When compiled upon not entering the right selection it doesnt display to re-enter or repeat the loop
int counterA = 0;
while (counterA < 0) {
if (conversionSelection.equalsIgnoreCase("binary"))
counterA++;
if (conversionSelection.equalsIgnoreCase("octal"))
counterA++;
else System.out.println("Error. Please enter weither to convert the Hex to Octal or Binary:");
conversionSelection = keyboard.nextLine();
}
The loop should not execute at all since the condition fails right from the get-go.
There is also a dangling else statement. So you need to have an else-if for your second if statement if you want the else to properly execute. While it may not currently produce errors, it is an important skill to prevent dangling 'else's'.
Your code will never enter the while loop since counterA = 0 and the while loop condition is for it to be < 0. You want counterA to be less than zero, or the condition to include counterA's value in its set (e.g. while (counterA <= 0)).
This loop will never start because 0 is not smaller than 0.
Related
I am wondering why one loop works while the other doesn't even though they are very similar.
The dmCyl loop I would like to loop until a positive int is entered. This is the loop that does not work. However, the hgtCyl loop, which loops until the user enters an int, does work. It seems like the (dmCyl < 0) loop is just completely ignored. Why is that?
Also, the loop that works does so whether I put hgtCyl = scnr.nextInt(); in the while body (after scnr.next()) or outside of it. Does it's location change anything?
Thanks!
EDIT:
Setting the first while to a do while loop solves the problem.
Thanks again!
Solution:
dmCyl = scnr.nextInt();
do{
System.out.print("please insert positive int");
dmCyl = scnr.nextInt();
}while (dmCyl < 0);
Original Code:
int dmCyl = 0;
int hgtCyl = 0;
Scanner scnr = new Scanner(System.in);
while (dmCyl < 0) {
System.out.print("Please enter an integer");
scnr.next();
}
dmCyl = scnr.nextInt();
And the loop that works:
while (!scnr.hasNextInt()){
System.out.print("Please enter an integer");
scnr.next();
}
hgtCyl = scnr.nextInt();
Use do/while
do {
System.out.print("Please enter an integer");
dmCyl = scnr.nextInt();
} while (dmCyl < 0);
You are setting dmCyl to zero initially then you are telling the loop to only go while it is less than zero. Since at the point it starts the loop it has already satisfied the condition and never enters the loop. Set dmCyl equal to -1 instead.
You initialize dmCyl = 0, so your statement dmCyl < 0 is false.. you should try to add something like this: while (dmcyl <= 0)
Here part of the official description of while from the Java Language Specification 14.12, relevant text marked (bold) by me:
A while statement is executed by first evaluating the Expression. If the result is of type Boolean, it is subject to unboxing conversion (§5.1.8).
If evaluation of the Expression or the subsequent unboxing conversion (if any) completes abruptly for some reason, the while statement completes abruptly for the same reason.
Otherwise, execution continues by making a choice based on the resulting value:
If the value is true, then the contained Statement is executed. Then there is a choice:
If execution of the Statement completes normally, then the entire while statement is executed again, beginning by re-evaluating the Expression.
If execution of the Statement completes abruptly, see §14.12.1.
If the (possibly unboxed) value of the Expression is false, no further action is taken and the while statement completes normally.
That means, the expression is checked before executing the loop content for the first time. If it results in false the content is not executed and the loop terminated.
I am having a heck of a time trying to figure out why I can't leave a loop. What I need to do is leave the loop if my Boolean, forward, is set to true. (The Boolean has been set to false above the while loop.
When I run the code snippet below I and enter a positive number I can only enter an unlimited amount a numbers. When I enter a negative number I get one prompt telling me that's not allowed and to try again. After than I am stuck in the similar situation above. It doesn't matter what I enter next it will just keep letting input over and over again.
{
while (forward == false)
if (n2 == 0)
{
System.out.println("Sorry, the 0 is not a valid entry for the second number, try again!");
n2 = in.nextInt();
}
else
{
forward = true;
}
}
You can get away with using no extra variables
n2 = in.nextInt();
while (input == 0){
System.out.println("Sorry, 0 is not valid input");
n2 = in.nextInt();
}
as the forward is a boolean, you can use it directly instead of compare
forward == false.
