What is the mistake in the following code?
while ((char t==(char) System.in.read())!='0')
You can not declare a new variable in a while loop.
while (boolean always = true) {
} // DOES NOT COMPILE!!!
You'd have to declare the variable before and outside of the loop, so perhaps something like this:
boolean always = true;
while (always) {
break;
} // compiles fine!
// always is still in scope after the loop!
always = !always;
In this sense, for loop is unique: you can in fact declare a new local variable whose scope is limited to that loop:
for (boolean always = true; always; ) {
break;
} // compiles fine!
// always is no longer declared after the loop!
always = !always; // DOES NOT COMPILE!
That said, looking at what you're doing, you may want to look at java.util.Scanner. I suspect that it will serve your need much better.
Example
Here's an example of using Scanner to read numbers from standard input, terminating at 0. It then prints the sum of those numbers. It handles invalid input gracefully using hasNextInt() instead of Integer.parseInt/NumberFormatException.
Scanner sc = new Scanner(System.in);
System.out.println("Enter numbers (0 to end):");
int sum = 0;
int number;
do {
while (!sc.hasNextInt()) {
System.out.println("I'm sorry, that's not a number! Try again!");
sc.next();
}
number = sc.nextInt();
sum += number;
} while (number != 0);
System.out.println("The sum of those numbers is " + sum);
Here's an example session:
Enter numbers (0 to end):
1
3
-1
five
I'm sorry, that's not a number! Try again!
2
0
The sum of those numbers is 5
This is probably what you intended to write.
char t;
while ((t = (char) System.in.read()) != '0') {
//...
}
while ((char t==(char) System.in.read())!='0')
// ^^ should be 'char t = ...'
This loop can be rewritten more clearly as
while (true) {
char t = (char) System.in.read();
if (t == '0')
break;
...
char t is a statement which declares a variable rather than an expression; it has no value to compare with the == operator. You probably also meant to use assignment rather than equality, but unlike C++ the declaration does not have a value.
The easiest way of restricting the scope of a variable in a loop is to use for instead, which specifically allows you to declare variables.
for (char t; ( t = (char) System.in.read() ) != '0'; )
// loop body involving t
However, some company's guidelines don't allow mutating operators in boolean expressions, such as the read and the assignment, and would prefer you to separate them out onto several lines. Personally I find restricting the scope more important.
Related
Fairly new to java and programming.
Wrote this recursive method, with the objective of asking for a valid string that is both an integer and greater than 0:
private int getDimension(String tableElement){
Integer Input= 0;
System.out.println("Define table rows "+tableElement+"'s."
+"Enter an integer >= 1:");
if( !Reader.hasNextInt() || (Input=Input.parseInt(Reader.nextLine())) <= 0)
return getDimension(tableElement);
return Input;
}
I'd like to stick to using a short and recursive method. It seems to handle the >= 0 logic fine, but blows up when i pass it something other than an integer.
Can someone explain why does that happen to me please?
hasNextInt() doesn't actually consume your input, so you're stuck with the same non-int input on your next call.
Simply spoken, your code doesn't make much (any?) sense.
First of all, there is not really a point in using a recursive method that asks the user for input; and that does not at all do anything about the argument passed to it!
private int getDimension(String tableElement){
Integer Input= 0;
Bad: you keep up mixing int and `Integer. They are not the same. And - read about java coding style guides. Variable names start lower case!
if( !Reader.hasNextInt() || (Input=Input.parseInt(Reader.nextLine())) <= 0)
The first condition gives:
true: when there is NO int ...
false: when there is an int
true leads to: calling your method again without retrieving a value from the reader.
false leads to parsing an int; and checking its value for <= 0.
In one case, you are doing a recursive call; completely ignoring the input you got from the reader; in the other case, you returning 0; or that value in input.
Solution: do something like:
while (true) {
if (reader.hasNextInt()) {
input = reader.nextInt();
break;
}
// there is no number!
read.nextLine(); // consume & throw away non-number!
print "Enter a number"
}
instead.
But seriously: start with throwing away this code.
Final side note: you do Input.parseInt() ... but that is a static method on the Integer class. Just call that as Integer.parseInt() instead! But as said; throw away your code; and learn how to properly use that Scanner class; start reading here.
Because the user can enter anything, you must always read in the line, then compare it:
String num = Reader.nextLine();
return num.matches("[1-9][0-9]*") ? Integer.parseInt(num) : getDimension(tableElement);
Here I've use regex to figure out if it's a positive number; the expression means "a 1-9 char followed by 0 or more of 0-9 chars"
I am trying to get my code to prevent a user input from having a number in it.
