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This chapter is from the book

This chapter is from the book

6.3 Loops

Loops are used to execute a segment of code repeatedly until some condition is met. JavaScript's basic looping constructs are

  • while
  • for
  • do/while

6.3.1 The while Loop

The while statement executes its statement block as long as the expression after the while evaluates to true; that is, nonnull, nonzero, nonfalse. If the condition never changes and is true, the loop will iterate forever (infinite loop). If the condition is false, control goes to the statement right after the closing curly brace of the loop's statement block.

The break and continue functions are used for loop control.

Format

while (condition) {
  statements;
  increment/decrement counter;
}

Example 6.5.

  <html>
    <head>
      <title>Looping Constructs</title>
    </head>
    <body>
      <h2>While Loop</h2>
      <font size="+2">
1     <script type="text/javascript">
2       var i=0;    // Initialize loop counter
3       while ( i < 10 ){      // Test
4         document.writeln(i);
5         i++;    // Increment the counter
6       }    // End of loop block
7     </script>
      </font>
    </body>
  </html>

Explanation

  1. The JavaScript program starts here.
  2. The variable i is initialized to 0.
  3. The expression after the while is tested. If i is less than 10, the block in curly braces is entered and its statements are executed. If the expression evaluates to false, (i.e., i is not less than 10), the loop block exits and control goes to line 6.
  4. The value of i is displayed in the browser window (see Figure 6.4).
    Figure 6.4

    Figure 6.4 Output from Example 6.5.

  5. The value of i is incremented by 1. If this value never changes, the loop will never end.
  6. This curly brace marks the end of the while loop's block of statements.
  7. The JavaScript program ends here.

6.3.2 The do/while Loop

The do/while statement executes a block of statements repeatedly until a condition becomes false. Owing to its structure, this loop necessarily executes the statements in the body of the loop at least once before testing its expression, which is found at the bottom of the block. The do/while loop is supported in Mozilla/Firefox and Internet Explorer 4.0, JavaScript 1.2, and ECMAScript v3.

Format

do
   { statements;}
while (condition);

Example 6.6.

  <html>
    <head>
      <title>Looping Constructs</title>
    </head>
    <body>
      <h2>Do While Loop</h2>
      <font size="+2">
      <script type="text/javascript">
1       var i=0;
2       do{
3         document.writeln(i);
4         i++;
5       } while ( i < 10 )
      </script>
      </font>
    </body>
  </html>

Explanation

  1. The variable i is initialized to 0.
  2. The do block is entered. This block of statements will be executed before the while expression is tested. Even if the while expression proves to be false, this block will be executed the first time around.
  3. The value of i is displayed in the browser window (see Figure 6.5).
    Figure 6.5

    Figure 6.5 Output from Example 6.6, the do/while loop.

  4. The value of i is incremented by 1.
  5. Now, the while expression is tested to see if it evaluates to true (i.e., is i less than 10?). If so, control goes back to line 2 and the block is re-entered.

6.3.3 The for Loop

The for loop consists of the for keyword followed by three expressions separated by semicolons and enclosed within parentheses. Any or all of the expressions can be omitted, but the two semicolons cannot. The first expression is used to set the initial value of variables and is executed just once, the second expression is used to test whether the loop should continue or stop, and the third expression updates the loop variables; that is, it increments or decrements a counter, which will usually determine how many times the loop is repeated.

Format

for(Expression1;Expression2;Expression3)
  {statement(s);}
for (initialize; test; increment/decrement)
  {statement(s);}

The preceding format is equivalent to the following while statement:

Expression1;
while( Expression2 )
  { Block; Expression3};

Example 6.7.

  <html>
    <head>
      <title>Looping Constructs</title>
    </head>
    <body>
      <h2>For Loop</h2>
      <font size="+2">
      <script type="text/javascript">
1       for( var i = 0; i < 10; i++ ){
2         document.write(i);
3       }
      </script>
      </font>
    </body>
  </html>

Explanation

  1. 1 The for loop is entered. The expression starts with step 1, the initialization of the variable i to 0. This is the only time this step is executed. The second expression, step 2, tests to see if i is less than 10, and if it is, the statements after the opening curly brace are executed. When all statements in the block have been executed and the closing curly brace is reached, control goes back into the for expression to the last expression of the three. i is now incremented by one and the expression in step 2 is retested. If true, the block of statements is entered and executed.
  2. The value of i is displayed in the browser window (see Figure 6.6).
    Figure 6.6

    Figure 6.6 Output from Example 6.7.

  3. The closing curly brace marks the end of the for loop.

6.3.4 The for/in Loop

The for/in loop is like the for loop, except it is used with JavaScript objects. Instead of iterating the statements based on a looping condition, it operates on the properties of an object. This loop is discussed in Chapter 9, "JavaScript Core Objects," and is only mentioned here in passing, because it falls into the category of looping constructs.

6.3.5 Loop Control with break and continue

The control statements, break and continue, are used to either break out of a loop early or return to the testing condition early; that is, before reaching the closing curly brace of the block following the looping construct.

Table 6.1. Control Statements

Statement

What It Does

break

Exits the loop to the next statement after the closing curly brace of the loop's statement block.

continue

Sends loop control directly to the top of the loop and re-evaluates the loop condition. If the condition is true, enters the loop block.

