I am wondering how I can stop the scanner from reading extra erroneous input from the user.
For example, I would like to read in the user input 2 one as 2 into the variable diam, which the program below achieves.
The issue is then, that the next loop automatically detects the one leftover from the input, and executes the if statement accordingly.
I was able to work around this by creating two scanners, but unfortunately this is not allowed for this particular assignment. In addition, we are required to use .hasNextInt() in our program.
How do I prevent this "spill-over" using only one scanner? I have to assume this question has been posed before, but I did not have much luck finding an answer.
Scanner scnr = new Scanner(System.in);
System.out.print("Enter the diameter of a "
+ "cylinder (in centimeters): ");
// BEGIN: diameter input verification loop
for (;;) {
if (!scnr.hasNextInt()) {
System.out.print("Please enter an integer value "
+ "(less than 2,147,483,648) as decimal digits: ");
scnr.nextLine();
continue;
}
diam = scnr.nextInt();
if (diam >= 0) {
//null statement
}
else {
System.out.print("Please enter a positive integer value: ");
continue;
}
break;
}
//END: diameter input verification loop
//prompts user for container height
System.out.print("Enter the height of a "
+ "cylinder (in centimeters): ");
// BEGIN: height input verification loop
for (;;) {
if (!scnr.hasNextInt()) {
System.out.print("Please enter an integer value "
+ "(less than 2,147,483,648) as decimal digits: ");
scnr.nextLine();
continue;
}
height = scnr.nextInt();
if (height >= 0) {
//null statement
}
else {
System.out.print("Please enter a positive integer value: ");
continue;
}
break;
}
//END: height input verification loop`
One option would be to just read the entire line of input from the Scanner, and then retain only the first word. For example, for the diameter of a cylinder you could use:
System.out.print("Enter the diameter of a cylinder (in centimeters): ");
String input = scnr.nextLine();
try {
int diam = Integer.parseInt(input.split(" ")[0]);
}
catch (NumberFormatException e) {
System.out.print("Please enter an integer value "
+ "(less than 2,147,483,648) as decimal digits: ");
}
I can think of a couple of approaches:
As Tim notes, you can use readLine() to read a complete line from the user, then parse the line. You could use split, or create a new Scanner to parse the line, or various other approaches.
You can stick with a single Scanner and call nextLine() to discard unconsumed characters up to and including the next end-of-line. Obviously, you need to do this after calling nextInt(). For example:
height = -1;
while (height < 0) {
if (!scnr.hasNextInt()) {
System.out.print("Please enter an integer value "
+ "(less than 2,147,483,648) as decimal digits: ");
} else {
height = scnr.nextInt();
if (height < 0) {
System.out.print("Please enter a positive integer value: ");
}
}
scanner.nextLine();
}
(The above version of your code has restructured things to get rid of the break and continue. The restructuring also allows me to put the readLine as an unconditional last statement for the loop. I think it makes the logic easier to understand ...)
// BEGIN: height input verification loop
for (;;) {
scnr.nextLine(); /* read fresh input, deleting the left over input */
if (!scnr.hasNextInt()) {
System.out.print("Please enter an integer value " + "(less than 2,147,483,648) as decimal digits: ");
scnr.nextLine();
continue;
}
height = scnr.nextInt();
if (height >= 0) {
// null statement
} else {
System.out.print("Please enter a positive integer value: ");
continue;
}
break;
}
You can simply add a scnr.nextLine() to remove the "old" scnr left over from the previous input. This fixes the spill-over problem.
Related
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;
}
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;
I'm working on a project that calculates the value of a bank account based on starting balance(b), interest rate(IR), and quarters to display. My entire code works perfectly, but the very last portion is to make sure the variables like interest rate are within the confines of the boundaries my professor gave me. I do need to display an error message if the user enters a value outside the boundaries and ask for the value again.
For example, the number of quarters to display needs to be greater than zero, and less or equal to 10.
As you can see, pretty much all of my program is in a do-while loop. I know I can have nested loops, but what would I be able to put in my do-while loop that would work in this situation? An if-else statement? Try and catch block? Another while loop?