Essentially I want the code to do as follows:
ask for input
receive input
test whether or not the input contains a number(ex: 5matt vs matt)
if contains a number I want to System.out.println("Error: please do not input a number");
Heres the kicker (and why it's not a duplicate question): I can't use loops or other statements we haven't learned yet. So far the only true statements we've learned are if/else/else if statements. That means I can not use for loops, like some of the answers are suggesting. While they're great answers, and work, I'll lose points for using them.
System.out.println("Please input the first name: ");
String name1 = in.next();
System.out.println("Please input the second name: ");
String name2 = in.next();
System.out.println("Please input the third name: ");
String name3 = in.next();
name1 = name1.substring(0,1).toUpperCase() + name1.substring(1).toLowerCase();
name2 = name2.substring(0,1).toUpperCase() + name2.substring(1).toLowerCase();
name3 = name3.substring(0,1).toUpperCase() + name3.substring(1).toLowerCase();
I have this already but I can't figure out how to test if the input only contains letters.
Okay, there are many ways to deal with this. A good thing would be to use Regex (text matching stuff). But it seems that you should only use very basic comparison methods.
So, let's do something very basic and easy to understand: We iterate over every character of the input and check whether it's a digit or not.
String input = ...
// Iterate over every character
for (int i = 0; i < input.length(); i++) {
char c = s.charAt(i);
// Check whether c is a digit
if (Character.isDigit(c)) {
System.out.println("Do not use digits!");
}
}
This code is very straightforward. But it will continue checking even if a digit was found. You can prevent this using a helper-method and then returning from it:
public boolean containsDigit(String text) {
// Iterate over every character
for (int i = 0; i < input.length(); i++) {
char c = s.charAt(i);
// Check whether c is a digit
if (Character.isDigit(c)) {
return true;
}
}
// Iterated through the text, no digit found
return false;
}
And in your main program you call it this way:
String input = ...
if (containsDigit(input)) {
System.out.println("Do not use digits!");
}
Use a regular expression to filter the input
Eg
str.matches(".*\\d.*")
See this link for more info
There are several ways you could do this, among others:
Iterate over all the chars in the string and check whether any of them is a digit.
Check whether the string contains the number 1, number 2, number 3, etc.
Use a regular expression to check if the string contains a digit.
(Java Regular Expressions)
If you're allowed to define functions, you can essentially use recursion to act as a loop. Probably not what your prof is going for, but you could be just inside the requirements depending on how they're worded.
public static boolean stringHasDigit(String s) {
if (s == null) return false; //null contains no chars
else return stringHasDigit(s, 0);
}
private static boolean stringHasDigit(String s, int index) {
if (index >= s.length()) return false; //reached end of string without finding digit
else if (Character.isDigit(s.charAt(index))) return true; //Found digit
else return stringHasDigit(s, index+1);
}
Only uses if/elseif/else, Character.isDigit, and String.charAt, but recursion might be off limits as well.
I'm making a program with Java that needs to involve some error checking. I can stop users from entering bad numerical inputs like this (assume the input scanner has already been created):
while (n == 0){
System.out.println("Can't use 0 as a denominator! Please enter a real, nonzero number");
n = input.nextInt();
}
But how do I stop users from entering an invalid string? I can't use !=, because strings can only be compared with the string.equals() method, right? So, is there a while not loop? ie:
while !(string.equals("y") || string.equals("n")){
//here have code
}
Or something of that nature?
While there is no such thing as a while-not loop, you can always invert the condition:
while (!(string.equals("y") || string.equals("n"))){
This is read, "while the string is not equal to "y" or "n"".
You could also apply DeMorgan's identity to rewrite this as:
while (!(string.equals("y")) && !(string.equals("n"))){
which is a bit clearer as "While the string isn't equal to "y" and isn't equal to "n"".
There isn't a while-not instruction, but you can simply negate the condition in a normal while loop. Try this:
while (!string.equals("y") && !string.equals("n"))
Or even better, to guard against the case where the string is null and/or it's in a different case:
while (!"y".equalsIgnoreCase(string) && !"n".equalsIgnoreCase(string))
You almost get it, just change where you position your !
like this:
while (!(string.equals("y") || string.equals("n")))
Why not try regex?
Scanner sc = new Scanner(System.in);
String string = sc.nextLine();
while (!string.matches("(?i)^(?:y|n|yes|no)$"))
{
System.out.println("Invalid input...");
string = sc.nextLine();
}
boolean answer = string.matches("(?i)^(?:y|yes)$");
I am working on a class assignment this morning and I want to try and solve a problem I have noticed in all of my team mates programs so far; the fact that spaces in an int/float/double cause Java to freak out.
