The FOR Loop - 06ForLoop.fla

The For Loop is used when you need to repeat an operation several times.

Which kind of loop ???

• If you know in advance how many repetitions you need - use FOR
• If you need to repeat zero or more times - use WHILE
• If you need to repeat one or more times - use DO

This program creates a movie clip and draws a graph with grid lines.

• Input - Button press to trigger the processing
• Process - Create a movie clip
• Output -
      X axis grid lines (not yet implemented)
      Y axis grid lines
      Graph plot

1. Start up Flash.
2. Create a new Flash document.
3. Save the file as 06ForLoop.fla.
4. Place a button onto your canvas.

Set up your Flash MX environment like this ...

Enter this code

on (release)
{
    _root.createEmptyMovieClip("myGraph", 1);
  
    with (_root.myGraph) {
    lineStyle(1, 0x000000, 100);    // 1 thickness, 0x000000 colour, 100% alpha 
	
    // HORIZONTAL GRID LINES
    //   START--   STOP----  STEP--------
    for (yy = 50;  yy < 400; yy = yy + 20)
    {
        moveTo(  0, yy);                // Start position
        lineTo(500, yy);                // Straight line to this position
    }
	
    moveTo(0, 50 + (-250 / 13) * (-250 / 13)); // Start position
	
    // PLOT THE GRAPH
    //   START----   STOP----   STEP-------
    for (xx = -250;  xx < 250;  xx = xx + 1)
    {
        yy = (xx / 13) * (xx / 13);     // yy = xx squared
        lineTo(250 + xx, 50 + yy);
    }	
}

Here is the example movie

You can download the source code.

Tasks

1. Draw the X axis grid lines.
2. Turn the graph the right way up.
3. Explain the co-ordinate system and why the graph was upside down in the first place.
4. Plot a graph of y = 4 * x + 2
5. Plot a graph of y = x * x * x

Notes

Example

//  START->  STOP-->   STEP------>
for(ii = 10; ii < 20;  ii = ii + 1)
{
  doThis();
}

The For Loop has three parts. These are start, stop and step. The example above counts from 10 to 19. On the first time round the loop, ii = 10. Next time round ii = 11. Each time round the loop, ii increases its value by one.