lecture thursday, 4/20

+ turn in exercise 7 in the provided piles [a-c, d-i, j-l, m-r, s-z]

for tuesday:

+ lecture is on inheritance: please review ahead of time
  pre-inheritance box/scope concepts

+ exercise 8

% assign to $table1$ the row vector of column sums of table2
% assign to $table5$ the column vector of row sums of table3
% assign to $dist$ the distance between table3 and table4
% modify table2: swap columns 2 and 5 of table2

some matlab basics

% online help -- try $help$ on : $help$, $lookfor$, $who$, $type$

    help help

% *scripts* (.m files) -- can type code into a file and run it
% (we will see functions later)

    help echo
    help run
    help script

% character, string: array of characters (characater = 1-by-1 array)

    'start the attack'
    'start the attack' + 1
    char('start the attack' + 1) % <-- messes up spaces

% scalars -- same as in java
% use $,$ to combine multiple statements with printed answers
% use $;$ to combine multiple statements with suppressed output

    2, -3.14, 4e2
    5; 6.28; 7e3;


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creating new arrays in matlab

% arrays: put entries inside brackets $[$, $]$
% horizontal concatenation: juxtaposition (side by side with spaces or commas)
% vertical concatenation: go to next line or use $;$  (grr!  dual use of $;$)

    [ 2 3 4 ]
    [ 2, 5, 8 10] % <-- not a typo
    [ 1
      8
     -7 ]
    [ 2 ; 8 ; 70 ]

    [ 1 2 3
      4 5 6
      7 8 9 ]

    [ 10 11 12 ; 31 32 33 ; 48 4 0 ]

    [[ 1 ; 2 ; 3 ] [ 4 ; 5 ; 6 ] [ 7 ; 8 ; 9] ]

% note on concatenation: dimensions must match up

    [ 1 ; 2 3 ]

    [ 1 [ 2 ; 3 ] ]

% evenly spaced intervals lo:hi, lo:step:hi, hi:-step:lo
% note: will *not* go past second bound

    1:10.3 % <-- does not go past 10.3

    1:2:10 % <-- does not go past 10

    10:1 % <-- empty!

    1:-1:10 % <-- empty!

    10:-1:1

    10:-2:1 % <-- does not go past 1

% rand, ones, zeros: with no size given, return 1 number
% with one size give, return square matrix
% with two sizes given, return rectangular matrix

    ones
    rand(5)
    zeros(2,7)


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matlab array operations

% scalar op scalar, f(scalar): as you expect

    2 + 7, 2 - 7, 2 * 7, 2 / 7, 2 ^ 7, abs(-7), sqrt(49)

% equal-sized arrays: $+$ and $-$ work element-wise

    % you may ignore code below for now, but look at output
    a = [1 2 3 4 5], b = [8 5 4 6 9], a+b
    fprintf('%4d+%d', [a ; b]), fprintf('\n')

    % now look at code again and make sure you understand output
    a = [ 1 2 3 ; 4 5 6 ], b = [ 3 1 4 ; 2 1 7], a - b

% $'$ is transposition

    a, a'

% scalar with array: $+$, $-$ work element-wise

    3 - (1:10), (1:10) - 3

% use .^, .*, ./ for element-wise operations with one or two arrays
% this is because $^$, $*$, $/$ do matrix (linear-algebra) operations
% you are not expected to know linear algebra for cs100,
% but you are welcome to use those operations if you know them.

    (1:5) .^ 2
    2 .^ (1:5)

% note: bad style to use .^, .*, or ./ with two scalars

% $sum$, $max$, $prod$ on vectors: as we saw

    a = [4 1 5 2 8 3], sum(a), max(a), prod(a)

% $sum$, $max$, $prod$ on 2-d arrays: operate on each *column*

    a = [ 4 1 5 ; 2 8 3], sum(a), max(a), prod(a)

% $cumsum$, $cumprod$: cumulative sums or prodcuts

    cumsum([1 1 1 1])
    cumsum([1 3 5])

% note: can find max value and also *where*:

    a = [ 4 1 5 2 8 3]; [ 1:length(a) ; a ]
    [maxv, where] = max(a) % <-- two results are returned!
    a(where)

% <, >, <=, >=, ==, ~=: element-wise, with true=1, false=0

    3 < (1:5)

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matlab array access and modification

% since matlab uses $[$, $]$ to create arrays
% it uses $($, $)$ to index into arrays

% note: matlab indices start at 1, not 0 (grr!)

% note: matlab uses row-major indexing

% ignore for now -- grr! the elements are stored column-major order!!

    a = [ 3 1 4 ; 1 5 9 ; 2 6 5 ; 3 5 0 ]

a(1,1), a(1,2), a(1,3)
a(2,1), a(2,2), a(2,3)

% remember, arrays are values -- no references, no aliases!

    b = a
    b(1,1) = -b(1,1)

    a % <-- unchanged! 

% indexing above is special case of more general indexing:
% can access elements in multiples rows and columns --

    a([1 2 3], [1 3]) % rows 1 2 3 (omit 4), columns 1 (omit 2) 3

    a([1 2 3], [1 3]) = [ 7 7 ; 7 7 ; 7 7 ]

% shorthand: $:$ by itself means "all of them"

    a(1, :) % row 1, all of the columns
    a(:, 2) % all of the rows, column 2

    a(4:-1:1, [1 3]) % rows 4 3 2 1 (reverse order!), columns 1 and 3    

    a(ones(1,5), :) % row 1 five times, all of the columns