The for-loop

Sometimes we need to repeat a block of code a certain number of times. To do this, Java provides the for-loop. This loop is often used to iterate over a range of values or through an array. If the number of iterations or the range borders are known, it is recommended to use the for-loop. If they are unknown, the while-loop may be a preferable solution.

The basic for-loop syntax

The for-loop has the following basic syntax:

for (initialization; condition; modification) {
    // do something
} 

Parts of the loop:

• initialization statement is executed once before the loop begins; usually, loop variables are initialized here;
• condition is a Boolean expression that determines the need for the next iteration; if it's false, the loop terminates;
• modification is a statement that changes the value of the loop variables; it is invoked after each iteration of the loop; usually, it uses increment or decrement to modify the loop's variable.

The order of execution for any for-loop is always the same:

1. the initialization statement;
2. if the condition is false then terminate the loop;
3. if the condition is true, then loop's body is executed;
4. the modification is performed;
5. go to the stage 2 (condition).

Let's write a loop for printing integer numbers from 0 to 9 in the same line.

int n = 9;
for (int i = 0; i <= n; i++) {
    System.out.print(i + " "); // here, space is used to separate numbers
}

The code displays:

0 1 2 3 4 5 6 7 8 9 

The variables declared in the initialization statement are visible only inside the scope that includes all parts of the loop: the condition, the body, and the modification. The integer loop's variables are often named as i, j, k or index.

Here’s another example. Let's calculate the sum of the integer numbers from 1 to 10 (inclusive) using the for-loop.

int startIncl = 1, endExcl = 11;

int sum = 0;
for (int i = startIncl; i < endExcl; i++) {
    sum += i;
}

System.out.println(sum); // it prints "55"

Skipping parts

The initialization statement, the condition, and the modification parts are optional, the for loop might not have all of them.

It is possible to declare a variable outside the loop:

int i = 10;
for (; i > 0; i--) {
    System.out.print(i + " ");
}

Moreover, it is also possible to write an infinite loop without these parts at all:

for (;;) {
    // do something
}

Nested loops