1. Comparing loops: for vs while

A while loop can be used anywhere a statement or group of statements needs to be performed several times. But among all the possible scenarios, one is worth highlighting.

We're talking about the situation when the programmer (the program's creator) knows in advance how many times the loop should be executed. This is usually handled by declaring a special counter variable, and then increasing (or decreasing) the variable by 1 with each iteration of the loop.

Everything seems to work as it should, but it isn't very convenient. Before the loop, we set the counter variable's initial value. Then in the condition we check whether it has already reached the final value. But we usually change the value at the very end of the lop body.

And what if the body of the loop is large? Or if we have several nested loops? In general, in these cases it's desirable to collect all this information about counter variables in one place. And that's why we have the for loop in Java. It also doesn't look very complicated:

for (statement 1; condition; statement 2)
{
   block of statements
}

A while loop has just a condition in parentheses, but a for loop adds two statements, separated by semicolons.

The reality is simpler than it sounds: the compiler converts a for loop into an ordinary while loop like this:

statement 1;
while (condition)
{
   block of statements
   statement 2;
}

Or better yet, let's demonstrate this with an example. The two code snippets below are identical.

Option 1 Option 2
for (int i = 0; i < 20; i++)
{
   System.out.println(i);
}
int i = 0;
while (i < 20)
{
   System.out.println(i);
   i++;
}

We just gathered into one place all the code that pertains to the i counter variable.

In a for loop, statement 1 is executed just once, before the loop itself begins. This can be seen clearly in the second code snippet

statement 2 is executed the same number of times as the body of the loop, and each time it is executed after the entire body of the loop has been executed


2. Where the for loop is used

The for loop is probably the most used type of loop in Java. It is used everywhere, for programmers it is just clearer and more convenient than a while loop. Virtually any while loop can be converted into a for loop.

Examples:

while loop for loop
int i = 3;
while (i >= 0)
{
   System.out.println(i);
   i--;
}
for (int i = 3; i >= 0; i--)
{
   System.out.println(i);
}
int i = 0;
while (i < 3)
{
   System.out.println(i);
   i++;
}
for (int i = 0; i < 3; i++)
{
   System.out.println(i);
}
boolean isExit = false;
while (!isExit)
{
   String s = console.nextLine();
   isExit = s.equals("exit");
}
for (boolean isExit = false; !isExit; )
{
   String s = console.nextLine();
   isExit = s.equals("exit");
}
while (true)
   System.out.println("C");
for (; true; )
   System.out.println("C");
while (true)
{
   String s = console.nextLine();
   if (s.equals("exit"))
      break;
}
for (; true; )
{
   String s = console.nextLine();
   if (s.equals("exit"))
      break;
}

Pay attention to the last example. The statements for working with the loop counter are missing. There is no counter and no statement.

In a for loop, Java lets you omit the "statement to initialize the counter" and the "statement to update the counter". Even the expression that defines the loop condition can be omitted.


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4
Task
New Java Syntax, level 4, lesson 6
Locked
Even numbers
Use a for loop to display even numbers from 1 to 100 inclusive. Display each value on a new line.

3. Nuances of using the for loop

An important point about using for loops and break and continue statements.

A break statement in a for loop works the same as in a while loop — it terminates the loop immediately. A continue statement skips the loop body, but not statement 2 (which changes the loop counter).

Let's take another look at how for and while loops are related.

for (statement 1; condition; statement 2)
{
   block of statements
}
statement 1;
while (condition)
{
   block of statements
   statement 2;
}

If a continue statement is executed in a for loop, then the rest of the block of statements is skipped, but statement 2 (the one that works with the for loop's counter variable) is still executed.

Let's return to our example with skipping numbers that are divisible by 7.

This code will loop forever This code will work fine
int i = 1;
while (i <= 20)
{
   if ( (i % 7) == 0) continue;
   System.out.println(i);
   i++;
}
for (int i = 1; i <= 20; i++)
{
   if ( (i % 7) == 0) continue;
   System.out.println(i);
}

The code that uses the while loop will not work — i will never be greater than 7. But the code with the for loop will work fine.


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4
Task
New Java Syntax, level 4, lesson 6
Locked
Drawing a rectangle
Use the keyboard to enter two numbers m and n. Using a for loop, display an n x m rectangle made of eights. Here's an example: m=2, n=4 8888 8888

4. Comparing for loops: Java vs Pascal

By the way, Pascal also has a For loop. In general, essentially every programming language has one. But in Pascal is it super clear. Examples:

Pascal Java
For i := 1 to 10 do
Begin
   Writeln(i);
End;
for (int i = 1; i <= 10; i++)
{
   System.out.println(i);
}
For i := 1 to 10 do step 2
Begin
   Writeln(i);
End;
for (int i = 1; i <= 10; i = i + 2)
{
   System.out.println(i);
}
For i := 10 downto 0 do step 2
Begin
   Writeln(i);
End;
for (int i = 10; i >= 0; i = i - 2)
{
   System.out.println(i);
}

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4
Task
New Java Syntax, level 4, lesson 6
Locked
Triangle of eights
Using a for loop, display a right triangle of eights, with a base of 10 and a height of 10. Example output: 8 88 888 8888 88888 888888 8888888 88888888 888888888 8888888888
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4
Task
New Java Syntax, level 4, lesson 6
Locked
Drawing lines
Using a for loop to display: - a horizontal line of 10 eights - a vertical line of 10 eights (do not count any of the eights in the horizontal line as part of this vertical line).
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4
Task
New Java Syntax, level 4, lesson 6
Locked
Chain letter
Enter a name from the keyboard and use a for loop to display the following 10 times: loves me. Example output for the name "Scarlett": Scarlett loves me. Scarlett loves me. Scarlett loves me. Scarlett loves me. Scarlett loves me. Scarlett loves me. Scarlett loves me. Scarlett loves me. Scarlett lo