MATH36032 Problem Solving by Computer. More Data Structure

Transcription

1 MATH36032 Problem Solving by Computer More Data Structure

2 Data from real life/applications How do the data look like? In what format?

3 Data from real life/applications How do the data look like? In what format? Open data websites: Historical weather data: Official Statistics of UK: or google flu trend data, or the purchasing order data for Project 3? MS excel, txt (plain text), csv (comma separated value), xlm (extensible Markup Language), json (JavaScript Object Notation),...

4 Data Structure Different data (numerical, symbolic, characters,...) in computer: require different memory for storage are organised differently in memory have different operations

5 Data Type we have seen so far Numeric Types: double, single, int8, int16, int32, int64, uint8, uint16, uint32, uint64 Symbolic: defined by syms or symfun Logical: true,false Function Handles: to allow function as input argument in other functions (like integral or fminbnd) Characters and strings: A = string 1 Warning: not all new data structures are implemented in Octave

6 Data Type we have seen so far Numeric Types: double, single, int8, int16, int32, int64, uint8, uint16, uint32, uint64 Symbolic: defined by syms or symfun Logical: true,false Function Handles: to allow function as input argument in other functions (like integral or fminbnd) Characters and strings: A = string Other common data types: Structures, Cell Arrays, user defined class (object oriented programming),... New data types introduced recently 1 : Time Series (2006b), Table (2013b), Categorical Arrays (2013b), Date and Time (2015b),... 1 Warning: not all new data structures are implemented in Octave

7 Data type: Characters and Strings Create string (a vector of characters) by single quotation marks: str = 'Hello World!'; get individual character by str(1), str(6),... conversion between characters and numbers: double(str), char([ ]) Character Array (vector of string characters, i.e., a matrix): proj = ['taxicab number '; 'catalan constant '; 'reciprocal square']. Notice the spaces and semicolon (to match the size of each string). Equivalent way (no need to put empty space in the input): proj = char('taxicab number','catalan constant','reciprocal square') Other related commands (the equivalent vector operation may be faster): ischar, strfind, strcat, strcmp,...

8 Data type: Cell arrays Used to hold array of mixed data types (Notice the curly brackets {} use here!): C = {[1 2 3], 'MATLAB', [3;2;1], ones(2)}; proj = {'logistic','motion','data'}; Two ways to access data: C(2) (which is still a cell array) and C{2} (which is the character string 'MATLAB'). Adding cell to C = {1, 2, 3 }: C{4}=4 or C(4)={4}; (notice these two equivalent ways) Deleting cell: C(2)=[]; notice that C{2}=[] does not delete the element (just make it empty). Try experiments (especially about the difference between {} and ()), instead of remembering the rules.

9 Structure array using struct Student array student(1) student(2) FirstName John LastName Doe BirthDate EntryYear 2013 General syntax: variable.field = value student (1). FirstName = 'John '; student (1). LastName = 'Doe '; student (1). BirthDate = ' '; student ( 1). EntryYear = 2013; The output sol from ODE solvers like sol = ode45(...) is also a structure array!

10 Structure array using struct, alternative method Student array student(1) student(2) FirstName John LastName Doe BirthDate EntryYear 2013 student = struct (... ' FirstName ', { 'John ',[],[]},... ' LastName ',{ 'Doe ',[],[]},... ' BirthDate ',{ ' ',[],[]},... ' EntryYear ',{2013,[],[]});

11 Structure array, related functions Access Data: student(1).firstname Concatenate Structures: struct1.a = 'first'; struct1.b = [1,2,3]; struct2.a = 'second'; struct2.b = rand(2); combined = [struct1, struct2] Make sure all structures to be concatenate have the same fields! Field of structure array: getfield(student(1),'firstname'), which is the same as student(1).firstname

12 Table: column-oriented heterogeneous data The excel spreedsheet in MATLAB LastName ={ ' Smith '; ' Johnson '; ' Jones '}; Gender ={ 'Male '; 'Male '; ' Female '}; Age =[24; 35; 39]; Smoker =[1;0;0]; T= table ( LastName, Gender,Age, Smoker );

13 Table: column-oriented heterogeneous data The excel spreedsheet in MATLAB LastName ={ ' Smith '; ' Johnson '; ' Jones '}; Gender ={ 'Male '; 'Male '; ' Female '}; Age =[24; 35; 39]; Smoker =[1;0;0]; T= table ( LastName, Gender,Age, Smoker ); All inputs are column vectors The cell arrays LastName and Gender can be defined using string arrays: LastName=char('Smith','Johnson','Jones'); Gender=char('Male','Male','Female');

14 Different ways to access Data in a Table Get elements (of data type table): T(1,:),T(:,2) Get elements (data type is the same as the extracted data ): T{1,2}, T{1,3}, Get the column: T.Gender Get the row: LastName ={ 'Smith '; ' Johnson '; 'Jones '}; T. Properties. RowNames = LastName ; T({ 'Jones '},2:3);

15 Different ways to access Data in a Table Get elements (of data type table): T(1,:),T(:,2) Get elements (data type is the same as the extracted data ): T{1,2}, T{1,3}, Get the column: T.Gender Get the row: LastName ={ 'Smith '; ' Johnson '; 'Jones '}; T. Properties. RowNames = LastName ; T({ 'Jones '},2:3); You need to read and manipulate data using table for the last project.

16 Summary of the key commands for table readtable: create table from delimited text files (*.txt, *.dat, *.csv) and spreadsheet files (*.xls, *.xlsx, *.xlsb, *.xlsm, *.xltm, *.xltx, *.ods) array2table, cell2table, struct2table, table2array, table2cell, table2struc: conversion between table and other data types height, width: Number of rows and variables summary: Print summary of table