Matlab revision #1

Start Matlab

Everything should work

Variables: a named placeholder for something
Matlab commands or functions do stuff to inputs and return an outputs
operators: really also just functions, but a convenient shortcut for very common jobs + - * / etc.

add, subtract, multiply, divide numbers like this:

>> 1 + 1 - 1    % + and -

>> 10 .* 5      % use .* and ./ to multiply and divide

>> 2 .^ 0

>> 2 .^ 8       % use .^ to raise to power

note that the operators for multiply and divide are .* and ./ (more on the extra “dot” later)
use parenthesess ( ) to make order of operations explicit:

>> (10 + 1) .* 9
>>  10 + 1  .* 9   % not the same, WHY?

>> a = 10   % stores the number 10 in a variable called "a"
>> b = 2+2  % stores the result of 2+2 in "b"

>> a .* 2.3 % uses "a"
>> c = a + b    % stores result of a+b in "c"

>> x = [1, 2, 3]  % a ROW vector
>> y = [4; 5; 6]  % a COLUMN vector

>> u = [ 1, 2, 3 ;
         4, 5, 6  ]  % a 2 by 3 matrix

>> ones(2,5)   % 2-by-5 matrix full of 1
>> zeros(3,3)  % 3-by-3 matrix full of 0
>> rand (100)  % a 100-by-100 matrix of uniform random numbers

>> randn(5) % gaussian random numbers (5-by-5 matrix)
>> nan(10)  % not-a-number ... useful in some cases

To illustrate, let’s make a special example matrix (it’s special: all rows and columns add to the same value)

M = magic(5)

sum(M, 1) % sum ACROSS dimension 1

sum(M, 2) % sum ACROSS dimension 2

Use parentheses after variable name. To e.g. get the 2nd row, 4th column:

>> M(2,4)

ans =

    14

Remember: you can use the : to get everything along a dimension.

A useful metaphor

Think of a high-lighter pen colouring in the row and column. Where colours overlap: selected

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

A useful metaphor

Think of a high-lighter pen colouring in the entries that meet the criteria: selected

Pretend your array is one big long list (go down rows first, then columns, …).

For M (which was 5 by 5), that means 25 entries.

M(23) % one but last entry
M(:)  % all entries in one big long list

A useful metaphor

Think of your array as a snake or maybe an unravelling knitwear sweater