COMP 116: Introduction to Scientific Programming

COMP 116: Introduction to Scientific Programming Lecture 25: Cell Arrays and Structures

Announcements and Reminders • Friday class will be taught by SachinPatil • This Wednesday’s quiz moved to Friday • This Wednesday office hours from 2-3pm • Midterm coming up

Today • How to store variables of different types (numbers, characters, strings, arrays, matrices) all in one single array? • Two options: • Cells • Structs

Data Structures • Arrays • Homogenous • Indexing by numbers • Cell Arrays • Heterogeneous • Indexing by numbers • Structures • Heterogeneous • Accessing by naming

REVIEW: ARRAYS

Multi-dimensional Arrays • [42] • Scalar • 0-dimensional • [1 2 3 4 5] • Vector • 1-dimensional • [1 2 3; 4 5 6] • Matrix • 2-dimensional • We can go higher: • Multidimensional array • n-dimensional

N-D arrays • 3-Dimensional arrays • e.g. temperature at every point in this room (x,y,z) • 4-Dimensional arrays • e.g. temperature in this room over time (x,y,z,t) >> A = ones(2,3,2); >> B(:,:,1) = [1:4; 5:8]; >> B(:,:,2) = [7:10; 13:16];

Most common 3D array • Images • 3 channels (red, green, blue) • Each channel is a 2D array

Most common 3D array

N-D Arrays: Arithmetic • Element-by-Element arithmetic • The same as before • + - .* ./ .\ .^ • Linear algebra (i.e., matrix) operations generally not used

Cell Arrays >> doc cell Under Help: MATLAB → Programming Fundamentals → Classes (Data Types) → Cell Arrays

Creating Cell Arrays Just like arrays, but use {} instead of [] • Here’s a list of names: • Alexander the Great • Winnie the Pooh • Jack the Ripper • Stephen the Colbert • Put these into a MATLAB data structure

Creating Cell Arrays Just like arrays, but use {} instead of [] • Does this work? • names = ['Alexander the Great'; 'Winnie the Pooh'; 'Jack the Ripper'; 'Stephen the Colbert'] • Simple modification: • names = {'Alexander the Great'; 'Winnie the Pooh'; 'Jack the Ripper'; 'Stephen the Colbert'}

Cell Arrays Note: Not the same concept as cells (%%) in publishable scripts

Creating Cell Arrays % Cell array of different length strings days = {'Monday'; 'Tuesday'; ...}; % Cell array of 4 elements (of different sizes, data types) some_guy = { 'Steve', 45; ... [8 19 1956], {'ice cream'; 'pizza'} }; % Using 'cell' command cB = cell(n) % creates n by n cell of empty elements cC = cell( m, n) % creates m by n cell of empty elements % Creating a Cell Array from an Array A = ones(2, 2); cA = cell(size(A)); % Note: all elements are empty

Accessing Cell Arrays: Two ways to access elements: names{2} gives you the contents of a call names(2) gives you a new cell array with those contents % Change content some_guy{1,1} = 'Chuck' some_guy{2,2} = 1:5; some_guy{1,2} = []; % Delete content in cell 1,2 % Grab 1st, 2nd elements some_guy(1:2) row2 = some_guy (2,:) % Grab 2nd row of cells

Another useful example names = {'Sine', 'Cosine'}; labels = {'0', 'pi', '2pi', '3pi', '4pi'}; t = 0:pi/32:4*pi; plot(t, [sin(t); cos(t)]); legend(names); set(gca, 'XTick', 0:pi:4*pi); set(gca, 'XTickLabel', labels);

Accessing Cell Arrays • Remember: crucial distinction between {} and () operators Example: A = { false, rand(3); 4.0, 'This is a string'}; • A{1} • A{1,1} • A(2) • A(2,1) • A(2,:) • A{:,2} Indices work just as for ‘standard’ arrays.

