Hi, I was testing the code here: =[1]========= | medMatrix | medMatrix := Matrix new. medMatrix numberOfColumns: 7; numberOfRows: 5. 1 to: medMatrix numberOfRows do: [:row | 1 to: medMatrix numberOfColumns do: [ :column | "medMatrix at: row at: column put: 0." Transcript show: row asString, ',', column asString, '|'. ]. Transcript show: cr. ]. medMatrix. =========== and I got a transcript like this: =[2]========== 1,1|1,2|1,3|1,4|1,5|1,6|1,7|nil2,1|2,2|2,3|2,4|2,5|2,6|2,7|nil3,1|3,2|3,3|3,4|3,5|3,6|3,7|nil4,1|4,2|4,3|4,4|4,5|4,6|4,7|nil5,1|5,2|5,3|5,4|5,5|5,6|5,7|nil'[Spotter] Exception while collecting processors for <Matrix>: Error: Instances of UndefinedObject are not indexable' '[Spotter] Exception while collecting processors for <Matrix>: Error: Instances of UndefinedObject are not indexable' '[Spotter] Exception while collecting processors for <Matrix>: Error: Instances of UndefinedObject are not indexable' '[Spotter] Exception while collecting processors for <Matrix>: Error: Instances of UndefinedObject are not indexable' ============= I tried also with SciSmalltalk matrix, but they seem to be only square ones. What's is the problem with matrices? Why I'm getting Undefined Objects while trying to iterate or populate them? Thanks, Offray