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JavaScript JavaScript Loops, Arrays and Objects Simplify Repetitive Tasks with Loops `do ... while` Loops

Do While Loop conditional statement question...

Please see the conditional statement in the while loop. If correctGuess is already set to false, why would I need to place an exclamation mark next to correctGuess? I thought we want the loop to run while correctGuess is false.

var randomNumber = getRandomNumber(10);
var guess;
var guessCount = 0;
var correctGuess = false;

function getRandomNumber( upper ) {
  var num = Math.floor(Math.random() * upper) + 1; 
  return num;

do { 
  guess = prompt('Guess a number between 1 & 10');
  guessCount += 1;
  if (parseInt(guess) === randomNumber){
   correctGuess = true; 
} while ( ! correctGuess )
  document.write('<h1>You guessed the number!</h1>');
  document.write('It took you ' + guessCount + ' tries to guess the number' + randomNumber);

2 Answers

The !correctGuess is telling the program to keep running the code within the while loop as long as correctGuess is false. The ! just means not, so it's basically saying 'while NOT correct guess, keep running'

But isn't correctGuess already set to false at the top of the code when it's declared as a variable?

So why wouldn't we just do the following...?

while (correctGuess)

I thought (!correctGuess) would evaluate to true.

Seth Kroger
Seth Kroger
56,413 Points

The while loop will only continue if the expression evaluates to true. Your code would end the loop when correctGuess is false because when you substitute the value you get while(false). That is why you need to invert the condition with !correctGuess (ie. NOT(false) equals true).

Okay that's what I was misunderstanding. Thanks, everyone!

Brian Mendez
Brian Mendez
7,734 Points

You can also do:

while ( correctGuess == false );

until you get more comfortable with '!'