5.2. The for Statement

The for statement executes a statement or a block of statements while a specified Boolean expression evaluates to true. Such evaluation involves comparing the local loop counter variable (an integer counter that increments/decrements over every loop) to a preset value. Since the number of loops is controlled by comparing the counter with the preset value, you need to specify the preset value to decide the number of times you want to run the code block when defining a for loop statement.

5.2.1. For Statement Syntax

for loops are usually favored for iteration because the all the information about the local loop variable are available in one place at the top, which helps quickly visualize the overall sequence in the loop.

The C# for loop statement syntax is:

for ( initializer ; condition ; iterator ) {

// code block (statements)

}

Let us practice in csharprepl:

> for (int i = 1; i <= 5; i++)
{
      Console.WriteLine(i);
}
1
2
3
4
5

You can see that the for loop has four parts, a three-section semicolon-separated header and a body:

• Initializer: This section is a variable declaration statement that declares a local loop variable (or iterator variable or the counter) and initialize it with an initial value.

• Condition: The condition section is a boolean expression that determines if the next iteration in the loop should be executed or should the loop be terminated. If it evaluates to true, the next iteration is executed; otherwise, the loop is exited.

• Iterator: The iterator (iteration expression) section that defines what happens to the counter after each execution of the body of the loop. A common behavior is reassigning (increment or decrement) the value of the counter.

• The body of the loop, which must be a statement or a block of statements.

In the example above, the initializer is i = 1, condition is i <= n, and iterator is i++ (see arithmetic operators).

Note that the initializer section is executed only once, before entering the loop. Also, the declared variable is local to the for loop heading and the loop body and can not be accessed from outside the for statement.

The two semicolons are always needed in the for heading, but any of the parts they normally separate may be omitted. If the condition part is omitted, the condition is interpreted as always true, leading to an infinite loop, that can only terminate due to a return or break statement in the body.

Header variation

The declaration of a for loop header is flexible. There may be several variables of the same type initialized, separated by commas. Also, at the end of the for loop heading, the iteration portion may include more than one expression, also separated by commas. For example:

for (int i = 0, j = 10; i < j; i = i+2, j++) {
   Console.WriteLine("{0} {1}", i, j);
}

Guess what this does, and then paste it into csharprepl to check.

5.2.1.1. Execution sequence

For the execution sequence of for loops, note that the different parts of the heading are used at different times:

• When starting the statement, the initialization is done, and then the test of condition (boolean expression).

• After finishing the body and returning to the heading, the iteration operations are done, followed by the test.

5.2.2. break and continue Statements

The basic syntax of the for statement is simplistic. Here we are using the break and continue statements to learn the pragmatic coding practice of the for loops by connecting to our understanding of conditional/selection statements.

When performing iteration, there will be times that you want to alter the code execution sequence by skipping part or all of the iterations. If you only want to break out of the enclosing loop, but not out of the whole method or the outer loop (in case of nested looping), use a break statement

break;

in place of return, since return will break out the current method. With the break statement, execution continues after terminating the enclosing iteration statement.

Note that the break and continue statements only make practical sense inside of an if statement that is inside the loop. In the following examples, you see a for statement with a break statement enclosed in a conditional/selection statement.

Assuming that variable target already has a string value and variable arr is an array of strings. With your knowledge about arr.Length and arr[i] from Strings, read the following code:

bool found = false;

for (int i = 0; i < arr.Length; i++)   // loop for arr.Length times
{
   if (arr[i] == target)               // if one of arr == targe
   {
      found = true;                    // set found to true
      break;                           // break out of the enclosing loop
   }
}

if (found)                             // if found == true (from the previous block)
{
   Console.WriteLine("Target found at index " + i);
}
else
{
   Console.WriteLine("Target not found");
}

When an element in arr is reached that matches target, execution breaks out of the for loop and move on to the if (found) statement block below.

Now, observe an alternate implementation with a compound condition (Compound Boolean Expressions) in the heading and no break is:

bool found = false;

for (int i = 0; i < a.Length && !found; i++) {
   if (a[i] == target) {
      found = true;
   }
}

if (found) {
   Console.WriteLine("Target found at index " + i);
} else {
   Console.WriteLine("Target not found");
}

As you can see, the code exit because the condition section of the if statement header has an expression !found (Logical Operators), meaning found is not true. The shows that since break statements rely on the logic of the conditional statement, if the condition can be embedded in the header of the loop, you don’t have to use break. However, if you are designing a loop that has multiple exit criteria, using break statements can make the code much less verbose in the header’s condition section, and hence easier to follow because the if statement conditions and the immediate break action may be clearly presented.

5.2.3. Nested for Loop

There will be times when nested loops are are required for the problem scenario. A nested loop can look like this:

outer-Loop
{
   // body of outer-loop
   inner-Loop
   {
      // body of inner-loop
   }
... ... ...
}

Continuing with our discussion on break, let’s say we are in a situation like the following:

for (....) {

   // some statements of outer for loop

   for (....) {
      ...
      if (...) {
      ...
      break;
      }
      ...
   }

   // some statements of outer for look
}

The break statement is in the inner loop. If it is reached, the inner loop ends, but the inner loop is just a single statement inside the outer loop, and the outer loop continues. If the outer loop continuation condition remains true, the inner loop will be executed again. As an example:

for (int i = 0; i <= 3; i++)
{
   for (int j = 0; j <= 3; j++)
   {

      if (i == 2)
      {
         break;
      }

      Console.WriteLine("{0} -- {1}", i, j);
   }
}

Can you determine the output of the preceding code? Try it in csharprepl or a test project in your tests folder.

continue Statement

For completeness we mention the much less used continue statement:

continue;

A continue statement: - does not break out of the whole loop statement. - break/skips the execution of the rest of the body in the current enclosing loop iteration. - starts the next enclosing loop iteration.

In the simplest situations, a continue statement just avoids an extra else clause. It can considerably shorten code if the test is inside of complicated, deeply nested if statements. As an example:

for (int i = 0; i <= 3; i++)
{
   for (int j = 0; j <= 3; j++)
   {
      if (i == 2)
      {
           continue;
      }

      Console.WriteLine("{0} -- {1}", i, j);
   }
}

Can you determine the output of the preceding code? Try it in csharprepl or a test project in your tests folder.