I looked through a bunch of other questions and online, but I can't find anything that answers this specifically. I'm trying to handle dealing with and determining valid input based on a Windows command line argument. As a sample, I was just seeing if an entered number was positive to be as easy as possible. The biggest issue, and what made me unable to find a specific answer, was I'm really trying to use recursion to make it keep asking until a valid input is entered, rather than just killing the program.
Algorithm:
If there is an argument provided and it's a positive integer
Display value
Otherwise
Until the argument is a positive number
Prompt for a positive integer
Display value
I wrestled with the code and eventually got this to work, but it seems really inefficient, repetitive and hacked together. At first, I had the while-loop inside the caught exceptions, but this allowed other things to slip through from the command line. How can I make this as efficient as possible and also prevent any logic errors or exceptions? What approach should I take with my algorithm when tackling this? Here's my code:
import java.util.Scanner;
public class Test
{
public static void main( String[] args )
{
String arg;
Scanner user_input = new Scanner(System.in);
int i = 0;
try {
arg = args[0];
i = Integer.parseInt(arg);
} catch( ArrayIndexOutOfBoundsException e ) {
arg = "";
} catch( NumberFormatException e2 ) {
arg = "";
}
while( i <= 0 )
{
System.out.print("Please type in a positive whole number. ");
arg = user_input.next();
try {
i = Integer.parseInt(arg);
} catch( NumberFormatException e2 ) {
System.out.print("That's a letter! ");
continue;
}
if( i <= 0 )
{
System.out.print("That's a negative. ");
}
}
System.out.println("Input is " + i);
}
}
Try this:
The code is quite lengthy this is because two separate try blocks are required; one for the command-line argument & the other for the argument provided via the scanner...
I had to create my own custom exception, "NegativeNumberException"...
import java.util.Scanner;
public class NegativeNumberException extends Exception{
NegativeNumberException(){
System.out.println(exceptionMessage);
}
String exceptionMessage = "Number must be positive";
static int num;
public static void main(String[] args) throws NegativeNumberException{
try
{
if(Integer.parseInt(args[0])<0){
throw new NegativeNumberException();
}
else{
int num = Integer.parseInt(args[0]);
System.out.println("Your number is: " + num);
}
}
catch(NumberFormatException ex){
System.out.println("That's not even a number.");
}
catch(NegativeNumberException ex){
ex.getMessage();
}
while(num==0){
try{
System.out.println("Enter a positive number:");
Scanner input = new Scanner(System.in);
int num1 = input.nextInt();
if(num1<0){
throw new NegativeNumberException();
}
num = num1;
break;
}catch(Exception ex){
System.out.println("Positive number only, try again...");
}
}//End While
System.out.println("Your number is:" + num);
}
}
Input: (Command-line): lol
Output
(Console):That's not even a number
Enter a positive int
(Console input via Scanner): -4
(Console):Number must be positive
Positive number only, try again...
Enter a positive number:
(Console input via Scanner): 3
(Console):Your number is: 3
I modified your code so that it runs the way you want. Try using this:
public static void main( String[] args ) {
String arg;
Scanner user_input = new Scanner(System.in);
int numTries = 0, value = 0;
try {
numTries = Integer.parseInt(args[0]); // get max number of tries
} catch (ArrayIndexOutOfBoundsException e) {
arg = "";
} catch (NumberFormatException e2) {
arg = "";
}
// try 'numTries' times to read a valid number input
for (int i=numTries; i > 0; --i) {
System.out.print("Please type in a positive whole number: ");
arg = user_input.next();
try {
value = Integer.parseInt(arg);
} catch(NumberFormatException e2) {
System.out.println("That's a letter! ");
continue;
}
if (value <= 0) {
System.out.println("That's a negative. ");
}
break; // exit loop if number input found
}
System.out.println("Input is " + value);
}
This code has been tested on IntelliJ and it runs with no problems.
