CS1371 Introduction to Computing for Engineers

1
CS1371Introduction to Computing for Engineers

• Arrays(soon to be Matrices)

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Administrivia

• HW4 yes Web Work crashed. Extended to midnight
  tonight 6
• Lab 1 posted due Friday.
• Homework 5 Soon to be posted due next
  Thursday.
• START THIS ONE EARLY TOO!!!!
• Web page "Lectures" page has class schedule
  including all lecture topics and EXAM
  dates(first exam Sept 26th)
• OK to send me email..
• Reading
• Arrays Smith, Chapter 11

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Review Arrays

• A Collection is a really a data structure into
  which you put stuff, and from which you get
  stuff.
• Our basic (ie Matlabs) type of collection is
  called an Array.
• Arrays are homogeneous collection of things.
• All are the same
• Are indexable
• Example a text string is an array of characters
• Big part of Matlabs power is operation on entire
  Arrays, both on an per element basis and in a
  summation like basis.
• Each element of an array has a value (and a
  position).
• A vector is an one dimensional array

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Matrices Two Dimensional Arrays

• MatLab is all about matrices originally
  that's all it was about.
• A Matrix is a 2-D array. Behave like vectors but
  two dimensional. A deck of columns.
• As an array, they are indexable and homogeneous
• In some ways, just like vectors
• Create, refer, subset
• Scalar ops, logical ops
• Per-element operations (.)
• But arrays can be N-D

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Array and Creation

• Create 2-D arrays
• A 1 2 3 4 5 6
• A ones(4,6) four rows, 6 columns
• A false(2,3) logical matrix (can also use
  for vectors)
• ROWS and COLUMNS are the names of the dimensions.
  ROWS is always before COLUMNS (a 2 by 3 2
  rows, 3 cols)
• Access by a(2,1) just like vectors a(r,c)
• Show transpose operator A'
• Useful creation rand(4,3) do more than once
• Also zeros(r,c), and diag(A) (be careful here)

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N-D arrays

• Arrays can be N-D easiest to show N is 3
• image zeros(16,16,3) rows, cols, planes
• image(,,1) rand(16,16) floating point
  image
• imshow(image) make sure showing color
• image(,,2) rand(16,16) set 2D set of
  3Darray
• image(,,3) .4
• imshow(image)
• Extraction
• green (,,1) only a 2 D array.

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Ops on 2-D arrays and matrices

• Create an array B 5 4 9 10 11 12
• Sum(a) doesnt do what you might think
• Try max(a) gives max of each column why
  because matrix is really a stacking of vertical
  columns
• Suppose grades of 6 students, four grads
• gr rand(6,4) gives btw 0 and 1
• if want 60 to 100 gr round( 60 40.gr) (round
  works on arrays)

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Subsets on arrays

• Gr(24,24) - gives subset
• Gr(24) works but is not the right thing uses
  that linearization that's usually bad
• If want rows 2 4
• Gr(24,) gives all cols of the rows
• Colon gives you the whole dimension
• If just want the 4th column Gr(,4)
• Odd columns gr(,12end)
• Suppose CourseGr is section by student
• CourseGr round( 40 60.rand(25,4))
• Suppose passing gt60
• Count of passing sum(sum(CourseGrgt60))