Develop Reference

if else condition is not executed properly

I have a condition where if the user inputs a negative number or a number which is more than 100, or a string, an error message should be printed "That wasn't a valid percentage, I need a number between 0-100. Try again." and ask the user to reenter a valid number. and if the user decided to just enter, all the input should be calculated and printed the average amount.
public static void main(String[ ] args) {
int count = 0; //count to stop loop
double[ ] aGrade = new double[SIZE];
String input = new String("");
Scanner scan = new Scanner(System.in);
double total = 0;
int gTotal = aGrade.length;
boolean exit = false;
while ((count < SIZE) && (!exit)) {
System.out.print("Enter number " + (count + 1) + ": " + "\n");
try {
input = scan.nextLine();
if (Double.parseDouble(input) > 0 && Double.parseDouble(input) < 100) {
aGrade[count] = Double.parseDouble(input); //put into the array
count++; //only increment count if success
} else
System.out.println("That wasn't a valid percentage,"
+ " I need a number between 0-100. Try again.");
} catch (NumberFormatException nfe) {
exit = true; //exit loop
}
}
System.out.println("number of grades entered: " + count + "\n");
for (int i = 0; i < count; i++) {
// print entered grade
System.out.println("grade " + (i + 1) + ": " + aGrade[i]);
}
for (int i = 0; i < count; i++) {
total += aGrade[i];
}
// calculate and print the average
System.out.println("\n" + "Average grade: " + total /count);
But when I run my code, if I input letters, it won't allow the user to reinput value but prints whatever is calculated. I think it is in my if-else statement, but I am not sure how

When we try to convert String to Double it will throw java.lang.NumberFormatException. So whenever you enter String or char at that time instead of else it will go to catch block. As per your code else block only executed when user enter negative number or grater then 100 number.
I updated your code. Please review it.
import java.util.Scanner;
public class Average {
public static void main(String[] args) {
int count = 0; // count to stop loop
double[] aGrade = new double[3];
String input = new String("");
Scanner scan = new Scanner(System.in);
double total = 0;
int gTotal = aGrade.length;
boolean exit = false;
while ((count < 3) && (!exit)) {
System.out.print("Enter number " + (count + 1) + ": " + "\n");
try {
input = scan.nextLine();
if (Double.parseDouble(input) > 0 && Double.parseDouble(input) < 100) {
aGrade[count] = Double.parseDouble(input); // put into the array
count++; // only increment count if success
} else
System.out
.println("That wasn't a valid percentage," + " I need a number between 0-100. Try again.");
} catch (NumberFormatException nfe) {
nfe.printStackTrace();
exit = true; // exit loop
}
}
if (!exit) {
System.out.println("number of grades entered: " + count + "\n");
for (int i = 0; i < count; i++) {
// print entered grade
System.out.println("grade " + (i + 1) + ": " + aGrade[i]);
}
for (int i = 0; i < count; i++) {
total += aGrade[i];
}
// calculate and print the average
System.out.println("\n" + "Average grade: " + total / count);
}else {
System.out
.println("That wasn't a valid percentage," + " I need a number between 0-100. Try again.");
}
}
}

If you type letter as an input, you will never end up in your else part of the if statement since code inside if throws an exception and you are then inside catch part. Also, you wrote inside catch part, when NumberFormatException happens(when you enter letter instead of number), set exit to true and that is the reason why program don't let you type again after you input letter. Fix those things and it will work. Also, take a look at how to debug your program, learn that skill, it will help you to solve this kind of problems in the future.