If you wanna use this variable (I will not go thru the path of the best way achieve your aim), you can do the follow:
while (!forward) {
if (n2 == 0) {
System.out.println("Sorry, the 0 is not a valid entry for the second number, try again!");
n2 = in.nextInt();
} else {
forward = true;
}
}
Do you have an outer while loop that makes you go back into your while loop? An efficient way to solve loop issues is to debug the code a line at a time.
If you are in Eclipse, set a breakpoint (a place where your program will pause) by clicking on the line number just before entering the while loop. Then run the program, and you will see the current line highlighted. Then move line by line by pressing F6 repeatedly. In the bottom pane you can also find the current values of variables.
Now if you inspect your code line by line you will have a better idea of what's going on.
This is a highly subjective question, so I'll be more specific. Is there any time that a do-while loop would be a better style of coding than a normal while-loop?
e.g.
int count = 0;
do {
System.out.println("Welcome to Java");
count++;
} while (count < 10);`
It doesn't seem to make sense to me to check the while condition after evaluating the do-statement (aka forcing the do statement to run at least once).
For something simple like my above example, I would imagine that:
int count = 0;
while(count < 10) {
System.out.println("Welcome to Java"); count++;
}
would be generally considered to have been written in a better writing style.
Can anyone provide me a working example of when a do-while loop would be considered the only/best option? Do you have a do-while loop in your code? What role does it play and why did you opt for the do-while loop?
(I've got an inkling feeling that the do-while loop may be of use in coding games. Correct me, game developers, if I am wrong!)
If you want to read data from a network socket until a character sequence is found, you first need to read the data and then check the data for the escape sequence.
do
{
// read data
} while ( /* data is not escape sequence */ );
The while statement continually executes a block of statements while a particular condition is true
while (expression) {
statement(s)
}
do-while evaluates its expression at the bottom of the loop, and therefore, the statements within the do block are always executed at least once.
do {
statement(s)
} while (expression);
Now will talk about functional difference,
while-loops consist of a conditional branch instructions such as if_icmpge or if_icmplt and a goto statement. The conditional instruction branches the execution to the instruction immediately after the loop and therefore terminates the loop if the condition is not met. The final instruction in the loop is a goto that branches the byte code back to the beginning of the loop ensuring the byte code keeps looping until the conditional branch is met.
A Do-while-loops are also very similar to for-loops and while-loops except that they do not require the goto instruction as the conditional branch is the last instruction and is be used to loop back to the beginning
A do-while loop always runs the loop body at least once - it skips the initial condition check. Since it skips first check, one branch will be less and one less condition to be evaluated.
By using do-while you may gain performance if the expression/condition is complex, since it is ensured to loop atleast once. In that casedo-while could call for performance gain
Very Impressive findings here,
http://blog.jamesdbloom.com/JavaCodeToByteCode_PartOne.html#while_loop
The do-while loop is basically an inverted version of the while-loop.
It executes the loop statements unconditionally the first time.
It then evaluates the conditional expression specified before executing the statements again.
int sum = 0;
int i = 0;
do
{
sum += ids[i];
i++;
} while (i < 4);
Reference material
Simply, when you want to check condition before and then perform operation while is better option, and if you want to perform operation at least once and then check the condition do-while is better.
As per your question a working example,
1. when I needed to find the field which could be declared in the same class or the super class or the super class of that super class and so on i.e. finding the field located in deep class hierarchy. (A extends B B extends C and so on)
public Field SearchFieldInHierarchy(Object classObj, String fieldName )
{
Field type = null;
Class clz = classObj.getClass();
do
{
try
{
type = clz.getDeclaredField(fieldName);
break;
} catch (NoSuchFieldException e)
{
clz = clz.getSuperclass();
}
} while(clz != null || clz != Object.class);
return type;
}
2. When reading input stream from Http response
do
{
bytesRead = inputStream.read(buffer, totalBytesRead, buffer.length - totalBytesRead);
totalBytesRead += bytesRead;
} while (totalBytesRead < buffer.length && bytesRead != 0);
You kind of answer the question yourself-when it needs to run at least once, and it makes sense to read it that way.
do - while loop allows you to ensure that the piece of code is executed at least once before it goes into the iteration.
