HTML Goodies: Script Tip: Week 58

Script Tip…

Today we look at the functions of the script that “check” the radio buttons for you.

A parser is a server looking to find an item. That’s what happens here. The functions know which buttons should be checked, so they run down the line of buttons, parsing and “checking” the correct ones.

Here’s the script again:

     Click a chord on the right and watch the dots provide the pattern.

There’s no good way to attack this parsing area of the script because there are multiple functions involved and they work off of one another. I’ll delve into it by attacking the function that is first triggered when the user chooses a chord. It’s a function called showChord().

function showChord()
{

var Item, Ret, Count, Skip;

for (Count=0; Count < 42; Count++)
{

document.guitar[Count].checked=false

}

Item = document.guitar.chord.selectedIndex;

if (Item != -1)
{

Text = document.guitar.chord.options[Item].text;

Frets = parser (chords[Text])

for (Count = 1; Count <= Frets[0]; Count++)
{



document.guitar[parseInt(Frets[Count])].checked=true;

}

}

}

The user chooses a chord from the right-hand side of the display. By doing this, the onChange Event Handler is engaged. That triggers this function. Read slowly, this gets hairy.

First off, the variables “Item,” “Ret,” “Count,” and “Skip” are brought into play.

Next, a For loop is set up to roll through 41 times. You may think that’s a mistake because there are 42 buttons, but no! Note that the loop will roll until the number is less than 42 (that means 41). Plus, the count starts at 0 rather than 1. It has to start at 0 because now we’re going to look at the radio buttons and there is a 0 button. (0 to 41 means all 42 buttons are parsed, get it?)

Now, we get to the first line that deals with checking the buttons:

document.guitar[Count].checked=false

This line of code will direct the attention of the function to each of the radio buttons. Remember that the name of the form itself is “guitar.” Inside the square brackets is the variable “count” which will start at 0 and count up by one until it hits 41. We know that because “count” was set to 0 in the For loop above.

So, now we have a loop that will check the buttons one by one, setting their checked status to false. But why? They’re already false, right? Well, what if you choose a chord and then choose another one? You would want the first to be erased right? Ta da! This sets them all to false. They’re erased.

Item = document.guitar.chord.selectedIndex;

The next line is connecting to the drop-down menu and grabbing the selected index number. That’s the chord the user chose from the list. That number is given the variable name “Item.”

if (Item != -1)
{

Text = document.guitar.chord.options[Item].text;

Frets = parser (chords[Text])

Now, we get into some exchanging of information between this function and the next one called parser().

The script sets up an If statement to get the ball rolling. This is a case of setting up an instance that will never be true just so it can trigger something to happen. It asks if “Item” (the choice the user made from the drop-down menu) is not equal to minus one (-1). Since for (Count = 1; Count <= Frets[0]; Count++)
{