OCTAVE/MATLAB refresher while preparing for Interviews

matlab_refresher.m

format compact

%
% OOP
%

% INFO
% Must be created in own file with class name equal to filename -> shape
a1 = Shape(10,20);
disp(a1)
Shape.setGetNumShapes()
a1.getArea

a2 = Shape(5,10)
a1 > a2
a2 > a1

a3 = Trapezoid(10,4,6);
disp(a3)
a3.getArea


%
% FUNCTIONS
%

% INFO
% Matlab: Have to come at the end of a file (wtf...)
% Octave: The other way around...

function vol = cylinderVol(radius, height)
  vol = pi * radius^2 * height
end

% Octave, as I'm using it
cylinderVol(20,30)


% locals / scope bound vars respect their scope
function changeVal()
  changeMe = 10
  class(changeMe)
end

changeMe = 5
changeVal()
disp(changeMe)


% argumentless
function randNum = getRandomNum
  randNum = randi([1,100])
end

getRandomNum


% multiple return vals
function [coneVol, cylinVol] = getVols(radius, height)
  cylinVol = cylinderVol(radius, height)
  coneVol = 1/3 * cylinVol
end

[coneV, cylVol] = getVols(10, 20)


% mutliple arguments
function sum = getSum(varargin)
  sum = 0;
  for k = 1:length(varargin)
    sum += varargin{k}(1); % matlab maybe can't += at least to Derek Banas Video Tutorial I'm watching
  end
end

theSum = getSum(1,2,3,4)


% variable number of outputs
function [varargout] = getNumbers(howMany)
  for k = 1:howMany
    varargout{1}(k) = k;
  end
end

listOfNums = getNumbers(10)


% anonymous functions (oneline functions)
cubeVol = @ (l, w, h) l * w *h;
cV = cubeVol(2,2,2)


% pass a function to a function
function result = doMath(func, op)
  result = func(op)
end

mult3 = @ (x) x*3;
sol = doMath(mult3, 4)


% return a function from a functions
function func = doMath2(op)
  func = @(x) x * op
end

mult4 = doMath2(4);
sol2 = mult4(5)


% recursive functions

% eval executes code which is saved as string
toExecute = sprintf("total = %d + %d", 5, 4)
eval(toExecute) % o_O

function val = factorial(num)
  % TODO edge cases...
  % base case
  if num == 1
    val =1;
  else
    val = num * factorial(num - 1);
  endif
end

factorial(4)

%
% TABLES - not yet implemented in Octave 7.1
%

 name = {'Jim'; 'Pam'; 'Dwight'}
 age = [28; 27; 31];
 salary = [35000, 26000, 75000];
 id = {'1', '2', '3'};

 employees = table(name, age, salary, ...
             'RowName', id)

%
% STRUCTURES
%

 dougSmith = struct('name', 'Dough Smith', ...
                    'age', 34, 'purchases', [12 23])

 disp(dougSmith.age)
 dougSmith.wife = 'Patty Smith' % can be extended dynamically

 dougSmith = rmfield(dougSmith, 'wife') % remove field
 isfield(dougSmith, 'wife')
 fieldnames(dougSmith)

 customers(1) = dougSmith;

 sallySmith = struct('name', 'Sally Smith', ...
                    'age', 34, 'purchases', [12 23])

 customers(2) = sallySmith;

 disp(customers(2).name) %


%
% STRINGS
%

 % basically works like an array
 strArray = strsplit("I am a string")
 disp('Class: ')
 class(strArray)
 strArray(1)

%
% Cellery - or cell arrays
%

 how to store values of different types?
 like a class, record or struct does?

 cA1 = {'Dough Smith', 34, [25 8 1]}
 cA2 = cell(5)
 cA3 = {'Doug' 'Patty'} % character matrix
 nameMat = char(cA3) % cell arrays into char matrices
 cA4 = cellstr(nameMat) % char matrix into cell arrays

 cA1{1} %get index
 cA1{3}(2) % get element at index (works because there is a row vector)
 cA1{4} = 'Patty Smith'
 disp('Length:')
 length(cA1)
 cA1(4) = [] % delete?

 for i = 1:length(cA1)
   disp(cA1{i})
 end



%
% MATRICES ENHANCED
%

% ; = don't display
m3 = [2 3 4; 4 6 8; 8 12 16; 16 24 32];

m4 = [1:3; 4:6];
m5 = [2:4; 5:7];

m6 = [1 2 3;
      4 5 6]

m7 = [1 1 1 1;
      2 2 2 2;
      3 3 3 3]

