Understanding Array syntax

Question

I'm looking at an array problem that I'm having trouble understanding how the values are being passed in the for loop:

var a = [ [ 1, 2 ], [ 3, 4] ];
a[ 1 ][ 1 ] = 5;
for ( var row = 0; row < a.length; row++ )
{
for ( var col = 0; col < a[ 0 ].length; col++ )
document.write( a[ row ][ col ] + " " );
document.write( "<br />" );

Running the program I see: 1 2 3 5

Are [ [ 1, 2 ], [ 3, 4] ] two separate arrays or one belonging to var a? I can see the first part of the array [ [ 1, 2 ] is being passed and printed, what happens to the second part?

***Sorry total newbie I'm just looking for a better explanation of arrays. Thanks you!

The initial array is an array with two elements. Each element is itself an array. — Pointy
– Pointy, Commented Jul 9, 2014 at 20:00

durbnpoisn · Accepted Answer · 2014-07-09 20:03:30Z

1

Look at it like this:

var a = [ [ 1, 2 ], [ 3, 4] ];

//means
a[0] = [1,2]
a[1] = [3,4]

//so
a[0][0] = 1 //first element in the first element
a[1][0] = 3 //first element in the second element
a[1][1] = 4 //second element in the second element

And so on.

At your second statement you changed the value of the second element of the second element.

answered Jul 9, 2014 at 20:03

durbnpoisn

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user1785556 Over a year ago

Thank you @durbnpoisn. This is a great breakdown, very helpful in understanding how this is structured. Thanks for your time and input!

Justin Niessner · Accepted Answer · 2014-07-09 20:02:59Z

0

That would be a jagged array. Visually, you could think of it as:

[
    [1, 2],
    [3, 4]
]

Your second statement a[1][1] = 5 changes the second element of the second array (since arrays are zero-indexed).

The last blocks loop over the inner arrays, and then over the elements of those arrays so if you walk the array above like you are reading it, you get your output of:

1 2 3 5

answered Jul 9, 2014 at 20:02

Justin Niessner

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1 Comment

user1785556 Over a year ago

Thanks @Justin Niessner this is helping me understand that second statement. Thank you for breaking it down and showing me how its affecting the values. Thanks!

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