Try something like this:
boolean ok = false;
try {
input = scan.nextLine();
if ("".equals(input)) {
ok = true;
exit = true;
} else if (Double.parseDouble(input) >= 0 && Double.parseDouble(input) <= 100) {
aGrade[count] = Double.parseDouble(input); //put into the array
count++; //only increment count if success
ok = true;
}
} catch (NumberFormatException nfe) {
// nothing
}
if (!ok) {
System.out.println("That wasn't a valid percentage,"
+ " I need a number between 0-100. Try again.");
}

Catch invalid input and return to input prompt JAVA

I am very new to Java and as part of my college course I have to write a program that carries out some basic functions. Part of this program is that it needs to calculate the factorial of a number that the user inputs. If the user inputs a negative number then it must prompt for a positive number. I have got it to do this.
But if the user enters a fraction such as 2.2 then the program should present the user with an error and prompt for valid data. I believe some sort or try-catch should be implemented but so far I have had no success in getting this to work, after spending many hours on it. Any ideas how to get the program to catch the InputMismatchException error and prompt user for input again?
The relevant block of code from the program is below...
public static void factorialNumber() {
int factorial = 1;
boolean valid;
int number = 0;
do {
System.out.println("Please enter a number: ");
number = sc.nextInt();
valid = number > 0;
if (!valid) {
System.out.println("ERROR Please enter a positive number");
}
} while (!valid);
if (number < 0) {
System.out.println("***Error***: Please enter a positive number ... ");
factorialNumber();
}
if (number > 0) {
System.out.print("The factorial is: " + number + " ");
}
for (int i = 1; i <= number; i++) {
factorial *= i;
if ((number - i) > 0) {
System.out.print("x " + (number - i) + " ");
}
}
System.out.println("= " + factorial);
}

You can use Double class to parse the user input and then get only correct values. Like this:
public static void factorialNumber() {
int factorial = 1;
boolean valid;
int number = 0;
String userInput;
do {
System.out.println("Please enter a number: ");
userInput = sc.nextLine();
valid = validateUserInput(userInput);
} while (!valid);
number = Double.valueOf(userInput).intValue();
System.out.print("The factorial is: " + number + " ");
for (int i = 1; i <= number; i++) {
factorial *= i;
if ((number - i) > 0) {
System.out.print("x " + (number - i) + " ");
}
}
System.out.println("= " + factorial);
}
private static boolean validateUserInput(String userInput) {
if (userInput == null) {
System.out.println("You should enter a number!");
return false;
}
Double userInputNumber;
try {
userInputNumber = Double.valueOf(userInput);
} catch (Exception e) {
System.out.println("Please enter a valid number value.");
return false;
}
if (userInputNumber <= 0) {
System.out.println("ERROR Please enter a positive number");
return false;
} else if (userInputNumber - userInputNumber.intValue() > 0) {
System.out.println("ERROR You entered a fractional number!");
return false;
}
return true;
}

error msgs after second loop attempt

I'm working on another assignment and am stuck. First off, I realize I probably won't get the results I want from this code, but at the moment I can't run the code to even see how close or far away I am. I'm sure I'm missing something simple and hope that something will pop out to someone here so I can move past this point. My mother tongue is English but I'm living in Sweden and trying to learn code in a Swedish class so I added //for translations.
Again, I am working with very basic code so am not looking for an easy hack, more of just some insight to where I have gone wrong.
My assignment is to ask the user to enter 10 numbers, store those as an array. Then, offer the user 4 options to calculate those numbers and a 5th option to quit the program.
Here's what I have so far:
package inlämningsuppgift3;
import java.util.Scanner;
import java.util.Arrays;
public class Inlämningsuppgift3 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int nr;
int antal = 0;
int[] num = new int[10];
int sum = 0;
int min = 0;
int max = 0;
for (nr = 0; nr < 10; nr++) {
System.out.println("Ange ett tal " + (nr+1) + " : ");
int tal = input.nextInt();
num[nr] = tal;
if (nr == 0) { //first number
min = tal;
max = tal;
}
else { // All other numbers
if (tal > max) { max = tal; }
if (tal < min) { min = tal; }
}
sum = sum + tal;
}
{
// What would you like to do?
System.out.println("Välj vad du vill göra?");
// Show largest number
System.out.println("1: Visa storsta talet.");
// Show smallest number
System.out.println("2: Visa minsta talet.");
// Show average
System.out.println("3: Visa medeltalet.");
// Show all numbers
System.out.println("4: Visa alla inmatade tal.");
// Quit
System.out.println("5: Avsluta");
}
do {
int k = input.nextInt();
if (k == 1) {
// Largest number is:
System.out.println("Storsta talet är: " + max);
}
else if (k == 2) {
// Smallest number is:
System.out.println("Minsta talet är: " + min);
}
else if (k == 3) {
// Average number is:
System.out.println("Medeltalet är: " + sum/10);
}
else if (k == 4) {
// All the entered numbers:
System.out.println("Alla tal: " + num[10] + ", ");
}
else if (k==5) {
// Goodbye
System.out.println("Hej då!");
break;
}
else {
System.out.println("Felaktigt, prova igen.");
// Unrecognized, try again.
}
while (k<5);
}
}
}
I'm getting error on the last 3 } and I'm not sure why. Are they in the wrong place? I've tried moving them around, I've tried deleting them (obviously, didn't help either) I tried changes to my {} placement above in the code and just haven't found a way around this error. Thank you in advance for any input!