If I used a try-catch, then could anyone give me an example of how I could do that? Thank you very much for your time, and all help is appreciated! The below is my code for reference.
import java.util.Scanner;
public class InterestCalculator
{
public static void main(String[] args)
{
Scanner scannerObject = new Scanner(System.in);
Scanner input = new Scanner(System.in);
int quartersDisplayed;
double b, IR;
do
{
Scanner keyboard=new Scanner(System.in);
System.out.println("Enter the numbers of quarters you wish to display that is greater than zero and less or equal to 10: ");
quartersDisplayed = keyboard.nextInt();
System.out.println("Next enter the starting balance. ");
System.out.println("This input must be greater than zero: ");
b = keyboard.nextDouble();
System.out.println("Finally, enter the interest rate ");
System.out.println("which must be greater than zero and less than or equal to twenty percent: ");
IR = keyboard.nextDouble();
System.out.println("You have entered the following amount of quarters: " + quartersDisplayed);
System.out.println("You also entered the starting balance of: " + b);
System.out.println("Finally, you entered the following of interest rate: " + IR);
System.out.println("If this information is not correct, please exit the program and enter the correct information.");
double quarterlyEndingBalance = b + (b * IR/100 * .25);
System.out.println("Your ending balance for your quarters is " + quarterlyEndingBalance);
System.out.println("Do you want to continue?");
String yes=keyboard.next("yes");
if (yes.equals(yes))
continue;
else
break;
}
while(true);
}
}
So here's some code to answer your questions and help get you started. However, there are problems with your logic that do not pertain to your question which I will address afterward.
Note: I have added comments to your code. Most of them start with "EDIT:" so that you can tell what I changed. I didn't use this prefix in all cases because some of it is new code and it's obviously my comment
import java.util.Scanner;
public class InterestCalculator {
public static void main(String[] args) {
// EDIT: you already have a scanner defined below with a more meaningful name so I removed this one
// Scanner scannerObject = new Scanner(System.in);
Scanner input = new Scanner(System.in);
//EDIT: defining userResponse outside the loop so we can use it everywhere inside
String userResponse = null;
do {
//EDIT: moved the variables inside the loop so that they are reset each time we start over.
//EDIT: initialize your variable to a value that is invalid so that you can tell if it has been set or not.
int quartersDisplayed = -1;
//EDIT: gave your variables more meaningful names that conform to java standards
double startingBalance = -1, interestRate = -1;
//EDIT: you don't need a second Scanner, just use the one you already have.
// Scanner keyboard = new Scanner(System.in);
do{
System.out.println("Enter the numbers of quarters you wish to display that is greater than zero and less or equal to 10: ");
userResponse = input.next();
try{
quartersDisplayed = Integer.parseInt(userResponse);
}catch(NumberFormatException e){
//nothing to do here, error message handled below.
}
if(quartersDisplayed <= 0 || quartersDisplayed > 10){
System.out.println("Sorry, that value is not valid.");
}else{
break;
}
}while(true);
do{
System.out.println("Enter the starting balance (must be greater than zero): ");
userResponse = input.next();
try{
startingBalance = Double.parseDouble(userResponse);
}catch(NumberFormatException e){
//nothing to do here, error message handled below.
}
if(startingBalance <= 0){
System.out.println("Sorry, that value is not valid.");
}else{
break;
}
}while(true);
do{
System.out.println("Enter the interest rate (greater than zero less than twenty percent): ");
userResponse = input.next();
try{
interestRate = Double.parseDouble(userResponse);
}catch(NumberFormatException e){
//nothing to do here, error message handled below.
}
//Note: I assume twenty percent is represented as 20.0 here
if(interestRate <= 0 || interestRate > 20){
System.out.println("Sorry, that value is not valid.");
}else{
break;
}
}while(true);
System.out.println("You have entered the following amount of quarters: "
+ quartersDisplayed);
System.out.println("You also entered the starting balance of: " + startingBalance);
System.out.println("Finally, you entered the following of interest rate: "
+ interestRate);
System.out.println("If this information is not correct, please exit the program and enter the correct information.");
double quarterlyEndingBalance = startingBalance + (startingBalance * interestRate / 100 * .25);
System.out.println("Your ending balance for your quarters is "
+ quarterlyEndingBalance);
System.out.println("Do you want to continue?");
//EDIT: modified your variable name to be more meaningful since the user's response doesn't have to "yes" necessarily
userResponse = input.next();
// EDIT: modified the logic here to compare with "yes" or "y" case insensitively.
