while

Definition

The while statement is used to create a loop that executes a block of code as long as (while) the test condition is true. The test condition is evaluted at the start of the loop.

Syntax

while

A simple while statement has the following syntax:

while ( condition ) {
    block;
  }

while ( condition ) {
    block;
  }

while condition :
    block

The condition is an expression that is evaluated at the start of each pass through the loop, and which evaluates to either true or false.
The loop will stop executing when the condition becomes false.
The block is a block of code that gets executed while the condition evaluates to true.

Examples

Example 1: Multiplication Tables

Imagine that you want to print out the multiplication table for a given number. E.g., the times table for 6:
    1 times 6 = 6
    2 times 6 = 12
    3 times 6 = 18
    etc.

Let's look at some code to achieve this.

We'll use a while loop and a variable timesLine will act as our counter. We want to go through the loop 12 times, so we will initialise timesLine to 1, and loop while it is less or equal to 12, and increment it by one at the end of each pass through the loop.
- JavaScript
- Java
- Python
```
var i = 1;  // Initialise loop counter
  
// Repeat 1 to 12 times
while (i <= 12 ) {
    // Do something here
    i++;   // Required to prevent an endless loop!
}
```
```
int i = 1;  // Initialise loop counter
  
// Repeat 1 to 12 times
while (i <= 12 ) {
    // Do something here
    i++;   // Required to prevent an endless loop!
}
```
```
i = 1  # Initialise loop counter

# Repeat 1 to 12 times
while i <= 12 :
    # Do something here 
    i = i + 1   # Required to prevent an endless loop!
```
Python Note: Short cut for adding one to a variable, i++, not used in Python.
Warning: Since the loop condition is i <= 12 we need to ensure that the i variable gets updated within the loop. Otherwise, it will always evaluate to true and will loop forever (an infinite loop)!

We will get the required number for the multiplication table from the user and store it in the timesTable variable.

var i = 1;  // Initialise loop counter
  
var timesTable;   // the times table value chosen
timesTable = prompt( "Enter the times table required: " );
  
// Repeat 1 to 12 times
while ( i <= 12 ) {
    document.write(i + " times " + timesTable + " = " + (i * timesTable) + "<br>");
    i++;
}

JavaScript Note: As in this example, the mark is entered in a text field, the data type will be a string. However, we want to convert it to a number (integer) using the parseInt() function.

int i = 1;  // Initialise loop counter

int timesTable;         // the times table value chosen
Scanner input = new Scanner(System.in);
System.out.println("Enter the times table required: ");
timesTable = input.nextInt();
  
// Repeat 1 to 12 times
while ( i <= 12 ) {
    System.out.print(i + " times " + timesTable + " = ");
    System.out.println((i * timesTable));  // the answer
    i++;
}

i = 1 # Initialise loop counter
timesTable = int(input( 'Enter the times table required: ' ))
	  
while i <= 12 :
    print(i, " times ", timesTable, " = ", i * timesTable)
    i = i + 1

Python Note: Method input() produces string values so we convert to an integer with the int() method.

Example 2: Year Average

Imagine that you want to calculate the year average (in this case, a module is a course component, as university students would know it). The year is composed of 120 credits. For the sake of simplicity, we'll consider that the year is composed of six 20-credit modules.

Let's create the code for this, one step at a time.

Note: there are many solutions to this problem and it is important that you write code that you feel comfortable with and that you tackle the problem using a logic that feels natural to you.

Let's create a variable that will hold the number of marks entered: marksEntered and the sum of all the marks entered: sumMarksEntered
- JavaScript
- Java
- Python
```
var marksEntered = 0, 
    sumMarksEntered = 0;
```
```
int marksEntered = 0, 
    sumMarksEntered = 0;
```
Java Note: Java requires that the variable type is defined before use.
```
marksEntered = 0
sumMarksEntered = 0
```