In a while loop, the condition is tested before it executes code in the loop. In a do while loop, the code is executed before the condition is tested, resulting in the code always being executed at least once. Example:
$value = 5;
while($value > 10){
echo "Value is greater than 10";
}
The above would never output anything. If we do the same again like this:
$value = 5;
do{
echo "Value is greater than 10";
}while($value > 10)
It would output Value is greater than 10 because the condition is tested after the loop is executed. After this it would not output anything further.
The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once.
For example do check this link: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html
If the looping condition can only be known after a first step of the loop (when you do not want a condition before you enter the loop).
Typically:
do {
expr = ...;
while (expr);
Use the while Statement when you have to check a condition repeatedly and only when the condition is satisfied execute the loop
while(condition) //eg. a>5
{
Body of Loop
}
If you see the flow of control here you can see that the condition is checked before the execution of the loop, if the condition is not met the loop will not execute at all
In the Do-While statement the program will execute the body of the loop once and then it will check if the statement is true or not
do
{
Body of Loop
}
while(condition); //eg. a>5
If you notice the flow of control here you will see that the body is executed once, then the condition is checked. If the condition is False the Program will break out of the loop, if True it will continue executing till the condition is not satisfied
It is to be noted that while and do-while give the same output only the flow of control is different
/*
while loop
5 bucks
1 chocolate = 1 bucks
while my money is greater than 1 bucks
select chocolate
pay 1 bucks to the shopkeeper
money = money - 1
end
come to home and cant go to while shop because my money = 0 bucks
*/
#include<stdio.h>
int main(){
int money = 5;
while( money >= 1){
printf("inside the shopk and selecting chocolate\n");
printf("after selecting chocolate paying 1 bucks\n");
money = money - 1 ;
printf("my remaining moeny = %d\n", money);
printf("\n\n");
}
printf("dont have money cant go inside the shop, money = %d", money);
return 0;
}
infinite money
while( codition ){ // condition will always true ....infinite loop
statement(s)
}
please visit this video for better understanding
https://www.youtube.com/watch?v=eqDv2wxDMJ8&t=25s
It is very simple to distinguish between the two. Let's take While loop first.
The syntax of while loop is as follows:
// expression value is available, and its value "matter".
// if true, while block will never be executed.
while(expression) {
// When inside while block, statements are executed, and
// expression is again evaluated to check the condition.
// If the condition is true, the while block is again iterated
// else it exists the while block.
}
Now, let's take the do-while loop.
The syntax of do-while is different:
// expression value is available but "doesn't matter" before this loop, & the
// control starts executing the while block.
do {
// statements are executed, and the
// statements is evaluated and to check the condition. If true
// the while block is iterated, else it exits.
} while(expression);
A sample program is given below to make this concept clear:
public class WhileAndDoWhile {
public static void main(String args[]) {
int i = 10;
System.out.println("While");
while (i >= 1) {
System.out.println(i);
i--;
}
// Here i is already 0, not >= 1.
System.out.println("do-while");
do {
System.out.println(i);
i--;
} while (i >= 1);
}
}
Compile and run this program, and the difference becomes apparent.
Is there ever a situation where you must use a do while loop? Is it an accepted practice? It seems like it is equivalent to a plain while loop except that its first iteration happens before checking the conditional, if that is even true.
int i = 3;
while ( i > 0 ) { // runs 3 times
i--;
}
vs
int j = 3;
do {
j --;
} while ( j > 0 ); // runs 3 times
The same?
EDIT: I have seen the java doc, but
the example in the java docs doesn't look like it requires that the particular routine inside of the do while loop must be run in the do while loop instead of inside of a regular while loop!
Is there ever a situation where you must use a do while loop?
