Matlab Revision #2

Nick Myers

2022-11-30

Start Matlab

Everything should work

  • on local machine or
  • on https://matlab.mathworks.com

Last week

  • variables, assignment, equal sign =
  • arrays (vectors, matrices, rows, columns)
  • indexing: 3 ways (index, logical, linear)
  • problem solving! . Breaking big problems into smaller ones and function…

Today

  • for loop (repeating things)
  • true(), false() and if statement (doing things only in certain conditions)
  • text, strings, …
  • cell arrays (using more than numbers)
  • any requests?

Reminder on variables

>> a = 10   % stores the number 10 in a variable called "a"
>> b = 2+2  % stores the result of 2+2 in "b"
  • you can now use the variables (or place holders)::
>> a .* 2.3 % uses "a"
>> c = a + b    % stores result of a+b in "c"

Repeating operations

writing repeated operations can become tedious (and error-prone!):

>> a(1,1) = 0;
>> a(1,2) = 1;
>> a(1,3) = a(1,1) + a(1,2);
>> a(1,4) = a(1,2) + a(1,3);
>> a(1,5) = a(1,3) + a(1,4);
>> a(1,6) = a(1,4) + a(1,5);
>> a(1,7) = a(1,5) + a(1,6);
>> a(1,8) = a(1,6) + a(1,7);

For loops

instead:

>> a(1,1) = 0;
>> a(1,2) = 1;
>> nIterations = 6;
>> for i = 1:nIterations
>>    a(1,i+2) = a(1,i) + a(1,i+1);
>> end;

Last week: Logical indexing

Find where the array fulfills certain rules. (true, false)

(M > 5) % where are the values > 5
idx = (M > 5) % store them in idx
M(idx) % use them

More on true/false

Matlab thinks of logical (true/false) statements in terms of binary numbers

true = 1

false = 0

>> a = [5 8 2]; % create a new variable, assign 1x3 vector of numbers to it
>> a>3 % test each element of a to see if it is larger than 3 
>> idx = a>3; % create a new variable, idx, and assign the outcome of a>3 to it
>> a>5
>> a<8

Yet more on true/false

what about finding exact matches?

>> a==5 % returns a 1 (true) for every entry in 'a' exactly equal to 5, 0 (false) otherwise
>> idx = a==5; % create a new variable, idx, and assign the outcome of a==5 to it

what about more complicated cases?

>> a = [5 8 2];
>> b = 4;
>> a>b
>>
>> b = [4 4 4]; 
>> a>b %identical to above
>>
>> b = [4 9 3];
>> a>b %compares elements in a to corresponding elements in b

if-else statements

execute code only if a particular condition is met

>> % make a random number between 0, 1
>> a = rand(1)
>>
>> % now test if this time it is > or < than 0.5
>> if a > 0.5
>>   disp('chance favours the prepared mind')
>> else
>>   disp('uhoh!')
>> end

working with text

So far we have seen numeric data types (and booleans):

myNum = [2 8 11];
myBoolean = myNum>2;

Matlab also deals with text:

myText = 'Why is Matlab so difficult?';

some functions apply to numbers and text:

numel(myNum)
numel(myText)

others dont:

log10(myNum)
log10(myText) %produces an error - cant take logarithm of string variables

working with lots of text

text and numbers dont mix easily

myNum  = [3 5 9 11.2 -5];
myText = 'Hello';
myArray = [myNum; myText] %produces wrong result - cant have numbers and text in same array

Solution - cell arrays

myNum  = [3 5 9 11.2 -5];
myText = 'Hello';

myCellArray = {};
myCellArray{1,1} = myNum;
myCellArray{1,2} = myText;

Thanks!