java do-while syntax is:
do {
// Statements
}while(Boolean_expression);
so, change it to:
int k = 0;
do {
k = input.nextInt();
if (k == 1) {
System.out.println("Storsta talet är: " + max);
} else if (k == 2) {
System.out.println("Minsta talet är: " + min);
} else if (k == 3) {
System.out.println("Medeltalet är: " + sum / 10);
} else if (k == 4) {
System.out.println("Alla tal: " + num[10] + ", ");
} else if (k == 5) {
System.out.println("Hej då!");//good bye
break;
} else {
System.out.println("Felaktigt, prova igen.");
}
} while (k < 5) ;
and after while line must be two } now.

Hej Hej!
I made the modifications I think like you asked to make it 'just about work'
package Dunno;
import java.util.Scanner;
public class NumberCollector { //No special characters in source files,
//Can get transformed during deployment
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int nr;
int antal =0;
int[] num = new int[10];
int sum = 0;
int min=0;
int max=0;
for (nr=0; nr<10; nr++)
{
System.out.println("Ange ett tal " + (nr+1) + " : ");
int tal = input.nextInt();
num[nr]=tal;
if(nr == 0) //first number
{
min=tal;
max=tal;
}
else //all other numbers
{
if(tal>max)max =tal;
if(tal<min) min=tal;
}
sum = sum + tal;
}
System.out.println("Välj vad du vill göra?");
//what would you like to do?
System.out.println("1: Visa storsta talet.");
//Show largest number
System.out.println("2: Visa minsta talet.");
//show smallest number
System.out.println("3: Visa medeltalet.");//show average
System.out.println("4: Visa alla inmatade tal.");
//show all numbers
System.out.println("5: Avsluta");//quit
while (true) //Moved the stop condition to start of the loop makes it easier to read logically.
{
int k = input.nextInt();
if (k==1)
{
System.out.println("Storsta talet är: " + max);
//largest number is:
}
else if (k==2)
{
System.out.println("Minsta talet är: " + min);
//smallest number is:
}
else if (k==3)
{
System.out.println("Medeltalet är: " + sum/10);
//average number is:
}
else if (k==4)
{
System.out.println("Alla tal: " + num[10] + ", ");
//all the entered numbers:
}
else if (k==5)
{
System.out.println("Hej då!");//good bye
break;
}
else
{
System.out.println("Felaktigt, prova igen.");
//unrecognized, try again.
}
};
}
}
I only had to move around some braces.
It seems to broadly do what you want? It throws an exception if you ask for result 4, I'll leave that to you :)
Maybe this will be a good starting point? Consider replacing the loop with a switch/case condition would be nicer to read and maintain?

How to loop the following code?

