Develop Reference

In Java, how to restrict a user to enter only one type?

In this case, I have asked the user to enter 1,2 or 3. If user enters string, it should ask the question again. when my code runs, every if statement is running, where as I want to run a specific if statement. It would be a huge favor if anyone could help me.
String question = "What is the color of banana? Write 1,2 or 3. Choose one of the following:";
String answerOne = "1. yellow";
String answerTwo = "2. red";
String answerThree = "3. blue";
while(true){
System.out.println(question + "\n" + answerOne + "\n" + answerTwo + "\n" + answerThree);
Scanner input = new Scanner(System.in);
if (input.hasNext()) {
System.out.println("Please select a NUMBER between 1 and 3.");
}
if (input.hasNextDouble()) {
System.out.println("Please select a NUMBER between 1 and 3.");
}
if (input.hasNextInt()) {
int x = input.nextInt();
if (x == 1) {
System.out.println("Congratulations!! your answer is correct");
break;
}
if (x == 2 || x == 3) {
System.out.println("Sorry!! the correct answer is" + answerOne);
break;
}
if (x > 3) {
System.out.println("Please choose the correct answer!!");
}
}
}
}

I think that your problems are as follows.
You create a new scanner each time around the loop
Immediately after the new scanner is created, you execute a couple of 'hasNext' tests. Nothing will have been typed. You're possibly going to spin in that loop forever.
An integer is also a double, so even if the user has typed an integer, it'll satisfy the 'if double' condition.
If an integer is not typed, the input is never read, so you're stuck at that point in the input stream. You'll loop forever.
The flow should look like this:
input = new Scanner();
while (true) {
print question...
try {
int x = input.nextInt(); // waits for user input
process as before;
if (correct answer) {
break out of loop;
}
else {
print "nope";
}
}
catch (InputMismatchException ex) { // something that is not an integer
print "type an integer";
input.nextLine(); // skip incorrect input
}
}
This is just a pseudocode sketch, of course.
It uses exception handling as the most convenient way to make sure an integer is input. By calling nextInt() on the scanner, we assume the user is going to type something that is convertible to integer. The try/catch structure handles the error case.
There are other ways to write this; my own inclination would be to read a line and parse it. But this is closest to what I think was the original intent.

Ok, so first that's not how .hasSomething() works, it used to check if there have more elements. So what you should do, is to read the input as a string and check that string:
String question = "What is the color of banana? Write 1,2 or 3. Choose one of the following:";
String answerOne = "1. yellow";
String answerTwo = "2. red";
String answerThree = "3. blue";
System.out.println(question + "\n" + answerOne + "\n" + answerTwo + "\n" + answerThree);
Scanner input = new Scanner(System.in);
String c = input.next();//read the input
while(!(c.charAt(0) >= '1' && c.charAt(0) <= '3') || c.length() != 1){ //check if it's a number between 1 and 3
System.out.println("Enter agin:");
c = input.next();
}
int x = Integer.parseInt(c);//change c to an integer
if (x == 1) {
System.out.println("Congratulations!! your answer is correct");
break;
}
if (x == 2 || x == 3) {
System.out.println("Sorry!! the correct answer is" + answerOne);
}

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"

How to continue looping while the condition is false

I want my program to do the certain code only if the actors variable is <=5 && >0
else I want to go back and ask different question again:
System.out.println("\nEnter actors number(max.5): ");
int actorsNumber = scanner.nextInt();
scanner.nextLine();
if (actorsNumber <= 5 && actorsnumber>0) {
for (int i = 1; i <= actorsNumber; i++) {
System.out.println("\nEnter name and surname of the " + i + " actor: ");
String[] parts2 = scanner.nextLine().split("\\s+");
String actName = parts2[0];
String actSurname = parts2[1];
actorName.add(actName);
actorSurname.add(actSurname);
}
}
So when the condition is false for example actors number = 7 then I want to tell the user: "Enter the valid actors number(1-5)" and do the for loop again if the actorsnumber is 1-5 instead ask again "Enter the valid actors number(1-5)"

You need to ask user to enter number in a loop. The condition of the loop will be the condition of your if. Then after exiting the loop (which means user enter a correct number), enter your for loop.
This will look like that :
int actorsNumber = -1; //force a wrong value at the beginning
while(actorsNumber < 1 || actorsNumber > 5) {
//Ask user to enter a number between 1 and 5
//store this number in actorsNumber
}
//As we exited the while-loop, actorsNumber is between 1 and 5
//Put your for-loop here

while(!scanner.nextLine().equals("quit")) {
if (actorsNumber <= 5 && actorsnumber>0) {
for (int i = 1; i <= actorsNumber; i++) {
System.out.println("\nEnter name and surname of the " + i + " actor: ");
String[] parts2 = scanner.nextLine().split("\\s+");
String actName = parts2[0];
String actSurname = parts2[1];
actorName.add(actName);
actorSurname.add(actSurname);
}
}
}
Or something like that, point is, use a while loop on the scanner.

