Array of Array issue

This works unless I add the Hi Mr. C element.

by Howard Brown

HTML

Array of Arrays problem demo.

JavaScript

var arr = [[],[]];

alert( arr.length );  // wrong!

var c = 0;
for( var i in arr )
  for( var j in arr[ i ] )
    if( arr[ i ][ j ] != undefined )
      ++c;
alert( c );  // correct

arr[0][2] = 'Hi Mr.A';
alert(arr[0][2]);

arr[1][3] = 'Hi Mr.B';
alert(arr[1][3]);

arr[2][4] = 'Hi Mr.C';  // At this point I'm getting VM558:62 Uncaught TypeError: Cannot set property '4' of undefined
alert(arr[2][4]);

var c = 0;
for( var i in arr )
  for( var j in arr[ i ] )
    if( arr[ i ][ j ] != undefined )
      ++c;
alert( c );