Cells and Structures

Cells and Structures Array Cells and Array Structures

Character Strings • planets=['Mercury','Venus','Earth'] • Matlab returns one concatenated character string • MercuryVenusEarth • planets(1) is therefore only the first character, that is 'M' • Planets={'Mercury','Venus','Earth'} • Matlab now returns 3 separate character strings • 'Mercury' 'Venus' 'Earth' • Planets(1) is now 'Mercury' • Planets{1} is then Mercury without the quotes

Array of one dimensional cells • ['Mercury' ; 'Venus' ; 'Earth'] would avoid the catenation, but it is illegal since the three strings are not of equal length. (It is also inefficient.) • {'Mercury','Venus','Earth'} is a one dimensional cell array • Content of each cell can be of different type • p={'Mars' , 3.7, '2005' , 2005} • but if they are different accessing the cells becomes difficult • Note p{3} and p{4} are different – see 5*p{3} and 5*p{4} • Use structures instead – see later

Creating Cell Arrays • A = cell(2) • creates a 2 by 2 array for cells • Each cell can have a different content • Note how content of a cell has be enclosed in curly brackets • A(1,1) = {'ENDF 121'} • A(1,2) = {'Fall 2005'} • A(2,1) = { [1:7] } • A(2,2) = { [22,15,22,23,24,25,21] }

Different way of accessing cells via 'content indexing' • B = cell(2,3) • creates a 2 by 3 array whose cells are accessed by using { , } for the indices instead of ( , ) • B{1,1} = 'special matrices' • B{1,2} = zeros(3) • B{1,3} = ones(3) • B{2,1} = 'their inverse' • B{2,2} = 'does not exist' • B{2,3} = ones(3)

Creating cell arrays • It is not necessary to use the cell function first • but if variable exists already an error will occur • Can use clear to avoid this problem • cells can be multi dimensional, i.e. m = cell( 2,3,4,2 ) • cells can be nested, i.e. a single cell element can reference another cell • Note difference between cell indexing • A(1,1) = {' This class '} • and content indexing • A{1,1} = 'This class' • and direct addressing • A(1,1) = 123; • in the last case the floating point number 123 is stored directly at the storage location associate with A(1,1). • the last case can not be mixed with the first two

Functions for working with cells • C = cell(m,n,p,…) • celldisp(C) • cellplot(C) • C = num2cell(A) • A numeric array – uses direct addressing • C array of cells – uses indirect addressing • iscell(C) returns 1 for true 0 for false • [X,Y,…] = deal(A,B,…) • same as X=A; Y=B; … • number of variables have to be the same on both sides • except for special case [X,Y,…] = deal(A)

Example for students in class • S{1,1} = 'Joe Smith' • S{1,2} = '123-45-6789' • S{1,3} = 'smithj@email.uc.edu' • S{1,4} = [67,78,45] ; • S{2,1} = 'Ann Schmidt' • S{2,2} = '234-45-1234' • S{2,3} = 'schmida@email.uc.edu' • S{2,4} = [78,95,67] ;

Difficulties with cells • Cells of previous example hold information in following order (second index) • name • social security number • e-mail address • scores for tests • First index refers to student number in class • When entering data need to remember all of this • Excel or Access or similar programs are a much better choice for this type of problem • Cells are useful for annotating graphs, identifying data, etc.

Cells and Structures