Example 6.8.

  <html>
    <head>
      <title>Looping Constructs</title>
    </head>
    <body>
 1     <script type="text/javascript">
 2       while(true) {
 3         var grade=eval(prompt("What was your grade? ",""));
 4         if (grade < 0 || grade > 100) {
             alert("Illegal choice!");
 5           continue;   // Go back to the top of the loop
          }
          if(grade > 89 && grade < 101)
 6           {alert("Wow! You got an A!");}
 7        else if (grade > 79 && grade < 90)
            {alert("You got a B");}
          else if (grade > 69 && grade < 80)
            {alert("You got a C");}
          else if (grade > 59 && grade < 70)
            {alert("You got a D");}
 8        else {alert("Study harder. You Failed.");}
 9        answer=prompt("Do you want to enter another grade?","");
10        if(answer != "yes"){
11          break;    // Break out of the loop to line 12
          }
12      }
      </script>
    </body>
  </html>

Explanation

1

The JavaScript program starts here.

2

The while loop is entered. The loop expression will always evaluate to true, causing the body of the loop to be entered.

3

The user is prompted for a grade, which is assigned to the variable grade.

4

If the variable grade is less than 0 or more than 100, "Illegal choice" is printed.

5

The continue statement sends control back to line 2 and the loop is re-entered, prompting the user again for a grade.

6

If a valid grade was entered, and it is greater than 89 and less than 101, the grade "A" is displayed (see Figure 6.7).

7

Each else/if branch will be evaluated until one of them is true.

8

If none of the expressions are true, the else condition is reached and "You Failed" is displayed.

9

The user is prompted to see if he or she wants to enter another grade.

10, 11

If the answer is not yes, the break statement takes the user out of the loop, to line 12.

Figure 6.7

Figure 6.7 The user enters a grade, clicks OK, and gets another alert box.

6.3.6 Nested Loops and Labels

Nested Loops

A loop within a loop is a nested loop. A common use for nested loops is to display data in rows and columns. One loop handles the rows and the other handles the columns. The outside loop is initialized and tested, the inside loop then iterates completely through all of its cycles, and the outside loop starts again where it left off. The inside loop moves faster than the outside loop. Loops can be nested as deeply as you wish, but there are times when it is necessary to terminate the loop owing to some condition.

Example 6.9.

  <html>
    <head>
      <title>Nested loops</title>
    </head>
    <body>
      <script type="text/javascript">
        <!-- Hiding JavaScript from old browsers
1       var str = "@";
2       for ( var row = 0; row < 6; row++){
3         for ( var col=0; col < row; col++){
            document.write(str);
          }
4         document.write("<br />");
        }
        //-->
      </script>
    </body>
  </html>

Explanation

  1. The variable str is assigned a string "@".
  2. The outer for loop is entered. The variable row is initialized to 0. If the value of row is less than 6, the loop block (in curly braces) is entered (i.e., go to line 3).
  3. The inner for loop is entered. The variable col is initialized to 0. If the value of col is less than the value of row, the loop block is entered and an @ is displayed in the browser. Next, the value of col will be incremented by 1, tested, and if still less than the value of row, the loop block is entered, and another @ displayed. When this loop has completed, a row of @ symbols will be displayed, and the statements in the outer loop will start up again.
  4. When the inner loop has completed looping, this line is executed, producing a break in the rows (see Figure 6.8).
    Figure 6.8

    Figure 6.8 Nested loops: rows and columns. Output from Example 6.9.

Labels

Labels allow you to name control statements (while, do/while, for, for/in, and switch) so that you can refer to them by that name elsewhere in your program. They can be named the same as any other legal identifier that is not a reserved word. By themselves, labels do nothing. Labels are optional, but are often used to control the flow of a loop. A label looks like this, for example:

topOfLoop:

Normally, if you use loop-control statements such as break and continue, the control is directed to the innermost loop. There are times when it might be necessary to switch control to some outer loop. This is where labels most often come into play. By prefixing a loop with a label, you can control the flow of the program with break and continue statements as shown in Example 6.10. Labeling a loop is like giving the loop its own name.

Example 6.10.

  <script type="text/javascript">
1   outerLoop: for ( var row = 0; row < 10; row++){
2     for ( var col=0; col <= row; col++){
        document.write("row "+ row +"|column " + col, "<br /4");
3       if(col==3){
          document.write("Breaking out of outer loop at column
4              " + col +"<br />");
5         break outerLoop;
        }
      }
6     document.write("************<br />");
7   }    // end outer loop block
  </script>

Explanation

  1. The label outerLoop labels the for loop that follows it. It's like giving the for loop its own name so that it can be referenced by that name later.
  2. This is a nested for loop. As the program executes the row and column numbers are displayed.
  3. If the expression is true, the break statement, with the label, causes control to go to line 8; it breaks out of the outer: loop. A break statement without a label would cause the program to exit just the loop to which it belongs.
  4. The value of row and col are displayed as the inner loop iterates.
  5. The break statement with the label causes control to go to line 8.
  6. Each time the inner loop exits, this row of stars will be printed (see Figure 6.9).

    Figure 6.9

    Figure 6.9 Using a label with a loop.

    Notice that the row of stars is not printed when the loop is exited on line 5.

  7. The closing curly brace closes the outer for loop block on line 1.
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