To solve this issue I had a very crazy idea but it does work under certain circumstances. However the problem is that is does not always work and I cannot figure out why. Here is my "main" method:
import java.util.Scanner; //needed for scanner class
public class Test2
{
public static void main(String[] args)
{
BugChecking bc = new BugChecking();
String i;
double i2 = 0;
Scanner in = new Scanner(System.in);
System.out.println("Please enter a positive integer");
while (i2 <= 0.0)
{
i = in.nextLine();
i = bc.deleteSpaces(i);
//cast back to float
i2 = Double.parseDouble(i);
if (i2 <= 0.0)
{
System.out.println("Please enter a number greater than 0.");
}
}
in.close();
System.out.println(i2);
}
}
So here is the class, note that I am working with floats but I made it so that it can be used for any type so long as it can be cast to a string:
public class BugChecking
{
BugChecking()
{
}
public String deleteSpaces(String s)
{
//convert string into a char array
char[] cArray = s.toCharArray();
//now use for loop to find and remove spaces
for (i3 = 0; i3 < cArray.length; i3++)
{
if ((Character.isWhitespace(cArray[i3])) && (i3 != cArray.length)) //If current element contains a space remove it via overwrite
{
for (i4 = i3; i4 < cArray.length-1;i4++)
{
//move array elements over by one element
storage1 = cArray[i4+1];
cArray[i4] = storage1;
}
}
}
s = new String(cArray);
return s;
}
int i3; //for iteration
int i4; //for iteration
char storage1; //for storage
}
Now, the goal is to remove spaces from the array in order to fix the problem stated at the beginning of the post and from what I can tell this code should achieve that and it does, but only when the first character of an input is the space.
For example, if I input " 2.0332" the output is "2.0332".
However if I input "2.03 445 " the output is "2.03" and the rest gets lost somewhere.
This second example is what I am trying to figure out how to fix.
EDIT:
David's suggestion below was able to fix the problem. Bypassed sending an int. Send it directly as a string then convert (I always heard this described as casting) to desired variable type. Corrected code put in place above in the Main method.
A little side note, if you plan on using this even though replace is much easier, be sure to add an && check to the if statement in deleteSpaces to make sure that the if statement only executes if you are not on the final array element of cArray. If you pass the last element value via i3 to the next for loop which sets i4 to the value of i3 it will trigger an OutOfBounds error I think since it will only check up to the last element - 1.
If you'd like to get rid of all white spaces inbetween a String use replaceAll(String regex,String replacement) or replace(char oldChar, char newChar):
String sBefore = "2.03 445 ";
String sAfter = sBefore.replaceAll("\\s+", "");//replace white space and tabs
//String sAfter = sBefore.replace(' ', '');//replace white space only
double i = 0;
try {
i = Double.parseDouble(sAfter);//parse to integer
} catch (NumberFormatException nfe) {
nfe.printStackTrace();
}
System.out.println(i);//2.03445
UPDATE:
Looking at your code snippet the problem might be that you read it directly as a float/int/double (thus entering a whitespace stops the nextFloat()) rather read the input as a String using nextLine(), delete the white spaces then attempt to convert it to the appropriate format.
This seems to work fine for me:
public static void main(String[] args) {
//bugChecking bc = new bugChecking();
float i = 0.0f;
String tmp = "";
Scanner in = new Scanner(System.in);
System.out.println("Please enter a positive integer");
while (true) {
tmp = in.nextLine();//read line
tmp = tmp.replaceAll("\\s+", "");//get rid of spaces
if (tmp.isEmpty()) {//wrong input
System.err.println("Please enter a number greater than 0.");
} else {//correct input
try{//attempt to convert sring to float
i = new Float(tmp);
}catch(NumberFormatException nfe) {
System.err.println(nfe.getMessage());
}
System.out.println(i);
break;//got correct input halt loop
}
}
in.close();
}
EDIT:
as a side note please start all class names with a capital letter i.e bugChecking class should be BugChecking the same applies for test2 class it should be Test2
String objects have methods on them that allow you to do this kind of thing. The one you want in particular is String.replace. This pretty much does what you're trying to do for you.
String input = " 2.03 445 ";
input = input.replace(" ", ""); // "2.03445"
You could also use regular expressions to replace more than just spaces. For example, to get rid of everything that isn't a digit or a period:
String input = "123,232 . 03 445 ";
input = input.replaceAll("[^\\d.]", ""); // "123232.03445"
This will replace any non-digit, non-period character so that you're left with only those characters in the input. See the javadocs for Pattern to learn a bit about regular expressions, or search for one of the many tutorials available online.
Edit: One other remark, String.trim will remove all whitespace from the beginning and end of your string to turn " 2.0332" into "2.0332":
String input = " 2.0332 ";
input = input.trim(); // "2.0332"
Edit 2: With your update, I see the problem now. Scanner.nextFloat is what's breaking on the space. If you change your code to use Scanner.nextLine like so:
while (i <= 0) {
String input = in.nextLine();
input = input.replaceAll("[^\\d.]", "");
float i = Float.parseFloat(input);
if (i <= 0.0f) {
System.out.println("Please enter a number greater than 0.");
}
System.out.println(i);
}
That code will properly accept you entering things like "123,232 . 03 445". Use any of the solutions in place of my replaceAll and it will work.
