%------------------------------------------------------------------------------
% CS100J, Spring 2000
% Lecture 26
% Thurs 4/26
%------------------------------------------------------------------------------
echo on
clc
% Arrays
% + MATLAB's law: "everything an array"
% + sometimes say MATRIX
% - an array is a more general quantity
% - MATLAB classifies some operations as array and others as matrix
pause
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% Scalar
% + 1 row and col
% + "0-dimensional" array
1, [1], [[1]] % MATLAB will remove redundant []s
% So, all output is just 1
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% One-Dimension
% + one row or one column
% + MATLAB stores information as rows, usually
% - to separate individual elements, use spaces or commas
% - to separate individual rows, use semicolons
[1 2 3] % 1-D array (row)
[1;2;3] % 1-D array (col)
pause
%------------------------------------------------------------------------------
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% Two-Dimension
% + multiple rows and columns
% + also called rectangular
% + see vectors for separating rows and columns
[1 2; 3 4] % square matrix with rows [1 2] and [3 4]
[1,2; 3,4] % same as [1 2; 3 4]
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% + rectangularity must be preserved
% [1,2; 3] % fails because second row has only 1 element
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% appending arrays
A=[1 2; 3 4]
[A,A] % creates an array of 4 columns and two rows
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% Transpose
% + to store using columns, gets a bit more complicated
% + use transpose operator $'$
% + 1-D:
% ri' = rj (rows become columns and vice versa)
r=[1 2 3]
r'
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% + 2-D:
% Aij = Aji
A
A'
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r1 = [1 3]; r2 = [2 4];
[r1' r2']
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%------------------------------------------------------------------------------
clc
% Matrices
% + same structure and look as an array
% + usually means rectangular or square, but could have more dimensions
% + a matrix is an array that represents a linear transformation
%
% Ax = b
% A: coefficient matrix
% x: solution vector
% b: source vector
%
% example)
% 2x-2y = 10 \ [ 2 -2] {x} = {10}
% >-> [-2 5] {y} {20}
% -2x+5y = 20 / A x b
%
% A transforms x into b
% Find x from solution of simultaneous equations
% (save for later --> see MATLAB Handouts online)
% pause
%------------------------------------------------------------------------------
% Everything is an array
% + suppose want to solve something multiple times:
sin(0), sin(pi/2), sin(pi), sin(3*pi/2), sin(2*pi)
pause
% + slightly quicker way
sin([0 pi/2 pi 3*pi/2 2*pi])
pause
% is there an even quicker way?
% + colon (see $help colon$)
% create vector: start:stop (increments by 1)
% create vector: start:inc:stop (increments by inc, but won't exceed stop)
% examples)
A = 0:pi/2:2*pi
sin(A)
% briefer:
sin(0:pi/2:2*pi)
% see also $help linspace$
% $linspace(start,stop,numberofvalues)$
sin( linspace(0,2*pi,5) )
pause
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% + Miscellaneous
% help zeros
% help ones
% help eye
% help length
% help size
% help rand
% help diag
%------------------------------------------------------------------------------
% Indexing
% + finding the location (or, position) of an element in an array
% + use index or subscript
%
% A
% ijk....
% 1st index (i) is 1st dimension (row)
% 2nd index (j) is 2nd dimension (col),
% 3rd index (k) is 3rd dimension (page)
% and so on....
%
% + useful help pages:
% help paren
clc
pause
% + examples)
A=['a' 'b' 'c'; 'd' 'e' 'f'] % create array of strings
pause
% + single index extraction for 2D array (might seem peculiar)
% see $help colon$
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A(:) % get all elements of A and put in column
% start at "first" element and work down columns, then rows
% yes, I understand this might seem strange
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A(1) % get first element from all the elements (as if in column)
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A(4) % get 4th element from all the elements (as if in column)
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A(1:3) % get first thru third elements
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% + "regular extraction"
A(1,2) % get element at row 1, col 2
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% + using a colon (this might hurt)
% for now, assume 2D array
% nameofarray(vectorofrows,vectorofcols)
% vectors for rows and cols must not violate dimensions of A
% (unless you are inserting something -- discussed in "Insertion" later)
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A([1 2],[1 2])
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A([1:2],[1:2])
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A(1:2,1:2)
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% A(1:2 1:2) % won't work!
%
% + more sophisticated use of colon
% say "for these columns and these rows"
%
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A(:,:) % extract all rows and all cols
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A(:,1) % for all rows of A, extract the elem from the 1st column
% shorter: get 1st column from A
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A(1,:) % for all cols of A, extract the first (or "top") elem
% for the 1st row of A, extract all cols
% even shorter: get 1st row of A
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% example)
B = rand(5,4)
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B([1 2 3],:) % for all cols, get 1st thru 3rd row elements
% extract first three rows
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B([3 2 1],:) % extract 3rd, 2nd, 1st elements from each col
% extract first three rows in reverse order
pause
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B(:,[4 1]) % for all rows, extract 4th and 1st elements
% extract 4th and 1st columns
pause
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B([2 5],[4 2]) % extract elements with locations (2,4) (2,2) (5,4) (5,2)
% extract subarray
pause
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B([1 2 4],[3 1]) % from rows 1 2 4, extract cols 3 1
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%------------------------------------------------------------------------------
% Insertion
clear
A=zeros(3,4)
pause
A(1) = 1
pause
A(2,3) = 23
pause
A(1,[1 2 3 4]) = [100 200 300 400]
pause
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% + when colon on left side of assignment, MATLAB takes elements from the
% right side and inserts in the left-side matrix
A(2,:) = [0.1 0.2 0.3 0.4]
% + can insert beyond dimensions!
% MATLAB fills in needed "spots" with zeros
clear
clc
C=[1 2; 3 4]
C(6,6) = 10
%------------------------------------------------------------------------------
% exercise
Z=[1:3 ; 6:8]
% swap rows
Z([2 1],:) = Z([1 2],:)
pause
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