Useful Commands • celldisp – shows contents of the cell in the command winow, one element at a time • cellplot – graphical display of all cells in a cell array

Inspecting Cell Arrays A = { false, rand(3); 4.0, 'This is a string'}; Try: • disp(A) • celldisp(A) • cellplot(A) Now try this: >> B = {A; A} >> D = { B B } >> cellplot(D);

Practice A = {4, [1:10]', 'demo'; false, eye(2), rand(3)}; A B = A(1) C = A{2} D = A(2,[2 3]) E = {A{1, [1 3]}, 20} F = {A(1, [1 3]), 20}

Examples • Collection of strings • Extract important words from a document

Exercise • Write a function get_the_wordsthat • Takes as an input a sentence (in the form of a string) e.g. ‘This is a true statement’ • And returns a cell array of the words in the sentence i.e. {‘This’, ‘is’, ‘a’, ‘true’, ‘statement’}

Structures Under Help … MATLAB → Programming Fundamentals → Classes (Data Types) → Structures

Structures • Structures are a common concept in most programming languages • Each element inside a structure is given a unique name • These elements are called fields

Structures • Same example as before, but now with more data

Structures • Use named ‘fields’ for each variable alex.name = 'Alexander the Great'; alex.occupation = 'Conqueror'; alex.birth = 356; alex.fictional = false;

Structures • Use named ‘fields’ for each variable • Use the struct() function, with name-value pairs alex.name = 'Alexander the Great'; alex.occupation = 'Conqueror'; alex.birth = 356; alex.fictional = false; alex = struct('name', 'Alexander the Great',... 'occupation', 'Conqueror', ... 'birth', 356, ... 'fictional' = false);

Structure Arrays • Each individual structure object is accessed with its index (just as in vectors) • Each field inside an individual structure is found by naming using the '.' operator for access. characters(1) = alex; characters(2).name = 'Winnie the Pooh'; characters(2).occupation = 'Bear'; ... characters(3) = struct('name', 'Jack the Ripper',...);

Structure Arrays • One way to initialize is to use a ‘template’ % create structure layout % note the use of default values and empty arrays template = struct( 'name', 'no name', ... 'nickname', 'no name', ... 'emails', [], ... 'department', 'undeclared', ... 'type', 'undergrad', ... 'year', 1 ); % create structure array students = repmat( template, 1, 30 ); % now fill in each structure in the array

Examples • Nested records • Structures can contain other structures (or cell arrays) as named fields. class.instructor = struct('name', {' Vishal' ‘Verma'}); class.students = students; % this is a struct array class.building = 'FB'; class.room = 7;

Examples • Built-in structures • Figures get(gcf) • File lists >> files = dir files = 8x1 struct array with fields: name date bytes isdir datenum

Examples • The dir function returns a structure array with 4 fields: name, date, byte, isdir • The input of dir is a path • How do we use this to find out the total size of all the files in a directory? • How do we get all of the names of all images in a folder?

Common Pitfalls • Remember the difference between {} and () when accessing cell • () returns cells • {} returns contents • Correctly access the field of a structure via the dot operator and the fieldname. • currStudent.name = 'Steve'; • Remember: you still need the index () operator for working with Structure arrays • students(7).score = 97;

Style Guidelines • Use arrays when all elements are the same type and in some sense represent the same thing (just different values) • Use cell arrays or structures when the values that make up the object are logically grouped but not the same type or the same thing. • Prefer cell arrays when you want to loop through all elements in each individual cell via indexing. • Prefer structures when you want to name each element individually • Use cell arrays for storing an array of strings.

Summary • Review Cell Arrays & Structures • doc cell • MATLAB → Programming Fundamentals → Classes (Data Types) → Cell Arrays • MATLAB → Programming Fundamentals → Classes (Data Types) → Structures • Practice working with • Cell Arrays • Structures • Operators for Cell Arrays & Structures • {}, (), '.'

Useful functions • rmfield – removes a field from a structure • isstruct – tests whether a variable is of type structure • isfield – tests whether a name string is a field in the specified structure. • fieldnames – returns names of all fields in a struture.

COMP 116: Introduction to Scientific Programming