Lesson 4 | Repeating code with for loops |
Objective | Find out how to repeat a section of program code using a loop. |
Repeating Code with for Loops
There are three types of loops that are used in Java:
for
loops
while
loops
do-while
loops
Getting redundant with for loop
The most structured Java loop is the
for loop, which repeats a section of code a fixed number of times.
for Statement
The for statement is used when the number of times the loop needs to be executed is known. There are two variations of the for loop. The first one is discussed in this section and is the traditional form. The for-each statement is the second form and was introduced in Java 5.
It is discussed in the at the bottom of this page. The for statement consists of the following three parts:
- Initial operation
- Terminal condition
- End loop operation
Following is the syntax of the
for
loop:
for (initialization-expression; boolean-condition; update-expression)
Statement
for Loop Body
The body of a for loop is typically a block statement. The initial operation takes place prior to the first iteration of the loop and is executed only once. The end loop operations take place at the end of each execution of the loop. The terminal condition
determines when the loop will terminate and is a logical expression. It is executed at the beginning of each repetition of the loop.
Thus, the body of the for loop may be executed zero times if the first time the terminal condition is evaluated, it evaluates to false.
A variable is normally used as part of the initial operation, terminal condition, and end loop operation. The variable is either declared as part of the loop or is declared external to the loop. The following code snippet is an example of declaring a variable, i, as part of the loop. An example of using an external variable is covered in the The for statement and scope section:
for (int i = 1; i <= 10; i++) {
System.out.print(i + " ");
}
System.out.println();
In this example we used a single statement in the body of the loop.
The variable i
was assigned an initial value of 1 and is incremented by 1 each time the loop
executes. The loop executed 10 times and produced 1 line of output. The statement,
i++, is a more concise way of saying i = i + 1. The output should be the following:
1 2 3 4 5 6 7 8 9 10
The
for
loop repeats the
Statement
a number of times as determined by the
- initialization-expression,
- boolean-condition,
- update-expression:
- The
initialization-expression
initializes a loop control variable.
- The
boolean-condition
compares the loop control variable to some limit value.
- The
update-expression
updates the loop control variable before the next iteration of the loop.
Following is an example that uses a
for
loop to count up from one to ten:
for (int i = 1; i <= 10; i++)
System.out.println(i);
In this example, the initializationExpression
is int i = 1
, which is evaluated initially before the loop begins.
The boolean-condition
, i <= 10
, is a Boolean test performed before each iteration of the loop.
If the Boolean test result is true
the Statement
is executed, which in this case prints the current value of i
.
The update-expression
(i++
) is evaluated after each iteration.
This statement increments i
after each iteration and establishes the count.
The loop continues to iterate and print numbers as i
counts up to 10
.
Once i
reaches 11
the loop condition test fails (i <= 10)
, and the loop exits.
Conditional Loops - Quiz
for Loop - Exercise
In this exercise, you will develop an application that calculates grade-point averages.
for Loop - Exercise
Java Virtual Machine