Select your option: ");
answer = keyboard.next();
if (answer.equalsIgnoreCase("A")){
System.out.println("Enter int: ");
input1 = keyboard.nextInt(); // add an int to grp1
}
else if (answer.equalsIgnoreCase("B")){
System.out.println("Enter int: ");
input2 = keyboard.nextInt(); // add an int to grp2
}
else if (answer.equalsIgnoreCase("X")){
} // exit and total
else {
System.out.println("Invalid option - Try again.");
} // Invalid input - restart
}
while (answer.equals("A") || answer.equals("B"));
grp1 += input1;
grp2 += input2;
keyboard.close();
System.out.println("Group 1's total is: + grp1);
System.out.println("Group 2's total is: + grp2);
}
I need the to add a qualifier for if the user does not input a valid option, I tried using else:
else {
System.out.println("Invalid option - Try again.")
}
but this just skips to printing the totals, and does not ask the user for another input. How would I best achieve this?
Also,
grp1 += input1;
grp2 += input2;
Only counts the lasted entered int, is there a way to have it add all the entered ints?
Any help would be greatly appreciated, even outside of the questions I asked.

I think you have two confusions.
1) The "while" line in your code applies to the "do" block above it. That means that based on where the grp1 += and grp2 += lines are, they will only ever be run once. I suggest moving those calls to the end of the loop. You could move each line inside the relevant if block so that the code is run every time the user successfully enters a number after A or B.
2) The while condition is asking if the user entered "A" or "B". It's saying if they did, continue looping by going back to "do". If they entered literally anything else (any invalid answer), it will stop and run the code after the "while" line. I think what you really want is while (!answer.equals("X")), which will continue the loop until the user correctly enters an "X" character.
You'll also want to move those grp += lines up a bit.

Just change the condition inside while And also shift the totalling logic
do{
System.out.println("Make a selection:\n");
System.out.println("A: Enter a number for Group 1.");
System.out.println("B: Enter a number for Group 2.");
System.out.println("X: Exit and total the numbers for each group.\n");
System.out.println("Select your option: ");
answer = keyboard.next();
if (answer.equalsIgnoreCase("A")){
System.out.println("Enter int: ");
input1 = keyboard.nextInt(); // add an int to grp1
grp1 += input1;
}
else if (answer.equalsIgnoreCase("B")){
System.out.println("Enter int: ");
input2 = keyboard.nextInt(); // add an int to grp2
grp2 += input2;
}
else if (answer.equalsIgnoreCase("X")){
} // exit and total
else {
System.out.println("Invalid option - Try again.");
} // Invalid input - restart
}
while (!answer.equals("X"));
keyboard.close();
This will make your do while loop running i.e showing options to user until they wishes to exit. And also group total would be updated properly. I have updated answer based on answer by #Devin Howard

While loop does not wait for if statement's completion [duplicate]

This question already has answers here:
How do I compare strings in Java?
(23 answers)
Closed 8 years ago.
I am working on a school project for Intro to CS and I can't post my code because our teacher fails us for getting code checked from StackOverflow. My issue is that in my code with the format :
while (condition){
Do something;
if (new condition){
Do something else;
Wait for input;
Depending on input make while's condition false;
}
This code should wait for input when the if statement is evaluated and it does something else. However, my code does not wait for "Wait for input" step and goes directly to the "Do something" step. Here's a bit of the code. Thank you for your help.
while (inum != 102) {
System.out.println("Enter a number between 1 and 100: ");
inum = input.nextInt();
else if (inum == 101) {
System.out.println("Are you sure you want to quit?");
confirm = input.nextLine();
if (confirm == "yes") {
inum = 102;
}
}
Here the code gives me this when I type in 101:
Are you sure you want to quit?
Enter a number between 1 and 100:
*The code does not wait for
confirm = input.nextLine();
if (confirm == "yes") {
inum = 102;
}
step.

The easiest way to solve your problem is calling
input.nextLine();
just below
inum = input.nextInt();
The reason is: when you type "101" in the console, you really type 101 and NEWLINE. nextInt() takes the 101 from the console buffer, but the NEWLINE character remains. For that reason, the second nextLine() in your code was skipped (the code assumed you entered a new empty line)

You should use confirm.equals("yes").
input is already use to input the number and declare another scanner object to input the confirm String.

Define an other input Scanner and don't use the same for both String and int
Try this code (I put System.exist(0) so the program exit when you enter yes)
import java.util.Scanner;
public class Test {
/**
* #param args
*/
public static void main(String[] args) {
int inum = 0;
Scanner input = new Scanner(System.in);
Scanner input1 = new Scanner(System.in);
while (inum != 102) {
System.out.println("Enter a number between 1 and 100: ");
inum = input.nextInt();
if (inum == 101) {
System.out.println("Are you sure you want to quit?");
String confirm = input1.nextLine();
if (confirm.equalsIgnoreCase("yes")) {
inum = 102;
System.exit(0);
}
}
}
}

if( StringUtils.equals(confirm, "yes") ) { ...

How to end a while Loop via user input

package cst150zzhw4_worst;
import java.util.Scanner;
public class CST150zzHW4_worst {
public static void main(String[] args) {
//Initialize Variables
double length; // length of room
double width; // Width of room
double price_per_sqyd; // Total carpet needed price
double price_for_padding; // Price for padding
double price_for_installation; // Price for installation
String input; // User's input to stop or reset program
double final_price; // The actual final price
boolean repeat = true;
// Create a Scanner object for keyboard input.
Scanner keyboard = new Scanner(System.in);
while (repeat)
{
//User Input
System.out.println("\n" +"What is the length of the room?: ");
length = keyboard.nextInt();
System.out.println("What is the width of the room?: ");
width = keyboard.nextInt();
System.out.println("What is the price of the carpet per square yard?: ");
price_per_sqyd = keyboard.nextDouble();
System.out.println("What is the price for the padding?: ");
price_for_padding = keyboard.nextDouble();
System.out.println("What is the price of the installation?: ");
price_for_installation = keyboard.nextDouble();
final_price = (price_for_padding + price_for_installation + price_per_sqyd)*((width*length)/9);
keyboard.nextLine(); //Skip the newline
System.out.println("The possible total price to install the carpet will be $" + final_price + "\n" + "Type 'yes' or 'no' if this is correct: ");
input = keyboard.nextLine();
}
}
}
How would I make it so when the user says yes the program stop and if the user says no then the program just repeats? I don't know why I'm having so much trouble. I've searched for well over 4 hours. I am only supposed to use a while loop, I think.

You have to assign repeat in your while-loop so it becomes false if the user says yes:
repeat = !input.equalsIgnoreCase("yes");

You just need to set repeat to true or false based on user input. So in the end, compare input with yes or no. Something like this would work for you :
if ("yes".equals(input))
repeat = true; // This would continue the loop
else
repeat = false; // This would break the infinite while loop

boolean repeat = true;
// Create a Scanner object for keyboard input.
Scanner keyboard = new Scanner(System.in);
while (repeat)
{
-----------------------
-------------------------
System.out.println("Do you want to continue:");
repeat = keyboard.nextBoolean();
}

you also if you want your code to be more systematic , go and search about the interrupt , specially thread interrupt , these answers above is correct , find the more organic code and implement it

You can use a break statement to exit a while loop.
while (...) {
input = ...;
if (input.equals("Y")) {
break;
}
}