While the number of marks entered is less than 6, ask for another mark.
- JavaScript
- Java
- Python
```
var marksEntered = 0, 
    sumMarksEntered = 0;
      
while ( marksEntered < 6 ) {
    prompt( "Enter mark for module number " + ( marksEntered + 1 ) + ": ");
}
```
```
int marksEntered = 0, 
    sumMarksEntered = 0;
      
while ( marksEntered < 6 ) {
    System.out.println( "Enter mark for module number " + ( marksEntered + 1 ) + ": " );
}
```
```
marksEntered = 0
sumMarksEntered = 0
  
while  marksEntered < 6 :
    input( 'Enter mark for module number ' + str(marksEntered + 1) + ': ' ) 
```
Python Note: As we cannot concatenate strings and integers in Python we convert marksEntered to a string with the str() method.
Warning: Since the conditions is marksEntered < 6 and the marksEntered variable never gets updated with the code written so far, if the condition marksEntered < 6 evaluates to true, it will loop forever! This is called an infinite loop. Infinite loops are bad! They will most likely crash your browser! We must therefore fix this straightaway by updating the value of marksEntered each time the user enters a new mark.
- JavScript and Java - We will increment the value of the variable inside the loop using marksEntered++;, which is equivalent to marksEntered = marksEntered + 1;
- Python does not use ++ (e.g., marksEntered++) so we will use marksEntered += 1 (again this is equivalent to marksEntered = marksEntered + 1;).

Each time the user enters a new mark, this should be added to the sumMarksEntered variable. We'll use an intermediate variable moduleMark to store the new mark entered by the user.
- JavaScript
- Java
- Python
```
var marksEntered = 0, 
    sumMarksEntered = 0,
    moduleMark= 0;
  
while ( marksEntered < 6 ) {
    moduleMark= prompt( "Enter mark for module number " + ( marksEntered + 1) + ": " );
    sumMarksEntered = sumMarksEntered + parseInt( moduleMark );
    marksEntered++;
}
```
JavaScript Note: As in this example, the moduleMark is entered in a text field, the data type will be a string. However, we want to convert it to a number (integer) using the parseInt() function.
```
int marksEntered = 0, 
    sumMarksEntered = 0, moduleMark;
Scanner sc=new Scanner(System.in);  
  
while ( marksEntered < 6 ) {
    System.out.print ("Enter mark for module number " +  (marksEntered + 1) + ": ");
    moduleMark= sc.nextInt(); 
    sumMarksEntered = sumMarksEntered + moduleMark;
    marksEntered++;
}
```
```
marksEntered = 0
sumMarksEntered = 0
moduleMark = 0
  
while  marksEntered < 6 :
    moduleMark= input( 'Enter mark for module number ' + str(marksEntered + 1) + ': ') 	
    sumMarksEntered = sumMarksEntered + int( moduleMark)
    marksEntered += 1
	
```
Python Note: Method input() produces string values so we convert moduleMark with the int() method.
Notice how the condition marksEntered < 6 will be true initially as the variable is initialised to 0. Inside the loop, this variable is incremented (one is added to its value). This means that the value of this variable is going to get bigger and bigger, until we reach the value 6, which is defined by the condition statement. This also ensures that we do not have an infinite loop. Once we reach the value 6, the condition for the while loop becomes false and the loop doesn't get executed again. Instead, the code that comes after the loop is.

Finally, once the user has entered the 6 marks and we come out of the while loop, we'll calculate the year average by dividing the sum of all the marks entered (sumMarksEntered) by 6. We will use the yearAverage variable for this, and will display the output.

var marksEntered = 0, 
    sumMarksEntered = 0,
    moduleMark= 0,
    yearAverage = 0;
  
while ( marksEntered < 6 ) {
    moduleMark= prompt( "Enter mark for module number " + ( marksEntered + 1) + ": " );
    sumMarksEntered = sumMarksEntered + parseInt( moduleMark);	
    marksEntered++;
}
yearAverage = sumMarksEntered / 6;
alert( "Your year average is " + yearAverage );

int marksEntered = 0, sumMarksEntered = 0, moduleMark;
double yearAverage;
Scanner sc=new Scanner(System.in);  
  
while ( marksEntered < 6 ) {
    System.out.print( "Enter mark for module number " + (marksEntered + 1) + ": ");
    moduleMark= sc.nextInt(); 
    sumMarksEntered = sumMarksEntered + moduleMark;
    marksEntered++;
}
yearAverage = sumMarksEntered / 6.0;
System.out.println( "Your year average is " + yearAverage );

Java Note: 6.0 (sumMarksEntered / 6.0) required to force the result into a double

marksEntered = 0
sumMarksEntered = 0
  
while  marksEntered < 6 :
    moduleMark= input( 'Enter mark for module number ' + str( marksEntered + 1) + ': ')  
    sumMarksEntered = sumMarksEntered + int( moduleMark)
    marksEntered += 1
yearAverage = sumMarksEntered / 6
print( 'Your year average is ', yearAverage )

To get a good understanding of the logic developed here and the while loop structure, view this in the Code Visualiser , or play with the code below (press the "Run Code" button to run the code): JS Bin on jsbin.com.