I'm really new to java and I cannot find a way around this. I want to make a program that tells you that a number is either positive or negative, regardless if it is int or double. But after the program is executed, I want it to loop and ask again for input from the user, to execute the code again and again and again, as long as there is user input. Can I do that in java?
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
String userInput = "Input your number: ";
System.out.println(userInput);
if (in.hasNextInt()) {
int z = in.nextInt();
if (z > 0) {
System.out.println(z + " is positive.");
} else if (z < 0) {
System.out.println(z + " is negative.");
} else {
System.out.println(z + " is equal to 0.");
}
} else if (in.hasNextDouble()) {
double x = in.nextDouble();
if (x > 0) {
System.out.println(x + " is positive.");
} else if (x < 0) {
System.out.println(x + " is negative.");
} else {
System.out.println(x + " is equal to 0.");
}
} else {
System.out.println("Hey! Only numbers!");
}
}
}

Here is a one of the approach which is good start for you to understand what wonders pattern matching can do in Java and it can be improved by testing it against exhaustive data points.
This also shows how to use while-loop, overloading methods and ternary operator instead of nested if-then-else.
As you are learning, you should also use debugging feature of editors and also use system.out.println to understand what code is doing.
I am ending the program when user presses just enter (empty string).
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
while (true) {
String userInput = "Input your number: ";
System.out.print(userInput);
String input = scanner.nextLine();
// look for integer (+ve, -ve or 0)
if (input.matches("^-?[0-9]+$")) {
int z = Integer.parseInt(input);
System.out.println(display(z));
// look for double (+ve, -ve or 0)
} else if (input.matches("^-?([0-9]+\\.[0-9]+|[0-9]+)$")) {
double z = Double.parseDouble(input);
System.out.println(display(z));
// look for end of program by user
} else if (input.equals("")) {
System.out.println("Good Bye!!");
break;
// look for bad input
} else {
System.out.println("Hey! Only numbers!");
}
}
scanner.close();
}
// handle integer and display message appropriately
private static String display(int d) {
return (d>0) ? (d + " is positive") : (d<0) ? (d + " is negative") : (d + " is equal to 0");
}
// handle double and display message appropriately
private static String display(double d) {
return (d>0) ? (d + " is positive") : (d<0) ? (d + " is negative") : (d + " is equal to 0");
}
}
Sample Run:
Input your number: 0
0 is equal to 0
Input your number: 0.0
0.0 is equal to 0
Input your number: -0
0 is equal to 0
Input your number: -0.0
-0.0 is equal to 0
Input your number: 12
12 is positive
Input your number: -12
-12 is negative
Input your number: 12.0
12.0 is positive
Input your number: -12.0
-12.0 is negative
Input your number: 12-12
Hey! Only numbers!
Input your number: ---12
Hey! Only numbers!
Input your number:

Use this code!
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Console console = new Console();
while(true) {
// Take your input
Scanner in = new Scanner(System.in);
String userInput = "Input your number: ";
System.out.println(userInput);
if (in.hasNextInt()) {
int z = in.nextInt();
if (z > 0) {
System.out.println(z + " is positive.");
} else if (z < 0) {
System.out.println(z + " is negative.");
} else {
System.out.println(z + " is equal to 0.");
}
} else if (in.hasNextDouble()) {
double x = in.nextDouble();
if (x > 0) {
System.out.println(x + " is positive.");
} else if (x < 0) {
System.out.println(x + " is negative.");
} else {
System.out.println(x + " is equal to 0.");
}
} else {
System.out.println("Hey! Only numbers!");
}
// Ask for exit
System.out.print("Want to quit? Y/N")
String input = console.readLine();
if("Y".equals(input))
{
break;
}
}
}
}

Finding even numbers using a recursive method

I am trying to have the code listed below receive an int input from user, then find amount of even numbers in that int. I'm getting an error when I try to print the return... any help?
import java.util.Scanner;
public class evenDigits {
public static int countEvenDigits(int number){
if (number == 0)
return 0;
else{
int lastDigit = number % 10;
int result = countEvenDigits(number / 10);
if (lastDigit % 2 == 0)
return result + 1;
else
return result;
}
}
public static void main(String[] args) {
System.out.println("Please enter an integer.");
Scanner keyboard = new Scanner(System.in);
int number = keyboard.nextInt();
countEvenDigits(number);
System.out.println("There are " + result + " even digits in " + number);
}
}
Specifically, there is an error in this statement:
System.out.println("There are " + result + " even digits in " + number);

In main you need to change:
countEvenDigits(number);
to:
int result = countEvenDigits(number);
Otherwise, you're accessing a nonexistent variable in your println