Related
I'm trying to create a loop by entering the number to be added and then blocking the loop with a "exit" input from the user..but it's not working properly.
import java.util.Scanner;
public class main {
public static void main(String[] args)
{
int i,n=0,s=0;
double avg;
{
System.out.println("Input the numbers : ");
}
for (i=0;i<100;i++)
{
String input = new java.util.Scanner(System.in).nextLine ();
if(input.equals("exit")){
break;
}
Scanner in = new Scanner(System.in);
n = in.nextInt();
s +=n;
}
System.out.println("The sum of numbers is : " +s);
}
}
You have a couple of problems. One (minor) is that you are creating two scanners. Another (medium) is that your loop is set up to only go up to 100 - this is a magic number, and there's no reason to put in this artificial constraint. But your biggest problem is that you are ignoring the first entry in the loop if it is a number and not 'exit'
{
int i,n=0,s=0;
double avg;
boolean adding = true;
System.out.println("Input the numbers : ");
Scanner sc = new java.util.Scanner(System.in);
while(adding)
{
String input = sc.nextLine ();
if(input.equals("exit")){ // should proably be "EXIT" or equalsIgnoreCase
adding = false;
} else {
try {
int val = Integer.parseInt(input);
s += val;
} catch (NumberFormatException nfe) {
System.err.println ("expecting EXIT or an integer");
}
}
}
System.out.println("The sum of numbers is : " +s);
}
After this line your console does not have any next line or entry to read.
String input = new java.util.Scanner(System.in).nextLine ();
I am just trying to get code to work where the code asks again for an answer, if text or a symbol is entered, instead of a required integer:
import java.util.Scanner;
class timecalc {
int hrs = 0;
int min = 0;
static int hourflag = 0;
static int minflag = 0;
Scanner sc = new Scanner(System.in);
public int getHours() {
try {
hourflag = hourflag + 1;
if (hourflag > 1) {
System.out.println("Invalid month Please enter hours again:");
}
System.out.println("Enter month:");
return hrs = sc.nextInt();
} catch (InputMisMAtchException e) {
System.out.println("entered invalid input " + e);
}
}
Have reviewed answers already given but cant get a workable solution
Any ideas?
I won't give you the entire code, but just a hint or psuedo-code. As an exercise you can implement it as per your requirement.
System.out.println("Enter month:");
while (true) {
try {
int min = sc.nextInt();
break;
} catch (InputMismatchException ex) {
System.err.println("Invalid input, please enter again");
sc.nextLine(); // <----- advance the scanner
}
}
Here the logic is to loop until we get the right input. If it is an invalid input, the loop never breaks.
Also as a side-note, I would recommend you to create just one method to fetch correct inputs and call it respectively from other methods. Rather than duplicating this logic everywhere.
This program is for computing the digits of an integer. So there is chances to enter the input by user may string("raju" whatever it may be), number(12334), combination(string & number i.e, 234dsd) and nothing(he doesn't enter anything), isn't it? There might be another chances too I don't know(If there is mention it here).Try out with various inputs and the problems here are when I entered number and nothing. If input is number "result not coming" cmd prompt not continuing further and input is nothing(not entered) if statement is not executing. when the cmd prompt goes like that?
//computing digits of integer.
import java.util.Scanner;
class Main
{
public static void main (String w[])
{
Scanner s=new Scanner(System.in);
System.out.print("Enter a number");
String g=s.nextLine();
System.out.println("Entered value is"+g);
if(g==null)
{
System.out.println("Enter atleast one number");
}
else
{
try
{
int st=Integer.parseInt(g);
int sum=0;
while(st>=0)
{
int value=st%10;
st=st/10;
sum=value+sum;
}
System.out.println("the sum of digits: "+sum);
}catch (NumberFormatException nfe)
{
System.err.println("Invalid input. Enter only number...");
}
}
}
}
It is hard to understand you are asking here, but if you are asking you code is not trying again when the user inputs invalid input, the answer is that it is because your code has no loop to do that.
