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For Loops

In this lesson we look at the for loop statement which is generally used for repeating some statements a set numer of times. The for statement is more compact than the while statement and we can apply variables to the loop within the for statement itself.

The for Statement


The for statement will loop through a section of code a set number of times. The for statement contains three parts. In the first part we initialize a counter variable with a value, this only happens on initial entry to the for loop. The second part is a condition which tests the variable value at the start of each loop and if the condition is no longer true the for loop is exited. The final part of the for statement is an expression to be evaluated at the end of each iteration of the loop. This normally takes the form of a counter that is decremented or incremented.

Following are examples of the for loop statement which increment and decrement the expression variable.



// execute the loop until loop condition is false.
for (var i=0; i<3; i++) {
  alert('The i variable is = ' + i);
}
alert('left first for loop');

// execute the loop until loop condition is false.
for (var i=10; i>9; i--) {
  alert('The i variable is = ' + i);
}
alert('left second for loop');

Press the button below to action the above code:



The break Statement


The break statement allows us to terminate the current loop or label statement. Execution is passed to the statement following the current loop or label statement.



// execute the loop until break.
for (var i=0; i<4; i++) {
  if (i == 2) {
    break; 
  }
  alert('The i variable is = ' + i);
}
alert('left for loop');

Press the button below to action the above code:



The continue Statement


The continue statement allows us to continue from the expression part of the loop or from a predefined label statement of a labelled loop.



// execute the loop until loop condition is false.
for (var i=1; i<4; i++) {
  if (i == 2) {
    continue; 
  }
  alert('The i variable is = ' + i);
}
alert('left for loop');

Press the button below to action the above code:



The label Statement


The label statement allows to insert a statement identifier we can refer to from a break or continue statement.



// execute the loop until loop condition is false.
for (var i=1; i<3; i++) {
  atLabel:
  for (var k=1; k<3; k++) {
    if (k == 2) {
      continue atLabel; 
    }
    alert('The i variable is = ' + i + ' and k variable is = ' + k);
  }
}
alert('left for loop');


Press the button below to action the above code:



Reviewing The Code

We used the for statement with an increment and decrement counter and with the break, continue and label statements.

Lesson 5 Complete

In this lesson we looked at the for loop statement.

Related Tutorials

JavaScript Intermediate Tutorials - Lesson 4 - While and Do....While Loops

What's Next?

In the next lesson we take a look some more maths functions.

Reference

JavaScript Reference - for statement
JavaScript Reference - break statement
JavaScript Reference - continue statement
JavaScript Reference - label statement





  

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