Matlab Its good Variables Doubles numbers String text

Matlab It’s good

Variables • Doubles (numbers) • String (text) • Cell: a 1 x 1 size space containing another variable, of any size, inside it • Structure (struct): a variable holding other variables attached to it

Variables: size • Variables can be considered a martix. They are always at least 2 -dimentional • A variable with a single number has a size of 1 x 1 • While a variable such as: 5 6 7. 6 8 7 12 55 4. 5 • Is 2 x 4 (rows x columns)

Accessing variable elements • Variable elements can be accessed (or defined) by the variable name, followed by indices in brackets. • Indices are (row, column) A= 5 6 7. 6 8 7 12 55 4. 5 • A(1, 1) is 5, A(2, 1) is 7, A(2, 3) = 55 • A(2, 3) is row 2, third value. • In a double variable, each number has a separate index

Defining Variables • Variables are defined by an = sign • E. g. A = 5. 3 makes a 1 x 1 double with a value of 5. 3 • Now if we say A(1, 2) = 6, we will have a 1 x 2 variable, [5. 3 6] • Try it!

Defining variables • Several values can be entered aty once using square brackets [] • A = [1 2 5 6 7] gives a 1 x 5. A(1, 5) is 7. • You can also, in simple variables like this, just say A(5) • A semi colon adds lines • A = [1 3 5 7; 2 4 6 8] 1 3 5 7 2 4 6 8 • Must have the same number of elements per line

Padding with zeros • Matlab will expand the size of variables as needed. In a double, zeros are padded as needed. • A=1 • A(6) = 2 • A is now 1 0 0 2

Colon operator • In matlab, a colon has two meanings: – By itself, it means ‘all’ – With numbers on each side it means ‘from x to y’ • E. g. 2: 6 = 2 3 4 5 6 • You can use colons inside variable indexs when calling them • E. g. A = [2 4 6 8 10 12 14 16] • B = A(3: 5); B is now [6 8 10]

Colon wackiness • A(1: 5) = 10; Ai is now [10 10 10] • A = 1: 5; A is now [1 2 3 4 5] • A = 2: 2: 10; the middle number becomes an incrimentor • A is now 2 4 6 8 10

Exercise 1: defining double variables • Make a double “A” with values 5, 10, 15, 20, 25, 30 • Change third value to 100 • Make a variable “B” which is equal to the first 3 values in A • Make a variable “C” which has the same value as the last value of A (by calling A, not cheating!) Pause here to contemplate the nature of existance

Exercise 1 b: a matrix, using colons • Clear – (this clears all variables from the matlab workspace) • A = [1 5 7 9; 11 33 55 77; 100 200 300 400] • Set Change the 4 th value on the first row (the 9) to 3 • Set B equal to the first row of A • Set C equal to the first 3 numbers in the last row of A • Set D as the first COLUMN of A

Error: In an assignment A(I) = B, the number of elements in B and I must be the same. • Matlab tries to fit values into the exact space you define. If you tell it to fit 4 things in a space of 3 or 5 items, it throws an error: “In an assignment A(I) = B, the number of elements in B and I must be the same. ” • For example, A(3: 5) = [1 2] – you are saying “in these 3 spaces, fit 2 things” • If you give a single value for multiple spaces, Matlab sets them all to that values • E. g. A(1: 4) = 10, A is [10 10].

Multiple Dimensions • Matlab variables are always treated as if they were AT LEAST 2 D, but can in fact have as much dimensionality as you wish. • A third dimension can be thought of a cube. • So: – A(: , 1) = [1 2 3; 7 8 9] – A(: , 2) = [11 12 13; 21 22 25] • Now we have a 3 D variable of size 2 x 3 x 2 • Extra variables can be added (e. g. f. MRI data is typically 4 D): a cube, in a ‘room’

Basic Matlab Math • Matlab treats double variables as matrices, and will often default to doing matrix math, if you provide it two matrices • If you give a matrix an a mathmatical operand that is a single number, it applies that to all elements of a matrix • E. g. A = [1 2 3 4] • B = A+2; B is now [3 4 5 6] <<Do some examples together>>

Calling matlab functions • Matlab has thousands of useful functions you can call. A function call looks like this: • Output = functionname(inputs, moreinput, etc) • Output is any variable name into which the functions return value is assigned • Multiple input values are separated by commas

HELP! • All built-in matlab functions have a help. Type the word help and the function name • Try “help sum” • Lets try calling sum • A = [1 2 3 4]; • B =sum(A) >> B is now 10 (1+2+3+4)

Sum part 2 • Sum has an option to put in a second input variable. By default, sum outputs data for each row of the input • A = [1 2 3; 4 5 6] • Sum(a) gives [6 15] • This is because by default it calls as sum(A, 1), working on the sum based off the first dimension • BUT sum(A, 2) gives [5 7 9] (1+4, 2+5, 3+6)

Sum of sums, of more sums? Want sum? • Since sum only works on one dimension, we can ‘nest it’ go get a total sum: • B = sum(A)) NOTE: brackets MUST be balanced!!!! • B is now 21. • Matlab evaluates functions inside-out. The ‘deepest’ thing gets evaluated first. • <<stop here and marvel at the wonders of nesting>> • Other basic math functions: mean, min, max, median

Strings • Matlab treats strngs as an array of charaters • Strings are defined by encasing in single quotes • A = ‘happy’ • Now has a 1 x 6 variable h • A(2) is ‘a’ a p p y

String variables: size matters. a lot. • One of the biggest issues with string variables is the issue of size during variable assignments. • Matlab pads strings with whitespace • When making lists, spaces at the end of names can be an issue (e. g. a list of file names to call) << examples of using string here>>

Strings are NOT numbers • A = ‘ 1’ is a string, not a number • A+1 is 50. Because when you do math on a string, it uses asci reference to the string. • There are useful fucntions for this • num 2 str • str 2 num • Examples!