Repetition of something (in this case, the task of asking for input) generally requires a loop of some kind.
If you indented your code properly, this would probably be more obvious to you.
Try this one
//computing digits of integer.
import java.util.Scanner;
public class Main {
public static void main(String w[]) {
Scanner s = new Scanner(System.in);
System.out.println("Enter a number");
String g = s.nextLine();
System.out.println("Entered value is " + g);
try {
int st = Integer.parseInt(g);
int sum = 0;
while (st > 0) {
int value = st % 10;
st = st / 10;
sum = value + sum;
}
System.out.println("the sum of digits: " + sum);
} catch (NumberFormatException nfe) {
System.err.println("Invalid input. Enter only number...");
}
}
}
None of the answers so far explicitly mentioned the problem: There is an endless loop here:
int st=Integer.parseInt(g);
int sum=0;
while(st>=0)
{
int value=st%10;
st=st/10;
sum=value+sum;
}
because st never becomes negative when you start with a positive value.
Hey guys so i've been trying to answer this question for hours:
Write a program that asks the user to input a set of floating-point values. When the
user enters a value that is not a number, give the user a second chance to enter the
value. After two chances, quit reading input. Add all correctly specified values and
print the sum when the user is done entering data. Use exception handling to detect
improper inputs.
I've tried a few different things but i always have the same problem. Once something that isn't a number is given as input, the program outputs the message prompting for another input however the chance is not given, that is to say after 1 incorrect input it prints that message and jumps straight to printing the sum. The best i could do is below i'm just not sure how to approach this problem. Any help is greatly appreciated.
import java.util.Scanner;
import java.util.InputMismatchException;
public class q6{
public static void main(String[] args){
Scanner in = new Scanner(System.in);
boolean firstChance = true;
boolean secondChance = true;
double sum = 0;
while (secondChance){
try{
while (firstChance){
try{
System.out.print("Please enter a number: ");
double input = in.nextDouble();
sum = sum + input;
}
catch (InputMismatchException ex){
firstChance = false;
}
System.out.print("Please enter a number to continue or something else to terminate: ");
double input = in.nextDouble();
sum = sum + input;
firstChance = true;
}
}
catch (InputMismatchException e){
secondChance = false;
}
}
System.out.print("The sum of the entered values is " + sum);
}
}
i'm just not sure how to approach this problem
Pseudocode could be as follows:
BEGIN
MAX_INPUT = 2;
i = 0;
WHILE i < MAX_INPUT
TRY
num = GET_NUM();
CATCH
continue;
FINALLY
i++
END WHILE
END
Since the parsing of input to double is not successful, the scanner does not go past the given input. From javadoc Scanner
Scans the next token of the input as a double. This method will throw
InputMismatchException if the next token cannot be translated into a
valid double value. If the translation is successful, the scanner
advances past the input that matched.
Since the translation is not successful, it does not advance. So, in the catch block you could call in.next() to skip the token.
You could use an integer counter instead of the boolean variables firstChance and secondChance and do something like:
int attempts = 0;
while(attempts < 2) {
try {
//Get user input - possible exception point
//Print sum
} catch(InputMismatchException e) {
attempts++;
//Continue?
