Repetition in Java

by Ben Davidson

Objectives

Know when and why to use repetition
Understand how to implement while loops
Understand how to implement do while loops
Understand how to implement for loops
Understand how to implement for each loops
Understand how to use nested loops

Purpose of Repetition

Sometimes it is necessary to repeat a block of code many times. It may seem like a simple solution to copy a block of code and paste it as many times as you need, but there is an alternative to this code-bloating idea. In all programming languages there is some type of mechanism for repeating a block of code. This is known as repetition or looping. Some examples of when you want to repeat a block of code are:

You want to print a list of student's names from an array
You want to find the factorial of a number

while Loops

A while loop has three parts: the keyword while, a boolean expression, and the code block to be repeated.
Syntax:
while (boolean expression) {
statements to be repeated;
}
while loops are useful to repeat a block of code until a condition is met.

In the following example, student names will be printed until the student "Kyle" is found.

String[] studentNames = {"Bill", "Susan", "Fred", "Kyle", "Sarah"};
int i = 0;
while (!studentNames[i].equals("Kyle")) {
	System.out.println(studentNames[i]);
	i++;
}

do while Loops

In a do while loop the block of code to be repeated comes before the boolean expression. This guarantees that the code in the body of the loop will be executed at least once.
Syntax:
do {
statements to be repeated;
} while (boolean expression);
Again, a do while loop is different from a while loop because the statements in the code block will be executed at least once.

Example:

String[] studentNames = {"Bill", "Susan", "Fred", "Kyle", "Sarah"};
int i = 0;
do {
   System.out.println(studentNames[i]);
   i++;
} while (!studentNames[i].equals("Kyle"));

for Loops

A for loop has some of the same parts as a while loop, but with a different order.
for loops are useful for counting and going through an entire array.
A for loop has five parts: the keyword for, an initial action, a boolean expression, an action to be done after each loop, and a block of code to be repeated:
Syntax:
for (initial-action; boolean expression; action after each iteration) {
statements to be repeated;
}
for loops are often implemented with arrays.

The following example will print all of the names in the studentNames array:

String[] studentNames = {"Bill", "Susan", "Fred", "Kyle", "Sarah"};
for (int i = 0; i < studentNames.length; i++) {
   System.out.println(studentNames[i]);
}

The following is an ordered list of how the loop is executed

The array is created and filled with student names
0 is assigned to the variable i (only done once)
The boolean expression is evaluted (it is true)
Because the boolean expression was true, the code in the body of the loop is executed
i is incremented by one
Steps 4 and 5 are repeated until the boolean expression is false

for each Loops

for each loops were designed to be used with arrays and collections.
A for each loop is similar to a for loop, except it only has three parts: the keyword for, a variable declaration in which a new variable is declared and assigned to each element in the array, and the block of code to be repeated.
Syntax:
for (variableType variableName: arrayName) {
statements to be repeated;
}

In the following example, all of the names in the studentNames array are printed to the screen.

String[] studentNames = {"Bill", "Susan", "Fred", "Kyle", "Sarah"};
for (String name : studentNames) {
   System.out.println(name);
}

Nested Loops

It is often useful to put a loop inside of a loop.
Nested loops are used to print two dimmensional arrays.

An example of a nested loop follows:

int[][] board = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
for (int i = 0; i < board.length; i++) {
   for(int j = 0; j < board[i].length; j++){
      System.out.print(board[i][j]);
   }
   System.out.println();
}