As #apexlol stated, your code should be in a loop. However, do-while loop is more convenient than while loop since that way, the loop executes at least once no matter what.

Correct user input not matching array values

I've written a portion of code to take a user input, match it to a string value and then use a related double value to make calculations:
double [] currency = new double[] {0.05,0.10,0.20,0.50,1.00,2.00,5.00,10.00,20.00,50.00,100.00};
String [] currencytext = {"$0.05","$0.10","$0.20","$0.50","$1.00","$2.00","$5.00","$10.00","$20.00","$50.00","$100.00"};
Scanner keyboard = new Scanner(System.in);
for (int i = 0; i < currencytext.length; i++) {
boolean valid = false;
while(!valid){
System.out.format("$%.2f remains to be paid. Enter coin or note: ",sum);
String payment = keyboard.next();
if(payment.equals(currencytext[i])){
sum = sum - currency[i];
if(sum == 0) {
System.out.print("You gave " + payment);
System.out.print("Perfect! No change given.");
System.out.print("");
System.out.print("Thank you" + name + ".");
System.out.print("See you next time.");
}
}
if(!(payment.equals(currencytext[i]))) {
System.out.print("Invalid coin or note. Try again. \n");
}
if(payment.equals(currencytext[i]) && currency[i] > sum){
System.out.print("You gave " + payment);
System.out.print("Your change:");
}
}
}
The problem is that when it gets to user input, it doesn't match any string values except for $0.05. It seems to me like its not iterating through the array properly but I can't figure out why. Is anyone able to see a problem here?

This is a possible solution for your problem
Scanner keyboard = new Scanner(System.in);
double [] currency = new double[] {0.05,0.10,0.20,0.50,1.00,2.00,5.00,10.00,20.00,50.00,100.00};
String [] currencytext = {"$0.05","$0.10","$0.20","$0.50","$1.00","$2.00","$5.00","$10.00","$20.00","$50.00","$100.00"};
String payment = keyboard.next();
double sum = 100; // <- Working example - Read sum from keyboard entry
while (sum > 0) {
boolean paymentFound = false;
for (int i = 0; i < currencytext.length; i++) {
if (payment.equals(currencytext[i])) {
sum = sum - currency[i];
paymentFound = true;
if (sum == 0) {
System.out.println("You gave " + payment);
System.out.println("Perfect! No change given.");
// System.out.print("Thank you" + name + ".");
System.out.println("See you next time.");
break;
} else if (sum < 0) {
System.out.println("You gave " + payment);
System.out.println("Your change:" + (-1 * sum));
break;
}
}
}
if (!paymentFound) {
System.out.println("Invalid coin or note. Try again. \n");
}
if (sum > 0) {
System.out.format("$%.2f remains to be paid. Enter coin or note: ", sum);
payment = keyboard.next();
}
}
while-loop will continue until the payment is fullfilled.
for-loop traverse the arrays until a suitable payment is found
If suitable payment is found we substract it from sum. We use break to exit the for-loop in both cases. There is no need to keep searching.
If no suitable payment is found [!paymentFound], we keep on asking.
if (!paymentFound) {
System.out.println("Invalid coin or note. Try again. \n");
}
if (sum > 0) {
System.out.format("$%.2f remains to be paid. Enter coin or note: ", sum);
payment = keyboard.next();
}
The program will end when (sum < 0), in which case the while-loop exits.
I have use println instead of print to improve message legibility.

Too many flaws to point out.
However,
When the currencytext[i] does not match payment, it executes this code:
System.out.print("Invalid coin or note. Try again. \n");
System.out.format("$%.2f remains to be paid. Enter coin or note: ",sum);
payment = keyboard.next();
So, it executes this for all the times that your input does not match currencytext[i].
And, in this block, you have
payment = keyboard.next();
So, it asks for new input, in this block itself. Hence, you get the said output for all inputs except $0.05.
As far as $0.05 is concerned, your first if block executes successfully, and prints no output. So, it moves to the next iteration of the while loop, where again, payment remains the same ($0.05), but currencytext[i] becomes $0.10. SO they do not match, and you get the said output.
How to correct this:
With this code, you need to do a lot of corrections.
I suggest you again start from scratch.