Scanner.nextFloat will split your input automatically based on whitespace. Scanner can take a delimiter when you construct it (for example, new Scanner(System.in, ",./ ") will delimit on ,, ., /, and )" The default constructor, new Scanner(System.in), automatically delimits based on whitespace.
I guess you're using the first argument from you main method. If you main method looks somehow like this:
public static void main(String[] args){
System.out.println(deleteSpaces(args[0]);
}
Your problem is, that spaces separate the arguments that get handed to your main method. So running you class like this:
java MyNumberConverter 22.2 33
The first argument arg[0] is "22.2" and the second arg[1] "33"
But like other have suggested, String.replace is a better way of doing this anyway.
I'm trying to input 3 different variables into an array inside a while loop, as long as i don't enter stop for any of the variables. the while loop is only suppose to let me input a second variable value if the 1st variable isn't stop, and likewise with inputting a third variable value
Right now, the first loop goes fine and i can input all 3 variables, but the 2nd and 3rd time, the for loop outputs the first variable, but doesn't allow me to input a value before skipping to the 2nd variable.
ex of what i mean:
name:afasdf
extra info:afdsaf
unit cost:123123214
name: extra info: adflskjflk
also, entering Stop isn't ending the loop either
unit cost:123217
i know that this loop works when there's only one variable, and i've tried using a for loop instead of a while loop, and adding tons and tons of else statements, but it seems to stay the same
is there something wrong with the way i set up my breakers?
is the way i set up the last breaker(the one that stops even when i put stop for a double variable) messing up the rest of hte loop?
thank you so much
here is my code
ArrayItem s = new ArrayItem();
String Name = null, ID = null;
double Money = 0;
boolean breaker = false;
while(breaker ==false)
{
System.out.print("Name:" + "\t");
Name = Input.nextLine();
if(Name.equals("Stop")) //see if the program should stop
breaker = true;
System.out.print("Extra Info:" + "\t");
Details = Input.nextLine();
if(ID.equals("Stop"))
breaker = true;
System.out.print("Unit Cost:" + "\t");
Money = Input.nextDouble();
// suppose to let me stop even if i input stop
// when the variable is suppose to be a double
if(Input.equals("stop") || Input.equals("stop"))
breaker = true;
else
s.SetNames(Name);
s.SetInfo(Details);
s.SetCost(Money);
}
A couple of things about the code: "Name:" + "\t" can be simplified ot "Name:\t". This is true for the rest of the code. In Java, it's customary to use camelcase where the first word is lowercase. For example, s.SetMoney would be s.setMoney. Also, variables follow the same rules where Money would be money, and ID would be id. If your teacher is teaching you otherwise, then follow their style.
The loop should also be a do-while loop:
do
{
// read each value in sequence, and then check to see if you should stop
// you can/should simplify this into a function that returns the object
// that returns null if the value should stop (requiring a capital D
// double for the return type)
if ( /* reason to stop */)
{
break;
}
s.setNames(name);
s.setId(id);
s.setMoney(money);
} while (true);
private String getString(Scanner input)
{
String result = input.nextLine();
// look for STOP
if (result.equalsIgnoreCase("stop"))
{
result = null;
}
return result;
}
private Double getDouble(Scanner input)
{
Double result = null;
// read the line is a string looking for STOP
String line = getString(input);
// null if it's STOP
if (line != null)
{
try
{
result = Double.parseDouble(line);
}
catch (NumberFormatException e)
{
// not a valid number, but not STOP either!
}
}
return result;
}
There are a lot of concepts in there, but they should help as you progress. I'll let you put the pieces together.
Also, you did need to fix the brackets, but that's not the only issue. Because Money is a double, you must read the value as a String. I suspect that Input is a Scanner object, so you can check Input.hasNextDouble() if it's not, then you can conditionally check the String value to see if it's "stop" (note: you are checking for "Stop" and "stop", which are not equal). Your last, no-chances check compares the Scanner to "stop", which will never be true. Check
System.out.print("Unit Cost:\t");
if (Input.hasNextDouble())
{
Money = Input.nextDouble();
// you can now set your object
// ...
}
// it's not a double; look for "stop"
else if (Input.nextLine().equalsIgnoreCase("stop"))
{
// exit loop
break;
}
// NOTE: if it's NOT a double or stop, then you have NOT exited
// and you have not set money
breaker = true;
while(breaker){
Name = readInput("Name");
Details = readInput("Details");
Money = Double.parseDouble(readInput("Money"));
if(Name.equals("stop") || Details.equals("stop"))
breaker = false;
else {
// set ArrayItem
}
}
private static String readInput(String title){
System.out.println(title+":");
//... read input
// return value
}