Develop Reference

infinite loop in a while statement

import java.util.Scanner;
public class Main {
public static void main(String[] args) {
System.out.println("\nThe sum of the numbers is: " + getSumOfInput());
}
public static int getSumOfInput () {
int counter = 0;
int sumOfNums = 0;
Scanner userInput = new Scanner(System.in);
while(counter <= 10) {
System.out.print("Enter the number " + counter + ": ");
boolean checkValidity = userInput.hasNextInt();
if(checkValidity) {
int userNum = userInput.nextInt();
userInput.nextLine();
System.out.println("Number " + userNum + " added to the total sum.");
sumOfNums += userNum;
counter++;
} else {
System.out.println("Invalid input. Please, enter a number.");
}
}
userInput.close();
return sumOfNums;
}
}
Hello everybody!
I just started java and I learned about control flow and now I moved on to user input, so I don't know much. The problem is this code. Works just fine if you enter valid input as I tested, nothing to get worried about. The problem is that I want to check for wrong input from user, for example when they enter a string like "asdew". I want to display the error from else statement and to move on back to asking the user for another input, but after such an input the program will enter in an infinite loop displaying "Enter the number X: Invalid input. Please, enter a number.".
Can you tell me what's wrong? Please, mind the fact that I have few notions when it comes to what java can offer, so your range of solutions it's a little bit limited.

Call userInput.nextLine(); just after while:
...
while(counter <= 10) {
System.out.print("Enter the number " + counter + ": ");
userInput.nextLine();
...

The issue is, that once you enter intput, which can not be interpreted as an int, userInput.hasNextInt() will return false (as expected). But this call will not clear the input, so for every loop iteration the condition doesn't change. So you get an infinite loop.
From Scanner#hasNextInt():
Returns true if the next token in this scanner's input can be interpreted as an int value in the default radix using the nextInt() method. The scanner does not advance past any input.
The fix is to clear the input if you came across invalid input. For example:
} else {
System.out.println("Invalid input. Please, enter a number.");
userInput.nextLine();
}
Another approach you could take, which requires less input reads from the scanner, is to always take the next line regardless and then handle the incorrect input while parsing.
public static int getSumOfInput() {
int counter = 0;
int sumOfNums = 0;
Scanner userInput = new Scanner(System.in);
while (counter <= 10) {
System.out.print("Enter the number " + counter + ": ");
String input = userInput.nextLine();
try {
int convertedInput = Integer.parseInt(input);
System.out.println("Number " + convertedInput + " added to the total sum.");
sumOfNums += convertedInput;
counter++;
} catch (NumberFormatException e) {
System.out.println("Invalid input. Please, enter a number.");
}
}
return sumOfNums;
}

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;

Product of Number input by user, program stops when user inputs 0

i want to make a program reads integers from the user one by one, multiply them and shows the product of the read integers. The loop for reading the integers
stops when the user presses 0. If the user enters a 0 as the first number, then user would not be able to provide any other numbers (Not adding the last 0 in the product). In this case, the program should display “No numbers entered!”
Heres my code right now
ProductNumbers.java
package L04b;
import java.lang.reflect.Array;
import java.util.Scanner;
public class ProductNumbers {
public static void main(String[] args) {
int num = -1;
boolean isValid = true;
int numbersEntered = 0;
int product = -1;
Scanner scnr = new Scanner(System.in);
System.out.println(
"This program reads a list of integers from the user\r\n"
+ "and shows the product of the read integers");
while (num != 0) {
System.out.print("Enter number = ");
int curNum = scnr.nextInt();
if (curNum == 0)
break;
numbersEntered++;
product *= num;
}
if (numbersEntered == 0) {
System.out.println("No numbers entered!");
} else {
System.out.println(product);
}
}
}
I know this is completely wrong, i usually setup a template, try to figure out what needs to be changed, and go by that way, i also need to start thinking outside the box and learn the different functions, because i dont know how i would make it end if the first number entered is 0, and if the last number is 0, the program stops without multiplying that last 0 (so that the product doesnt end up being 0)... i need someone to guide me on how i could do this.
Heres a sample output of how i want it to work
This program reads a list of integers from the user
and shows the product of the read integers
Enter the number:
0
No numbers entered!
and
This program reads a list of integers from the user
and shows the product of the read integers
Enter the number:
2
Enter the number:
-5
Enter the number:
8
Enter the number:
0
The product of the numbers is: -80

You have a nested for loop, why?
You only need the outer while loop that gets the user's input until the input is 0.Also this line:
product *= i;
multiplies i, the for loop's counter to product and not the user's input!
Later, at this line:
if (isValid = true)
you should replace = with ==, if you want to make a comparison, although this is simpler:
if (isValid)
Your code can be simplified to this:
int num = -1;
int product = 1;
int counter = 0;
Scanner scnr = new Scanner(System.in);
System.out.println(
"This program reads a list of integers from the user\r\n"
+ "and shows the product of the read integers");
while (num != 0) {
System.out.print("Enter a number: ");
num = scnr.nextInt();
scnr.nextLine();
if (num != 0) {
counter++;
product *= num;
System.out.println(product);
}
}
if (counter == 0)
System.out.println("No numbers entered");
else
System.out.println("Entered " + counter + " numbers with product: " + product);