}
}
import java.util.*;
public class q6 {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
double inputNumber, sum = 0.0;
int correctCount = 0, wrongCount = 0;
while(wrongCount <=1) {
System.out.println("Please enter a numeric floating value:");
try {
inputNumber = in.nextDouble();
correctCount++;
sum += inputNumber;
if(correctCount >= 2)
break;
} catch(InputMismatchException e) {
wrongCount++;
in = new Scanner(System.in);
continue;
}
}
System.out.println(sum);
}
}
You need to break your while loop when you encounter a "bad" input. Then, you'll need to set firstChance to true again, so you can access the second while, you also will need a counter that counts the number of the attempts (I named it chances):
int chances = 0;
while (secondChance){
firstChance = true;
try{
while (firstChance){
try{
System.out.print("Please enter a number: ");
double input = in.nextDouble();
sum = sum + input;
}
catch (InputMismatchException ex){
chances ++;
in = new Scanner(System.in);
firstChance = false;
}
if(!firstChance && chances < 2)
break;
if(chances >= 2) {
System.out.print("Please enter a number to continue or something else to terminate: ");
double input = in.nextDouble();
sum = sum + input;
firstChance = true;
}
}
}
catch (InputMismatchException e){
secondChance = false;
}
}
As stated in my comment, in order to not deal with Scanner wrong read, it would be better to use Scanner#nextLine() and read the data as String, then try to parse it as double using Double#parseDouble(String) since this method already throws NumberFormatException and you can handle this error. Also, it would be better that your code could handle more than 1 request (if your teacher or somebody else asks to do this):
final int REQUEST_TIMES = 2;
double sum = 0;
for(int i = 1; i <= REQUEST_TIMES; i++) {
while (true) {
try {
if (i < REQUEST_TIMES) {
System.out.print("Please enter a number: ");
} else {
System.out.print("Please enter a number to continue or something else to terminate: ");
}
String stringInput = in.nextLine();
double input = Double.parseDouble(stringInput);
sum += input;
} catch (NumberFormatException nfe) {
System.out.println("You haven't entered a valid number.");
break;
}
}
}
The logic is to read the input once, if it is correct then display the sum, else read the input again and display the sum if the input is correct.
public class q6 {
static int trial = 0;
public static void main(String[] args) {
double sum = 0;
while (trial <= 2) {
Scanner in = new Scanner(System.in);
try {
trial++;
System.out.print("Please enter a number: ");
double input = in.nextDouble();
sum = sum + input;
trial=3;
} catch (InputMismatchException ex) {
trial++;
if (trial == 4){
System.out.println("You have entered wrong values twice");
}
}
}
if (trial <= 3){
System.out.print("The sum of the entered values is " + sum);
}
}
The above program is just a rough idea, you can improve this to adapt to your need.
I have been trying to stop the exceptions but I cannot figure out how.
I tried parseInt, java.util.NormalExceptionMismatch etc.
Does anyone have any insight how to fix this problem? Formatting is a bit off due to copy and paste.
do
{
System.out.print(
"How many integers shall we compare? (Enter a positive integer):");
select = intFind.nextInt();
if (!intFind.hasNextInt())
intFind.next();
{
// Display the following text in the event of an invalid input
System.out.println("Invalid input!");
}
}while(select < 0)
Other methods I have tried :
do
{
System.out.print(
"How many integers shall we compare? (Enter a positive integer):");
select = intFind.nextInt();
{
try{
select = intFind.nextInt();
}catch (java.util.InputMismatchException e)
{
// Display the following text in the event of an invalid input
System.out.println("Invalid input!");
return;
}
}
}while(select < 0)
It seems to me that you want to skip everything until you get an integer. This code here skips any input except an integer.
As long as there is no integer available (while (!in.hasNextInt())) discard the available input (in.next). When integer is available - read it (int num = in.nextInt();)
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
while (!in.hasNextInt()) {
in.next();
}
int num = in.nextInt();
System.out.println("Thank you for choosing " + num + " today.");
}
}
Quick sample of how to catch exceptions:
int exceptionSample()
{
int num = 0;
boolean done = false;
while(!done)
{
// prompt for input
// inputStr = read input
try {
num = Integer.parseInt(inputStr);
done = true;
}
catch(NumberFormatException ex) {
// Error msg
}
}
return num;
}
IMO, the best practice is to use nextLine() to get a String input, then parseInt it to get the integer. If unparsable, just complain back to the user and request re-entry.
Remember you may have to do a second nextLine() (discard the input) to clear up the buffer.