2 newbie questions regarding JavaScript Arrays?

[unibumps]

First Question -- I am running through a 2 dimensional array, and in the array I am adding an image to img sources in the body based on whether the array reads as true or false.

My question is concerning an example I found that runs through the row and column to check whether it is true or false, and it reads as:

document.getElementById("imagename" + (i * imagetruefalse.length + j)).src = "true.png";

WHY is it using the * to run through the array? I'm trying to figure out why it's using that operator to run through and [j] but I just can't figure it out.


Second Question: I've noticed in some examples of arrays people use +arrayname+ to speak back to it -- why put the array name through the + signs? I can't find an explanation on this. For example:

the -- document.write("<b>"+dayarray[day]+", "+montharray[month]+" "+daym+", "+year+"</b>") -- part of the following example.

<script type="text/javascript">
var mydate=new Date()
var year=mydate.getFullYear()
var day=mydate.getDay()
var month=mydate.getMonth()
var daym=mydate.getDate()
//if the current date is less than 10, pad it.
if (daym<10)
daym="0"+daym
var dayarray=new Array("Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday")
var montharray=new Array("January","February","March","April","May","June","July","August","September","October","November","December")
//write out the final results
document.write("<b>"+dayarray[day]+", "+montharray[month]+" "+daym+", "+year+"</b>")
</script>

All insight is greatly appreciated!

 

[Silent]

First question: The code snip never really addresses the array as imagetruefalse[j], so I think it's a 1-dimensional array that stores 2D data. That explains the multiplication, like this:

[1, 1, 1]
[2, 2, 2] = [1, 1, 1, 2, 2, 2, 3, 3, 3]
[3, 3, 3]

To find the index of (x,y) in the 1-dimensional array, just do (length is 3 in this example):
index = y*length + x

Second question: The + is a string concatenation operation. It adds two or more strings (or, as in your example, many small strings and many variables) together as one long string.

var s = "foo";
var t = "bar";
var text = s + t + " " + t + s; // becomes "foobar barfoo"