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How to Write a While Loop in Computer Programming

Updated on March 26, 2013

Why do we need loops?

Loops are one of the basic things you need to start using if you are learning computer programing. Without loops, programmers job would be much more difficult and in some cases even impossible.

This is because programmers use loops any time when it is necessary to do the same thing few times. This way, code can be written only once and surrounded with a loop.

There are few types of loops over there, such as for, while, go while etc.

Why While?

But makes while loop different than others? When and why should you use it?

For example, most commonly used loop is For. When we use it? We use it when we know exact number of iterations that should happen.

But sometimes, we don't know how many times something should happen. All we know is that there is some condition that should be met in order to stop executing the loop. In this case it is necessary to use While!

Difference Between While and Do While

One of the less popular loops, that can also be used in the similar situations as while, is do while loop. Difference between those two loops is how many times code inside the loop will be executed.

Do while loop is always executed at least one, even if condition for entering this loop is not met. This is because when this loop is used, it will first be executed, and then check the condition. In case when while loop is used, condition is checked first, and then, if it is met, code inside can be executed.

Example

Imagine following trivial situation: Your task is to read some number, let's call it X, and to divide it by two, until it's less than one. For example, if X is 5, you should divide it once, and get 2.5. Then you should divide 2.5 and get 1.25. In next iteration, you will finally get 0.67, which is lower than 1.

But initially, you don't know how much is X, so you can't tell how much iterations will it take. And this is why we use while loop in this case. Following code example is written in Java:

Code in Java

 public void divideByTwo(int X){
   double var = X;
   while(var>1){
     var = var/2;
  }
 }

Explanation of the example

Let's examine every line of this method:

1. public void divideByTwo(int X){ - declaration of the method. This method has one input parameter: X. This is the initial number that should be divided by two.

2. double var = X; - declaration of the variable var. Initially, this variable has the same value as X. In the future, it will be used to 'save' the value that we get by dividing it by two.

3. while(var>1){ - setting the condition and definition of the while loop. When writing this loop in Java, you need to write keyword while, and then to set condition inside the brackets. Our condition here means that we should iterate trough loop (repeat code written inside it) as many times as it is needed to make var lower than 1.

4. var = var/2; - divide var with 2 in every iteration. Var now gets new value that is half of the previous one. If var is still higher than 1, it will become it's own half once more.

5. } - we are closing while block.

6. } - we are closing the divideByTwo method.


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