No: every do-while loop can be written as a while-loop by running the body once before the loop begins. However, there are certainly cases where it makes more sense to use the do-while construct (i.e. if you always want the loop to iterate at least once), which is why it exists in the first place.
Is it an accepted practice?
If you use it appropriately, then yes absolutely.
It seems like it is equivalent to a plain while loop except that its first iteration happens before checking the conditional, if that is even true.
That's right. You can read more about do-while in its tutorial.
This example maybe help you be clearer:
int i = 3;
System.out.print("while: ");
while (--i > 0){
System.out.print("x");
}
System.out.print("\ndo-while: ");
int j = 3;
do
{
System.out.print("x");
}while (--j > 0);
This prints
while: xx
do-while: xxx
A real time example.
There is a contest with 5 level.
In each level if you score 100 you can proceed to next level.
Less code for do while, but not for while.
boolean playContest()
{//do while
int level = 1;
int score;
do
{
score = 0;
score = play();
}while(score>99 && level++<6)
if(level>4 && score>99)
isWinner = true;
else
isWinner = false;
return isWinner;
}
boolean playContest()
{//while
int level = 1;
int score;
while(level <6)
{
score = 0;
score = play();
if(score < 100)
break;
level++;
}
if(level>4 && score>99)
isWinner = true;
else
isWinner = false;
return isWinner;
}
basic difference between while and do-while is do while will be executed at least once.
when do-while is best option?
in case when you want to execute some actions till you meet condition, of course you could achieve same thing by using while but early termination of loop with break, is nasty and ugly solution
When you want to execute the statement inside do for at least once, then you can go for it.
Directly from Docs
The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, the statements within the do block are always executed at least once,
do {
statement(s)
} while (expression);
No, there is no time a do-while loops is the only option, it is used for convenience when you do not want to repeat code.
I saw this keyword for the first time and I was wondering if someone could explain to me what it does.
What is the continue keyword?
How does it work?
When is it used?
continue is kind of like goto. Are you familiar with break? It's easier to think about them in contrast:
break terminates the loop (jumps to the code below it).
continue terminates the rest of the processing of the code within the loop for the current iteration, but continues the loop.
A continue statement without a label will re-execute from the condition the innermost while or do loop, and from the update expression of the innermost for loop. It is often used to early-terminate a loop's processing and thereby avoid deeply-nested if statements. In the following example continue will get the next line, without processing the following statement in the loop.
while (getNext(line)) {
if (line.isEmpty() || line.isComment())
continue;
// More code here
}
With a label, continue will re-execute from the loop with the corresponding label, rather than the innermost loop. This can be used to escape deeply-nested loops, or simply for clarity.
Sometimes continue is also used as a placeholder in order to make an empty loop body more clear.
for (count = 0; foo.moreData(); count++)
continue;
The same statement without a label also exists in C and C++. The equivalent in Perl is next.
This type of control flow is not recommended, but if you so choose you can also use continue to simulate a limited form of goto. In the following example the continue will re-execute the empty for (;;) loop.
aLoopName: for (;;) {
// ...
while (someCondition)
// ...
if (otherCondition)
continue aLoopName;
Let's see an example:
int sum = 0;
for(int i = 1; i <= 100 ; i++){
if(i % 2 == 0)
continue;
sum += i;
}
This would get the sum of only odd numbers from 1 to 100.
If you think of the body of a loop as a subroutine, continue is sort of like return. The same keyword exists in C, and serves the same purpose. Here's a contrived example:
for(int i=0; i < 10; ++i) {
if (i % 2 == 0) {
continue;
}
System.out.println(i);
}
This will print out only the odd numbers.
Generally, I see continue (and break) as a warning that the code might use some refactoring, especially if the while or for loop declaration isn't immediately in sight. The same is true for return in the middle of a method, but for a slightly different reason.
As others have already said, continue moves along to the next iteration of the loop, while break moves out of the enclosing loop.
These can be maintenance timebombs because there is no immediate link between the continue/break and the loop it is continuing/breaking other than context; add an inner loop or move the "guts" of the loop into a separate method and you have a hidden effect of the continue/break failing.
