Properties vs Instance Variables

[Guy Steven]

I am currently learning objective c and cocoa

I believe I understand the difference between properties and instance
variables, and the effect of using properties as opposed to instance
variables viz a viz KVO and memory management.

What I can't understand is why you would access instance variables directly
within a class. My quess is speed. Is there any other reason?

And related to this, why give instance variables a name different to the
name of the property.

Again, I suspect it may be a matter of style, but would like to know if
there are cases where it is a necessity.
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computing values within NSTableView using bindings

[Guy Steven]

I have an NSTableView with 3 columns.


The first contains an NSPop-up which takes as its datasource an array of
objects with two properties, name and cost. The name is displayed.

The user enters a quantity in the 2nd column.

How do I go about displaying the cost in the third column? I can bind that
column to the quantity key, but don't then know the cost, or I can bind the
column to get the cost, but don't then know the quantity?
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