# Matrix Multiplication bug and workaround

Sun Jul 30 15:36:29 EDT 2017

```Using LC 8.1.3, I just found a bug in the Matrix Multiply function.

Could you check if this bug was fixed in more

Follow this recipe to reproduce this bug:

1) Create a new stack with a button, a text field
and a check box named "showbug"

2) paste these two lines into the text field:

TM 1,0.6,-1.2,1,40,10
CO 0,30

You could find this and other examples in this webpage:

https://stackoverflow.com/documentation/svg/3249/transformation#t=20170730054300495488

3) paste this script into the button:

----------------------------------------------------------------

local tMatrix1
local tMatrix2

on mouseUp
put empty into tMatrix1
put empty into tMatrix2
put empty into tNewMatrix
put empty into tNewCoordinatePoint

put fld 1 into tMatrixList

repeat for each line tMX in tMatrixList

switch word 1 of tMX
case TM
TransformTMatrix(word 2 of tMX)
break
case CO
CoordinatePointMatrix(word 2 of tMX)
break
end switch

end repeat

put matrixMultiply(tMatrix1, tMatrix2) into tNewMatrix
combine tNewMatrix with comma
answer item 1 to 2 of tNewMatrix

end mouseUp

on TransformTMatrix pArgs
local tA
local tB
local tC
local tD
local tE
local tF

local tMatrix

if the hilite of btn "showbug" then
put item 1 of pArgs into tA
put item 2 of pArgs into tB
put item 3 of pArgs into tC
put item 4 of pArgs into tD
else -- use a workaround
put item 1 of pArgs into tA
put item 2 of pArgs into tC -- change position
put item 3 of pArgs into tB -- change position
put item 4 of pArgs into tD
end if

put tA into tMatrix[0,0]
put tB into tMatrix[0,1]
put tC into tMatrix[1,0]
put tD into tMatrix[1,1]

if (the number of items in pArgs) is 6 then
put item 5 of pArgs into tE
put item 6 of pArgs into tF
else
put 0 into tE
put 0 into tF
end if

put tE into tMatrix[0,2]
put tF into tMatrix[1,2]

put 0 into tMatrix[2,0]
put 0 into tMatrix[2,1]
put 1 into tMatrix[2,2]

if tMatrix1 is empty
then
put tMatrix into tMatrix1
return empty
end if

end TransformTMatrix

on CoordinatePointMatrix tArgs
put item 1 of tArgs into tG
put item 2 of tArgs into tH

put tG into tCoordinatePoint[0,0]
put tH into tCoordinatePoint[1,0]
put 1 into tCoordinatePoint[2,0]

if tMatrix1 is empty
then
put tCoordinatePoint into tMatrix1
return empty
end if

if tMatrix2 is empty
then
put tCoordinatePoint into tMatrix2
return empty
end if

end CoordinatePointMatrix

----------------------------------------------------------------

4) Hilite the check box "showbug" and click
the button... the result is 58,40
This result is wrong.

5) Unhilite the check box "showbug" and click
the button... the result is 4,40
This result is correct.

If this bug is still present in more recent LC versions.
Could you report it to the Quality Center?