Loops

Loops are important for calculations that require repeated steps and for processing input consisting of many data items.

The While loop

It has the form

          while (condition)
          {

                 statements

          }

As long as the condition remains true, the statements inside the while statement are executed. These statements are called the body of the while statement. 

When you declare a variable inside the loop body, the variable is created for each iteration of the loop and removed after the end of each iteration.

Infinity loops

A very annoying loop error is an infinite loop: a loop that runs forever and can be stopped only by killing the program or restarting the computer.

A common reason for infinite loops is forgetting to update the variable that controls the loop.

The for loop

The for loop is used when a value runs from a starting point to an ending point with a constant increment or decrement.

for( initialization; condition; update)                                           {                                                                                                             statements;                                                                    } 

for (int counter = 1; counter <= 10; counter++)
{
       System.out.println(counter);
} 

For loop is often called count-controlled loop. In contrast, while loop can be called event-controlled loop because it executes until an event occurs;

• The initialization is executed once, before the loop is entered.
• The condition is checked before each iteration.
• The update is executed after each iteration.

Enhanced for loop:

Traditional example:

for (int i = 0; i < myArray.length; i++) {
    System.out.println(myArray[i]);
}

Enhanced for example:

for (int myValue : myArray) {
    System.out.println(myValue);
}

The do loop

The do loop is appropriate when the loop body must be executed at least once.

do                                                                                               {                                                                                                          statements                                                                           } while (condition);

The body of the do loop is executed first, then the condition is tested.

Misc Notes:

• To compute an average, keep a total and a count of all values.
• To count values that fulfill a condition, check all values and increment a counter for each match.
• If your goal is to find a match, exit the loop when the match is found. 
• To find the largest value, update the largest value seen so far whenever you see a larger one.
• To compare adjacent inputs, store the preceding input in a variable. 
• When the body of a loop contains another loop, the loops are nested. A typical use of nested loops is printing a table with rows and columns. 
• You can introduce randomness by calling the random number generator.