IMHO, it's best to use them as a measure of last resort, and then to make sure their use is grouped together tightly at the start or end of the loop so that the next developer can see the "bounds" of the loop in one screen.
continue, break, and return (other than the One True Return at the end of your method) all fall into the general category of "hidden GOTOs". They place loop and function control in unexpected places, which then eventually causes bugs.
"continue" in Java means go to end of the current loop,
means: if the compiler sees continue in a loop it will go to the next iteration
Example: This is a code to print the odd numbers from 1 to 10
the compiler will ignore the print code whenever it sees continue moving into the next iteration
for (int i = 0; i < 10; i++) {
if (i%2 == 0) continue;
System.out.println(i+"");
}
As already mentioned continue will skip processing the code below it and until the end of the loop. Then, you are moved to the loop's condition and run the next iteration if this condition still holds (or if there is a flag, to the denoted loop's condition).
It must be highlighted that in the case of do - while you are moved to the condition at the bottom after a continue, not at the beginning of the loop.
This is why a lot of people fail to correctly answer what the following code will generate.
Random r = new Random();
Set<Integer> aSet= new HashSet<Integer>();
int anInt;
do {
anInt = r.nextInt(10);
if (anInt % 2 == 0)
continue;
System.out.println(anInt);
} while (aSet.add(anInt));
System.out.println(aSet);
*If your answer is that aSet will contain odd numbers only 100%... you are wrong!
Continue is a keyword in Java & it is used to skip the current iteration.
Suppose you want to print all odd numbers from 1 to 100
public class Main {
public static void main(String args[]) {
//Program to print all odd numbers from 1 to 100
for(int i=1 ; i<=100 ; i++) {
if(i % 2 == 0) {
continue;
}
System.out.println(i);
}
}
}
continue statement in the above program simply skips the iteration when i is even and prints the value of i when it is odd.
Continue statement simply takes you out of the loop without executing the remaining statements inside the loop and triggers the next iteration.
Consider an If Else condition. A continue statement executes what is there in a condition and gets out of the condition i.e. jumps to next iteration or condition. But a Break leaves the loop.
Consider the following Program. '
public class ContinueBreak {
public static void main(String[] args) {
String[] table={"aa","bb","cc","dd"};
for(String ss:table){
if("bb".equals(ss)){
continue;
}
System.out.println(ss);
if("cc".equals(ss)){
break;
}
}
System.out.println("Out of the loop.");
}
}
It will print: aa cc Out of the loop.
If you use break in place of continue(After if.), it will just print aa and out of the loop.
If the condition "bb" equals ss is satisfied:
For Continue: It goes to next iteration i.e. "cc".equals(ss).
For Break: It comes out of the loop and prints "Out of the loop. "
The continue statement is used in loop control structure when you need to jump to the next iteration of the loop immediately.
It can be used with for loop or while loop.
The Java continue statement is used to continue the loop. It continues the current flow of the program and skips the remaining code at the specified condition.
In case of an inner loop, it continues the inner loop only.
We can use Java continue statement in all types of loops such as for loop, while loop and do-while loop.
for example
class Example{
public static void main(String args[]){
System.out.println("Start");
for(int i=0; i<10; i++){
if(i==5){continue;}
System.out.println("i : "+i);
}
System.out.println("End.");
}
}
output:
Start
i : 0
i : 1
i : 2
i : 3
i : 4
i : 6
i : 7
i : 8
i : 9
End.
[number 5 is skip]
I'm a bit late to the party, but...
It's worth mentioning that continue is useful for empty loops where all of the work is done in the conditional expression controlling the loop. For example:
while ((buffer[i++] = readChar()) >= 0)
continue;
In this case, all of the work of reading a character and appending it to buffer is done in the expression controlling the while loop. The continue statement serves as a visual indicator that the loop does not need a body.
It's a little more obvious than the equivalent:
while (...)
{ }
and definitely better (and safer) coding style than using an empty statement like:
while (...)
;
continue must be inside a loop Otherwise it showsThe error below:
Continue outside the loop