If it doesn't fit, it sets valid to true, so the code just has the chance to check against the first item at currencytext[0], which is $0.05. Then !payment.equals(currencytext[i]) is also true, and your code prints the lines there. Your else ifs are also not properly nested.

I don't know how you are reading input. One improvement you can do is write reading input code in for loop.
Scanner scanner = new Scanner(System.in);
for (... ) {
....
String payment = scanner.nextLine();
....
}

Java Loop: Why does this loop not stop as expected?

I need help getting this loop to work. It may be because I don't really know how to use loops or the if statements. But I think I needed to use the If statements when it comes to receiving a free yogurt after 10.
import java.util.Scanner;
public class Foothill {
public static void main(String args[]){
Scanner scanner = new Scanner(System.in);
int numPurchases, totalPurchased;
boolean correct;
String menu;
numPurchases = 0;
totalPurchased = 0;
correct = false;
while(!correct)
{
System.out.println("Menu:");
System.out.println(" P (Process Purchase)");
System.out.println(" S (Shut down)");
menu = scanner.nextLine();
if (menu.charAt(0)=='S' || menu.charAt(0)=='s'){
System.exit(0);
} else if (menu.charAt(0)=='P' || menu.charAt(0)=='p'){
continue;
}
System.out.println("Your choice: " + menu.charAt(0));
System.out.println("How many yougurts would you like to buy?");
numPurchases = scanner.nextInt();
totalPurchased += numPurchases;
if (totalPurchased > 0 || totalPurchased < 10){
System.out.println("you have purchased " + totalPurchased + " yogurts");
} else if (totalPurchased >= 10){
System.out.println("Congradulations you have recieve a free Yogurt");
System.out.println("Remainding yogurt credits: " + (totalPurchased-10));
}
}
}
}
I don't really know how to use loops that well so that may be my problem because I have never made a loop using false and true

Just remove the the following statement, then the loop will work:
else if (menu.charAt(0)=='P' || menu.charAt(0)=='p'){
continue;
}

You had a number of problems with your code.
Here is the solution:
import java.util.Scanner;
public class Foothill {
public static void main(String args[]){
Scanner scanner = new Scanner(System.in);
int numPurchases, totalPurchased;
boolean correct;
String menu;
numPurchases = 0;
totalPurchased = 0;
correct = false;
while(!correct){
System.out.println("Menu:");
System.out.println(" P (Process Purchase)");
System.out.println(" S (Shut down)");
menu = scanner.next();
if (menu.charAt(0)=='S' || menu.charAt(0)=='s'){
System.exit(0);
}
else if (menu.charAt(0)=='P' || menu.charAt(0)=='p'){
System.out.println("Your choice: " + menu.charAt(0));
System.out.println("How many yougurts would you like to buy?");
numPurchases = scanner.nextInt();
totalPurchased += numPurchases;
}
if (totalPurchased > 0 && totalPurchased < 10){
System.out.println("you have purchased " + totalPurchased + " yogurts");
}
else if (totalPurchased >= 10){
System.out.println("Congradulations you have recieve a free Yogurt");
System.out.println("Remainding yogurt credits: " + (totalPurchased-10));
}
}
scanner.close();
}
}
Like mentioned above, you probably need some way to break out of the while loop by setting correct to true.

As user2357112 has stated in the comments, the || operator is inappropriate for testing if a number is in a given range since that condition would be true for any number.
You could change the condition to use && instead but I don't know why you'd only display that message on a certain condition. You could print it at the end of each purchase and then print an extra message if the user has received a freebie.
System.out.println("you have purchased " + totalPurchased + " yogurts");
if (totalPurchased >= 10){
System.out.println("Congradulations you have recieve a free Yogurt");
System.out.println("Remainding yogurt credits: " + (totalPurchased-10));
}
You also need to know that Scanner#nextInt() does NOT consume the trailing new-line character from user input. Scanner#nextLine() however, trims this character from the read input. As a result you'll get a StringIndexOutOfBoundsException after making a purchase since the Scanner#nextLine() to read in the user's choice at the menu will eat up the trailing new-line character and your program will try to get the first character of an empty String.
As stated in the linked thread you can fix this by calling Scanner#nextLine() again after reading integer input to remove the new-line character from the buffer like so:
numPurchases = scanner.nextInt();
scanner.nextLine();