CS100J Lecture 22

1
CS100J Lecture 22

• Previous Lecture
• MatLab
• Learn MatLab
• Implement (a piece of MatLab) in Java
• Get an appreciation for what MatLab does for you
• Learn to implement a class of some complexity
• Handout The CS100 MatLab Syllabus
• Java Concepts
• Overloading
• this as a constructor
• The Next Several Lectures
• MatLab and its implementation, continued.

2
Sample Client Code

• // ones(2,3) ones(2,3)
• ML.add( ML.ones(2,3), ML.ones(2,3) )
• // 5 ones(2,3)
• ML.add( new ML(new ml(5)), ML.ones(2,3) )
• The second example suggest that it might be
  useful to add an additional overloaded
  constructor for ML
• // Construct scalar matrix v.
• public ML(int v)
• this(1,1)
• values00 new ml(v)
• so the example could be
• // 5 ones(2,3)
• ML.add( new ML(5), ML.ones(2,3) )

3
Critique of ML Method add

• Most of the code (all but the 3 invocations of
  ml.add) has nothing to do with the fact that this
  is the method for addition.
• To implement subtraction, almost all code (except
  for the 3 invocations of ml.add) would be
  identical.
• We could implement each of the other
  element-by-element matrix operations in about 30
  seconds each
• Copy the definition of ML.add
• Replace add with sub, or mult, or div,
  etc.
• Write ml.add, ml.mult, ml.div, etc.
• Terrible
• Grotesque duplication of code.
• Loss of single point of change.
• No abstraction.
• Surely, there must be a way to avoid duplicating
  the common code.
• Coming later.

4
MatLab

• Matrices with the same number of rows can be
  concatenated horizontally (comma optional).
• gtgt 1 2 3
• ans
• 1 2 3
• gtgt ones(2,3) , (2 . ones(2,2))
• ans
• 1 1 1 2 2
• 1 1 1 2 2
• Matrices with the same number of columns can be
  concatenated vertically.
• gtgt 1 2 3
• ans
• 1
• 2
• 3
• gtgt ones(2,3) (2 . ones(1,3))
• ans

5
MatLab

• A constant matrix can be written with a
  combination of horizontal and vertical matrix
  concatenations.
• gtgt 1, 2, 3 4, 5, 6 7, 8, 9
• ans
• 1 2 3
• 4 5 6
• 7 8 9

6
Implementation Strategy

• ML.row(x,y) implements binary row concatenation,
  i.e., x, y
• ML.col(x,y) implements binary column
  concatenation, i.e., x y
• Defer concatenation of more than 2 rows or
  columns at a time. Until then
• / a, b, . . ., y, z /
• ML.row(a, ML.row(b, ... ML.row(y, z)) ... )
• / a b . . . y z /
• ML.col(a, ML.col(b, ... ML.col(y, z)) ... )
• (Actually ML.row and ML.col are associative, so
  the grouping is arbitrary.)

7
class ML Method row

• // x, y
• static ML row(ML x, ML y)
• if (x.h ! y.h)
• throw new RuntimeException
• ("operands must have same height")
• else
• ML result new ML(x.h, x.wy.w)
• for (int r 0 r lt x.h r)
• for (int c 0 c lt x.w c)
• result.valuesrc x.valuesrc
• for (int c 0 c lt y.w c)
• result.valuesrx.wc
• y.valuesrc
• return result

8
class ML Method col

• // x y
• static ML col(ML x, ML y)
• if (x.w ! y.w)
• throw new RuntimeException
• ("operands must have same width")
• else
• ML result new ML(x.hy.h, x.w)
• for (int c 0 c lt x.w c)
• for (int r 0 r lt x.h r)
• result.valuesrc x.valuesrc
• for (int r 0 r lt y.h r)
• result.valuesx.hrc
• y.valuesrc
• return result