// if (userResponse.equals(userResponse))
if("y".equalsIgnoreCase(userResponse) || "yes".equalsIgnoreCase(userResponse))
continue;
else
break;
} while (true);
Now to address other issues - your interest calculation doesn't seem correct to me. Your formula does not make use of the quartersDisplayed variable at all. I assume you're compounding the interest quarterly so you will definitely need to make use of this when calculating your results.
This may be beyond the scope of your project, but you should not use double or float data types to represent money. There is a stackoverflow question about this topic that has good information.
Possible improvements - since you're asking the user for two values of type double you could create a method to ask for a double value and call it twice instead of repeating the code. This is a better approach because it helps reduce the chance of mistakes and makes testing and maintenance easier.
You can do something like this in your do/while loop:
do
{
Scanner keyboard = new Scanner(System.in);
do
{
System.out.println("Enter the numbers of quarters you wish to display that is greater than zero and less or equal to 10: ");
quartersDisplayed = keyboard.nextInt();
}
while (quartersDisplayed < 1 || quartersDisplayed > 10);
System.out.println("Next enter the starting balance. ");
do
{
System.out.println("This input must be greater than zero: ");
b = keyboard.nextDouble();
}
while (b < 1);
// rest of code ...
}
With the Scanner#hasNextInt (and the equivalent for double), you can avoid having exceptions thrown, and thus don't need try-catch clauses. I think in general if you can avoid try-catch, it's good, because they are clumsy - but I might be wrong.
However, my approach is like this. Inside your outer do-while, have three other do-while-loops to get the three values. The reason is that you want to keep looping until you get a correct value. The explanation of why keyboard.nextLine() is important is covered here.
I didn't include all of your code, only the part in question. Here's my take on it:
import java.util.Scanner;
public class InterestCalculator {
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in);
int quartersDisplayed = -1;
double b = -1.0;
double IR = -1.0;
do {
do {
System.out.println("Enter the number of quarters.");
if(keyboard.hasNextInt()) {
quartersDisplayed = keyboard.nextInt();
keyboard.nextLine(); //important
} else {
System.out.println("You need to enter an integer.");
continue;
}
} while(quartersDisplayed < 1 || quartersDisplayed > 10);
do {
System.out.println("Enter the starting balance.");
if(keyboard.hasNextDouble()) {
b = keyboard.nextDouble();
keyboard.nextLine();
} else {
System.out.println("You must enter a number.");
continue;
}
} while(b <= 0);
do {
System.out.println("Enter the interest rate.");
if(keyboard.hasNextDouble()) {
IR = keyboard.nextDouble();
keyboard.nextLine();
} else {
System.out.println("You must enter a number.");
continue;
}
} while(IR <= 0 || IR > 20.0);
//... rest of code
} while(true);
}
}
I want it to loop again when the user enters "Y" or "y" and quit when they enter "N" or "n". The quitting option works, however, when they enter Y/y, it shows the first system out, but does not let the user pick which operation they wish to do. Instead the option to continue pops up again and inhibits the user from making any choice.
Here is the code:
import java.util.Scanner;
public class Calc2 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
double numOne, numTwo, ans;
String option;
do {
System.out.println(
"For addition press '1', for subtraction press '2', for division press '3', for multiplication press '4'");
String choice = input.nextLine();
if (choice.contains("1")) {
System.out.println("Enter the first number : ");
numOne = input.nextDouble();
System.out.println("Enter the second number : ");
numTwo = input.nextDouble();
ans = numOne + numTwo;
System.out.println("The answer is: " + ans + " ya bish.");
}
else if (choice.contains("2")) {
System.out.println("Enter the first number : ");
numOne = input.nextDouble();
System.out.println("Enter the second number : ");
numTwo = input.nextDouble();
ans = numOne - numTwo;
System.out.println("The answer is: " + ans + " ya bish.");
} else if (choice.contains("4")) {
System.out.println("Enter the first number : ");
numOne = input.nextDouble();
System.out.println("Enter the second number : ");
numTwo = input.nextDouble();
ans = numOne * numTwo;
System.out.println("The answer is: " + ans + " ya bish.");
} else if (choice.contains("3")) {
System.out.println("Enter the first number : ");
numOne = input.nextDouble();
System.out.println("Enter the second number : ");
numTwo = input.nextDouble();
ans = numOne / numTwo;
System.out.println("The answer is: " + ans + " ya bish.");
}
System.out.println("Press 'Y' to continue or 'N' to quit.");
option = input.next();
} while (option.equals("Y") || option.equals("y"));
if (option.equals("N") || option.equals("n")) {
System.out.println("Thank you. ");
}
}
}
If anyone can help me, it'd be greatly appreciated. Thanks!
