for loop

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 loop 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.

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:

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.

6

Task

New Java Syntax, level 6, lesson 3

Locked

String array in reverse order

1. Create an array of 10 strings. 2. Enter 8 strings from the keyboard and save them in the array. 3. Display the contents of the entire array (10 elements) on the screen in reverse order. Each element on a new line.

Open

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.

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.

6

Task

New Java Syntax, level 6, lesson 3

Locked

Array of numbers in reverse order

1. Create an array of 10 numbers. 2. Enter 10 numbers from the keyboard and write them to the array. 3. Display the elements of the array in reverse order. Display each value on a new line.

Open

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:

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);
}