% dot productish multiplication
m8 = m6 * m7


gT3M = m4 > 3
%sum(gT3M, 'all') %
% logical array / boolean array should be output_precision

isequal(m4, m4)
isequal(m4, m5)
find( m3 > 24 )
prod(m3) % multiply column values
cumsum(m3) % cummulative sum - ascending is default
cumprod(m3)
fliplr(m3) %columns
flipud(m3) %rows
rot90(m3)
rot90(m3, 2) % 180 degrees :-D
reshape(m3, 2, 6) % 2 by 6

m4 + m5 % add element wise
m4 .* m5 % multiply element wise

sqrt(m3)
m3 = m3 * 2

sqrt(m3)

sum(m3)


 LOOPS


 = 1
hile i < 20
 if(mod(i,2) == 0)
   disp(i)
   i = i + 1;
   continue % go back to while loop
 end
 %disp("else: ")
 %disp(i)
 i = i + 1;
 if i >= 10
   break % jump out of while loop
 end
nd


 FOR LOOPS
4 = [ 2 3 4; 4 6 8] % 2 row matrix

 how to loop over it?

or row = 1:2
 for col = 1:3
   disp(m4(row, col))
 end
nd

 and a vector?
Vect = [ 6 7 8 ]

or i:length(IVect)
 disp(IVect(i))
nd


for i = [ 2 3 4] % loop over 'vector'
  disp(i)
end

for i = 10:-1:1 %backwards w/ start:step:end
  disp(i)
end

for i = 1:10 % loops over 10 values from 1 to 10 inclusive
  disp(i)
end



 matrices


m1 = [ 2 3 4; 4 5 6]
mNRV = length(m1) % number of values in row
mNV = numel(m1) % number of all values
mS = size(m1) %dimensions
[nRows, nCols] = size(m1) %tuple save

% random stuff :)

m2 = randi([10, 20], 3) % values between 10-20 and 2x2
m2(1,2) = 22 % changes values
m2(1,:) = 25 % change all row values at row 1
m2(:,1) = 36 % change all col values at col 1
mR1Last = m2(end, 1)
MR2Last = m2(2, end)
m2(:,2) = []; % this clears colum 2
m2



 vectors


% row or column vectors

% row vectors:
vt1 = [ 5 3 2 1 ]
vL = length(vt1)
vt1 = sort(vt1) %asc
vt1 = sort(vt1, 'descend')
vt2 = 5:10 % stepsize=1 (default)
vt3 = 2:2:10 % stepsize=2
vt4 = [vt1 vt2] % concat
% index starts at 1 omg
vt4(1)

vtEnd = vt4(end)
vt4(1) = 12 % overwrite
vt4
vt4(15) = 33 % gabs are filled with default vals
vt4(1:3) % range
vt4([ 2 4 6 ]) % specific indices

% column vectors

vt5 = [2;3;4]
vt6 = [1 2 3] %row

multiplication
  1 2 3
2
3
4

results in matrix!
vtMult = vt5 * vt6

% dot product
vt7 = [4 5 6]
% (1*4) + (5*2) + (3*6) = 32
vtDot = vt6 * vt7' % transpose!
vtDotP2 = dot(vt6, vt7)

% cross product
vtCross = cross(vt6, vt7)

% create linearly spaced vector
vt8 = linspace(1,20,4) % 4 elements in space 1..20
vt9 = logspace(1,3,3)


 if, else, elseif, switch, case


ge = 12

witch age
 case 5
   disp('You go to kg')
 case num2cell(6:13) % this is incluse! and I found it out by experiment. :)
   disp('You go to middle school')
 case {14,15,16,17,18}
   disp('High School')
 otherwise
   disp('Stay at home')
 end



f age <=5
 disp('You go to Kindergarden')
lseif age >5 && age <= 12
 disp('You go to elem. school')
lse
 disp('You are lost, because you are growing up')
nd

INFO: To get help type the following into the command window
doc
help


 OPERATIONS AND OPERATORS


 INFO
 default precision is 15 decimal places

 interesting operators
 abs, floor, ceil, round, exp, log, log10, log2, sqrt, deg2rad, and so on...
printf('5^4 = ', 5^4)
printf('5 %% 4 = ', mod(5,4)) %escaping is %

 everything is stored into 'ans' by efault
andi([10,20])
ns

 I/O second part
 sprintf('5 + 4 = %d\n', 5 + 4)
 formats to string


 DATA TYPES

primitive types
nt 8, 16, 32, char logical double single uint8...

% chars and ints
v4 = double('A')
v5 = char(64)

% everything is double by default!
v2 = 8
class(v2)
v3 = int8(v2)
class(v3)

% ranges for types
intmin('int8')
intmax('int8')

% multiline expression
v1 = 1 + 2 + 3 ...
    +4

dynamic typing
c1 = 'A'
class(c1)

s1 = 'A string'
class(s1)

s2 = "A string"
class(s2) %string?


vInput = input("Enter a vector: ");
disp(vInput)


name = input('What''s your name?', 's')

if ~isempty(name)
  fprintf("Hello %s\n", name)
end