Please change below line in your code
String choice = input.nextLine();
from this code
String choice = input.next();
There trouble you see here is the use of nextLine after nextDouble. Check here [Scanner is skipping nextLine() after using next(), nextInt() or other nextFoo() methods
Your problem appears to be at the beginning of your do-while loop as such:
System.out.println(
"For addition press '1', for subtraction press '2', " +
"for division press '3', for multiplication press '4'");
String choice = input.nextLine();
This is the only place where you use nextLine method (rahter than next or nextDouble and so on). This means that after you've read the option argument at the end of the iteration:
option = input.next();
there's still a new line character that hasn't been read by the scanner. When you do nextLine() in the next iteration it reads the new line character before the user has any chance to input something).
Either change that first line to input.next() as well, or make sure every time you read a value, you clear the new line character (for instance by reading nextLine and then casting the value - this would also allow you to do input validations).
I have a little problem with this do while loop; when I run the program it is working, at least partially, what I mean is first you need to make a choice for convertion from C to F or from F to C and after you enter the values the program stops what I want to do is to keep asking for values until you enter 3. I tried to do it with a do while loop but it is not working so if someone has any ideas I would be grateful. Here is the code:
import java.util.Scanner;
public class DegreesInConversion2 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Conversion table: ");
int choice = input.nextInt();
do {
System.out.println();
System.out.println("1 for convertion from Celsious to Fahrenhait: ");
System.out.println("2 for convertion froom Fahrenheit to Celsious: ");
System.out.println("3 for Exit: ");
System.out.println();
System.out.println("Make a choice between 1 - 3 ");
choice = input.nextInt();
System.out.println();
switch (choice) {
case 1:
System.out.println("Enter temperature in Celsious: ");
double cel = input.nextDouble();
if (cel < -273.15) {
System.out.println("Invalid values, please enter temperature greater than -273.15 in C:");
} else {
System.out.println("You enetered " + cel + "C " + "which is " + (((cel * 9) / 5) + 32) + "F");
}
break;
case 2:
System.out.println("Enter temperature in Farhneit: ");
double far = input.nextDouble();
if (far < -459.67) {
System.out.println("Invalid values, please enter temperature greater than -459.67 in F:");
} else {
System.out.println("You enetered " + far + "F " + "which is " + (((far - 32) * 5) / 9) + "C");
}
break;
case 3:
System.out.println("Goodbyu have a nice day: ");
break;
default:
System.out.println("Invalid entry: Please enter a number between 1-3:");
}
} while (choice != 3);
}
}
Like in your other question, here you're scanning for input before prompting the user for input.
You need to remove the second line below:
System.out.println("Conversion table: ");
int choice = input.nextInt();
do
With your code as is, it outputs
Conversion table:
and then blocks waiting for input. Whereas you want it instead to continue into the while loop and output
1 for convertion from Celsious to Fahrenhait:
2 for convertion froom Fahrenheit to Celsious:
3 for Exit:
Make a choice between 1 - 3
before blocking to scan for input.
As is, if you enter any number at the first block, your program enters the loop and behaves as you wanted. So you're nearly there!
The code does work. the problem is most likely the
int choice = input.nextInt();
before the do
Remove this, and change
choice = input.nextInt();
to
int choice = input.nextInt();
Besides the fact that you have: int choice = input.nextInt(); outside of the loop which is unnecessarily getting input before showing the menu, it seems to all work relatively fine. You can just declare int choice inside the loop where you have choice = input.nextInt(); (ie. just change that to intchoice = input.nextInt();).
I tested your code, and it works fine if you change the line int choice = input.nextInt(); (just before your do{} while() block) into int choice;.
As others have already mentioned, you should not read input before your do{} while() block, since the question has not been asked yet.
you forgot the break; after your default case