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We often need to repeat actions.
For example, outputting goods from a list one after another or just running the same code for each number from 1 to 10.
Loops are a way to repeat the same code multiple times.
The “while” loop
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The while
loop has the following syntax:
While the condition
is truthy, the code
from the loop body is executed.
For instance, the loop below outputs i
while i < 3
:
A single execution of the loop body is called an iteration. The loop in the example above makes three iterations.
If i++
was missing from the example above, the loop would repeat (in theory) forever. In practice, the browser provides ways to stop such loops, and in server-side JavaScript, we can kill the process.
Any expression or variable can be a loop condition, not just comparisons: the condition is evaluated and converted to a boolean by while
.
For instance, a shorter way to write while (i != 0)
is while (i)
:
If the loop body has a single statement, we can omit the curly braces {…}
:
The “do…while” loop
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The condition check can be moved below the loop body using the do..while
syntax:
The loop will first execute the body, then check the condition, and, while it’s truthy, execute it again and again.
For example:
This form of syntax should only be used when you want the body of the loop to execute at least once regardless of the condition being truthy. Usually, the other form is preferred: while(…) {…}
.
The “for” loop
The for
loop is more complex, but it’s also the most commonly used loop.
It looks like this:
Let’s learn the meaning of these parts by example. The loop below runs alert(i)
for i
from 0
up to (but not including) 3
:
Let’s examine the for
statement part-by-part:
part | ||
---|---|---|
begin | i = 0 | Executes once upon entering the loop. |
condition | i < 3 | Checked before every loop iteration. If false, the loop stops. |
body | alert(i) | Runs again and again while the condition is truthy. |
step | i++ | Executes after the body on each iteration. |
The general loop algorithm works like this:
That is, begin
executes once, and then it iterates: after each condition
test, body
and step
are executed.
If you are new to loops, it could help to go back to the example and reproduce how it runs step-by-step on a piece of paper.
Here’s exactly what happens in our case:
Here, the “counter” variable i
is declared right in the loop. This is called an “inline” variable declaration. Such variables are visible only inside the loop.
Instead of defining a variable, we could use an existing one:
Skipping parts
Any part of for
can be skipped.
For example, we can omit begin
if we don’t need to do anything at the loop start.
Like here:
We can also remove the step
part:
This makes the loop identical to while (i < 3)
.
We can actually remove everything, creating an infinite loop:
Please note that the two for
semicolons ;
must be present. Otherwise, there would be a syntax error.
Breaking the loop
Normally, a loop exits when its condition becomes falsy.
But we can force the exit at any time using the special break
directive.
For example, the loop below asks the user for a series of numbers, “breaking” when no number is entered:
The break
directive is activated at the line (*)
if the user enters an empty line or cancels the input. It stops the loop immediately, passing control to the first line after the loop. Namely, alert
.
The combination “infinite loop + break
as needed” is great for situations when a loop’s condition must be checked not in the beginning or end of the loop, but in the middle or even in several places of its body.
Continue to the next iteration
The continue
directive is a “lighter version” of break
. It doesn’t stop the whole loop. Instead, it stops the current iteration and forces the loop to start a new one (if the condition allows).
We can use it if we’re done with the current iteration and would like to move on to the next one.
The loop below uses continue
to output only odd values:
For even values of i
, the continue
directive stops executing the body and passes control to the next iteration of for
(with the next number). So the alert
is only called for odd values.
A loop that shows odd values could look like this:
From a technical point of view, this is identical to the example above. Surely, we can just wrap the code in an if
block instead of using continue
.
But as a side-effect, this created one more level of nesting (the alert
call inside the curly braces). If the code inside of if
is longer than a few lines, that may decrease the overall readability.
Please note that syntax constructs that are not expressions cannot be used with the ternary operator ?
. In particular, directives such as break/continue
aren’t allowed there.
For example, if we take this code:
…and rewrite it using a question mark:
…it stops working: there’s a syntax error.
This is just another reason not to use the question mark operator ?
instead of if
.
Labels for break/continue
Sometimes we need to break out from multiple nested loops at once.
For example, in the code below we loop over i
and j
, prompting for the coordinates (i, j)
from (0,0)
to (2,2)
:
We need a way to stop the process if the user cancels the input.
The ordinary break
after input
would only break the inner loop. That’s not sufficient–labels, come to the rescue!
A label is an identifier with a colon before a loop:
The break <labelName>
statement in the loop below breaks out to the label:
In the code above, break outer
looks upwards for the label named outer
and breaks out of that loop.
So the control goes straight from (*)
to alert('Done!')
.
We can also move the label onto a separate line:
The continue
directive can also be used with a label. In this case, code execution jumps to the next iteration of the labeled loop.
Labels do not allow us to jump into an arbitrary place in the code.
For example, it is impossible to do this:
A call to break/continue
is only possible from inside a loop and the label must be somewhere above the directive.
Summary
We covered 3 types of loops:
while
– The condition is checked before each iteration.do..while
– The condition is checked after each iteration.for (;;)
– The condition is checked before each iteration, additional settings available.
To make an “infinite” loop, usually the while(true)
construct is used. Such a loop, just like any other, can be stopped with the break
directive.
If we don’t want to do anything in the current iteration and would like to forward to the next one, we can use the continue
directive.
break/continue
support labels before the loop. A label is the only way for break/continue
to escape a nested loop to go to an outer one.
Loops can execute a block of code as long as a specified condition is true.
The While Loop
The while
loop loops through a block of code as long as a specified condition is true.
Syntax
// code block to be executed
}
Example
In the following example, the code in the loop will run, over and over again, as long as a variable (i) is less than 10:
Example
text += 'The number is ' + i;
i++;
}
If you forget to increase the variable used in the condition, the loop will never end. This will crash your browser.
The Do/While Loop
The do/while
loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.
Syntax
// code block to be executed
}
while (condition);
Example
The example below uses a do/while
loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:
Example
text += 'The number is ' + i;
i++;
}
while (i < 10);
Do not forget to increase the variable used in the condition, otherwise the loop will never end!
Comparing For and While
If you have read the previous chapter, about the for loop, you will discover that a while loop is much the same as a for loop, with statement 1 and statement 3 omitted.
The loop in this example uses a for
loop to collect the car names from the cars array:
Example
var i = 0;
var text = ';
for (;cars[i];) {
text += cars[i] + '<br>';
i++;
}
The loop in this example uses a while
loop to collect the car names from the cars array:
Example
var i = 0;
var text = ';
while (cars[i]) {
text += cars[i] + '<br>';
i++;
}