One way to solve this is to utilize the break; keyword to escape from a loop, and then you can process the final result after the loop.
Something like this:
int numbersEntered = 0;
while (num != 0) {
int curNum = // read input
if (curNum == 0)
break;
numbersEntered++;
// do existing processing to compute the running total
}
if (numbersEntered == 0)
// print "No numbers entered!
else
// print the result
I think the key is to not try and do everything inside of the while loop. Think of it naturally "while the user is entering more numbers, ask for more numbers, then print the final result"

Generating Randoms but a few minor errors

I did look at the threads pertaining to randoms and implemented them into this assignment but I have 2 questions.
1) I need my program to generate random numbers (and print them) and count the iterations. I have counting the iterations down but I don't know why the random numbers don't print out. Does it have something to do with my guess = 0? Here's an example if I'm not clear.
Example:
Enter a number: 13
85
89
73
94
13
This took 5 tries
2) I have no clue why my program always ends up stuck at one number for the answer. The program immediately ends after entering the number 86.
import java.util.*;
public class FeelingLucky {
public static void main (String [] args) {
Scanner sc = new Scanner (System.in);
int tries = 0;
int guess = 0;
Random random = new Random(1);
int num = random.nextInt(100) + 1;
System.out.print("Pick a number between 1 and 100:");
while (guess != num) {
guess = sc.nextInt();
tries++;
}
System.out.println("It took " + tries + " tries to match");
sc.close();
}
}

Random(1) uses seed in constructor which is always the same. Use just Random() - no parameter constructor.
import java.util.*;
public class FeelingLucky {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int tries = 0;
int guess = 0;
Random random = new Random(); // No seed
int num = random.nextInt(100) + 1;
System.out.print("Pick a number between 1 and 100:");
while (guess != num) {
guess = sc.nextInt();
tries++;
}
System.out.println("It took " + tries + " tries to match");
sc.close();
}
}
See Java random always returns the same number when I set the seed?

You've called nextInt() on the Random object only once, so you've only generated the one random number. Inside your loop you call nextInt() on the scanner, which is reading from System.in, so your program is halting and waiting for the user to input a number again each time around the loop.
If you want the user to enter a single number once, and then Random to keep generating numbers until they match, you'd need to swap which one is called inside the loop. To print the random numbers being generated, you'd need to add a print statement inside the loop that prints that current number.

while (guess != num) {
num = random.nextInt(100) + 1;
guess = sc.nextInt();
System.out.printf("you guessed: %d the number was %d%n",guess, num);
tries++;
}
This one will print out each time, and guess a new random number each time.

do while loop with try catch [duplicate]

This question already has answers here:
Problem with "scopes" of variables in try catch blocks in Java
(3 answers)
Closed 6 years ago.
What is wrong with my code? I'm trying request prompt a user to enter a number if it is not between 1 and 6 or a number it should say invalid and prompt another try. The user may only have 3 tries.
import java.util.Scanner;
public class Game {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int[] numbers = new int[6];
System.out.println("Enter a number on a die from 1-6:");
String dieinput = input.next();
int invalidcount = 0;
System.out.println("Your number is: " + dieinput);
do{
try
{
// the String to int conversion
int dienum = Integer.parseInt(dieinput.trim());
int dienumOE = 0;
int count=0;
//test number input
if ((dienum >= 1) && (dienum<= 6))
System.out.println("number is " + dienum + oddoreven(dienum));
else
System.out.println("Invalid number, please enter a number 1-6");
count++;
System.out.println("Count" + count);
}
catch (NumberFormatException nfe){
System.out.println("Invalid number, please enter a number 1-6");
System.out.println("Count" + count);
}
while (count <= 3 && count >=0);
}
}
// Check if odd or even
public static String oddoreven(int num) {
String result;
if (num % 2 == 0)
result = "even";
else
result = "odd";
return result;
}
}

Your problem is "wrong" scoping: You can't use a variable within the catch block that is declared within the scope of the try block!
In other words; you need:
int counter = 0;
try { ...
} catch (...
if you want to use counter within both try and catch!
The "rule" is very simple: a variable is only visible up to that } that closes the "block" it is in.
That prevents you from actually compiling your code!
So, the real lesson here is more like: do not write 100 lines of code to then run the compiler. Only write as few lines as possible - every time when you think: this is "enough" to compile .. then compile. And then fix the errors reported to you. The java compiler is really good at giving you specific messages that tell you what you did wrong; but the more messy code you put up, the harder that gets!
And for the record: SO is not a "remote" compiler service that you should use to get your messy code to compile!