[Newbies] understanding selectors

[Jonathan Wright]

On Sat, 30 Jul 2011 08:52:53 -0500
Jonathan Wright <j.c at jondw.com> wrote:

> 
> Hello,
> 
> I'm trying to extend the Matrix class by making it able to add and
> subtract matrices.  So far I've implemented the following code to add:
> 
> + aMatrix
> 	| newMatrix rowCount columnCount sum |
> 	rowCount := self rowCount.
> 	columnCount := self columnCount.
> 	newMatrix := Matrix rows: rowCount columns: columnCount.
> 	
> 	1 to: rowCount do: [ :selectRow | 
> 		1 to: columnCount do: [ :selectColumn |
> 		sum := (self at: selectRow at: selectColumn) + 
> 					(aMatrix at: selectRow at:
>   selectColumn). newMatrix at: selectRow at: selectColumn put: sum.]].
> 	^newMatrix.
> 
> Now I want to implement a method for subtracting matrices.  However,
> I'd like to use the same code.  I tried to implement an operand
> selector, however, it errors out.
> 
> Something like this:
> 
> operand: operand matrix: aMatrix
> 	| newMatrix rowCount columnCount sum |
> 	rowCount := self rowCount.
> 	columnCount := self columnCount.
> 	newMatrix := Matrix rows: rowCount columns: columnCount.
> 	
> 	1 to: rowCount do: [ :selectRow | 
> 		1 to: columnCount do: [ :selectColumn |
> 		sum := (self at: selectRow at: selectColumn) operand 
> 					(aMatrix at: selectRow at:
>   selectColumn). newMatrix at: selectRow at: selectColumn put: sum.]].
> 	^newMatrix.
> 
> I know this is not SmallTalk convention, but how should I pursue
> something like this?
> 
> Thank You,
> Jonathan
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> 

I answered my own question.  How I love being able to look at all the
source code in this handy Browser in Squeak!

I implemented the following:

perform: anOpperator with: aMatrix
	| newMatrix rowCount columnCount result |
	rowCount := self rowCount.
	columnCount := self columnCount.
	newMatrix := Matrix rows: rowCount columns: columnCount.
	
	1 to: rowCount do: [ :selectRow | 
		1 to: columnCount do: [ :selectColumn |
		result := (self at: selectRow at: selectColumn)
perform: anOpperator with: (aMatrix at: selectRow at: selectColumn).
		newMatrix at: selectRow at: selectColumn put: result.]].
	^newMatrix.


where the subtraction/addition looks like this:
subtractMatrix := matrixOne perform: #- with: matrixTwo.