Concatenating strings • When we assigned double variables, we used square brackets. • This can be thought of as concatenating a series of numbers • We can also concatinate strings (or other variables) using [] • E. g. direc = ‘/data/cool/f. MRI/’ • file = ‘nicebrain. nii’ • fullfile = [direc file]

Cell: take a variable, and fit it into a space of 1 x 1 • Cells are a type of ‘container’ variable to hold other variables. • They can be especially useful for strings which can vary in length. • Cells are defined by { } • A{1} = [1 2 3 4] gives a 1 x 1 sized cell. A(1) is a cell with a value {1 2 3 4} • A cell can be ‘extracted’ using curly braces • B = A{1} gives a 1 x 4 double variable [1 2 3 4]

Error: Cell contents assignment to a non-cell array object. • If you tried the above code you got this • This is because A is probably defined in your workspace as a string or double. • You can only put a cell inside a cell variable • Do some cell practice here

Saving variables • Matlab always has a working directory, shown up top • You can save data with the save command • “Save mydata” will create a file names mydata. mat in the current directoy, with all variables from the workspace • Load mydata will load this variable IF YOU ARE IN THAT DIRECTORY • You can also specify a path – Load c: /scientceisfun/mydata. mat • Or as a function call with a string – Load(‘c: /scientceisfun/mydata. mat’) • Specific variables can also be saved (see help save)

Scripts and functions • Matlab has a great editor which allows you to make scripts and functions • Scripts just act as if you had copy/pasted the data into the main matlab windows • Functions create a new workspace, and only have access to variables you send to it. – Functions do not modify variables in your main workspace, except the output variable!

Script example • Type edit myscript • His will open the editor with a file called myscript. m, which isn’t saved • Type some commands in there and save • Now if you type myscript in the matlab prompt, it will run those commands

Functions • Functions are defined by having the word function on the first line, and the function call e. g. edit myfunc Function out = myfunc(in) Out = in* 10; • Functions do not require an end statement or anything. They run line by line until the end • A ‘return’ commend exists the function

Commenting scripts/functions • Any content starting with a % is treated as a comment and not run • A comment can be a while line, or after a command • E. g. A = B %set A to B because SCIENCE • Comments at the top of a function, immediately before the function line, will become the help for that function.

Basic complex syntax • Most important things in matlab are completed via only the following steps: • Define and cotnrol variables (covered, more or less) • Calling functions (covered) • If statements • For/while loops

If statements • In Matlab, if statements make code conditional • The code following the if will only execute when the if condition is true • For a mathmatical equals, use “==“, or “isequal” (which also works on strings) • The if code applied until it hits an end statement

If example If A == 1 B=A; A = 2; end ------------------If A> 1 A = 1; end -------------------If A >=2 A=1 end s. Some allowable pp[erands: ==, >, <, <= (less or equal), >=, ~= (not equal)

Else • An if statements can also have else. In this case, when the ‘if’ is untrue, an ‘else’ will be run e. g. if A == 1 B=A; A = 2; else B =1; end

elseif • Elseif runs a new if, ONLY when the first if is not true • Can be chained • Lower elseif only ever run if no above command is run • Once any if/elseif is true, onlt that sub-block of code is run, and then it skips to the end startment

elseif example If a < 10 b=1 elseif a > 100 b=3 else %only runs if a is between 11 and 99 b=2 end *there is no limit on the number of elseifs, though an else has to be the last condition. **In a chain of elseif’s it is possible none get run. If there is an if and else, at least one is going to run

For loops • Probably the most important code in matlab • Iterates a variable, usually a double for idx = 1: 10 %is use idx by convention, any name is OK %code goes here end If can also call a variable Or a specified series of numbers, e. g. For ii = [1 2 5 6 8 10] • Like if, the loop terminates with an ‘end’ statement. A ‘break’ can also kill a loop

Loops in loops • For loops and if statement can be nested count =1; for idx = 1: 20 a(idx) = sum(b(idx, : )); if a(idx) > 100 for jdx = 1: 5 c(idx, jdx) = a(idx) + jdx; end elseif a(idx) > 1000 break end %end of if d(idx) = a(idx); count = count+1; End %end for idx

While loops • While works are in other programming formats, only running while/if the condition is true • When it hits the end, it returns to the while and checks the condition n=1; a=1; while n < 10 a = a+b(n); %lets assume “b” was defined above somewhere if a < 10 n = n+1; end %end if end %will now check is n is still less than 10

Try and switch • ‘try’ will try to run the code. If the code fails, instead of crashing matlab will go the end • Switch allows testing multiple conditions switch method case {'linear', 'bilinear'} disp('Method is linear') case 'cubic' disp('Method is cubic') end

Structure variable • Structures are variables encasing other variables by name. • Like pointers • Attached “sub-variable” defined by a “. ” e. g. A. dat = [1 2 3] Creates a struct variable A, with sub variable “dat” which is a 1 x 3 double. • Structures are useful to save lots of data into a single variable, keeping thing organized • Can be nested • E. g. – a. dat. name = ‘funfun’ – a. dat. num = [1 2 3 4]

Appendix of useful functions find disp length size