CS105 Lab Discussion 12 Loops

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CS105 Lab Discussion 12Loops

• Announcements
• MP5 is posted. Its due Sunday, November 12th at
  1130pm.
• Midterm 2 feedback sheets will be handed out this
  week in lab.

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Objectives

• An introduction to loops.
• Review module-level variables
• Download Lab 12 worksheet at http//www.cs.uiuc.e
  du/class/cs105

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Plan

• We want to implement a very simple painting
  program that allows brushes of various sizes.
• Buttons provided
• Change Brush Size Used to change brush size
• Paint Used to paint on the given canvas
• Clear Canvas Clears the canvas

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Coloring a Cell

• cmdPaint currently just gets the ActiveCell row
  and column number. Now lets color in the Active
  Cell.
• We can change the color of a cell by using the
  following notation Cells.Interior.Color
  color
• This painting program has only one color, Blue.
  To change the color of the cell
• Cells(intRow,intCol).Interior.Color vbBlue

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Coloring multiple cells

• Let's say that the default brush size is 5. Then
  the paint button should color 5 cells in one row
  starting from the Active Cell.
• Cells(intRow,intCol).Interior.Color vbBlue
• Cells(intRow,intCol 1).Interior.Color vbBlue
• Cells(intRow,intCol 2).Interior.Color vbBlue
• Cells(intRow,intCol 3).Interior.Color vbBlue
• Cells(intRow,intCol 4).Interior.Color vbBlue
• This code is redundant, is there a better way to
  do it?

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For-Loop

• We can do the same thing using a For-Loop
• Syntactically, a For-Loop looks like this
• For intIndex ltstartValuegt To ltendValuegt
• ltloop bodygt
• Next intIndex
• intIndex is the counter that keeps track of how
  many times we have executed the loop.
• Note intIndex is an integer variable that you
  must declare before the beginning of the loop

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Flowchart of a For-Next Loop
Initialize counter to start value


Start
Is counter gt end value?
No
Yes
End
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Code for cmdPaint

• We rewrite the code using a For-Loop.
• For intOffset 0 To 4
• Cells(intRow, intCol intOffset).Interior.Color
  _ vbBlue
• Next intOffset
• What does the "Next" statement do?
• How many cells will be colored blue?

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Steps in a For-Next Loop

• If we do not mention the step-size then the
  counter in a For-Next Loop is always incremented
  by 1.
• What if we want to increase the counter by 2 each
  time?
• For intOffset 0 To 4 Step 2
• Try this code!

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Change Brush Size

• Currently the brush size is set at 5. We want the
  user to be able to set the brush size.
• If we do this in cmdPaint, then the user will be
  forced to set the size every time he/she wants to
  paint
• We will do this in the Brush Size button instead
• Since the brush size will be used in both
  buttons, we need to define it as a module-level
  variable

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Module level variables

• Scoping rules Usually, variables are available
  only within the procedure (or function) they are
  defined in. (local variable)
• If we want a variable to persist, and be used by
  more than one procedure, then, in VBA, we define
  it outside any one procedure.
• Naming convention we prefix an m to the
  variable name when its a module level variable.
• Caution Use module level variables sparingly
  only when asked to on the MPs.

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Declaring module-level variables

• Since module-level variables are available to all
  subprocedures, we will declare these variables at
  the top of all the code, right after Option
  Explicit.
• Dim mintBrushSize As Integer

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Code for cmdBrush

• Look at the code for cmdBrush.
• Currently it gets the brush size from the user
  and checks that it is a number. Now we need to
  store it in mintBrushSize
• mintBrushSize vntBrushSize

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Brush Size

• Now we need to change cmdPaint so it doesn't
  always color the same number of cells
• Change the for-loop to
• For intIndex 0 to mintBrushSize - 1
• Why is the end value mintBrushSize 1 ?

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Module-level variables

• This works because the variable mintBrushSize is
  a module-level variable. If we had declared the
  variable inside cmdBrush_Click(), it would not
  work. Why?

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Some More Things to Try

• Make the brush draw vertically instead of
  horizontally.
• Make the brush draw diagonally down and right.
• Make the brush draw from right to left.
• Make the brush stroke two cells wide.