1. Cells and Structures Array Cells and Array Structures

2. Creating Cell Arrays A = cell(2) creates a 2 by 2 array for cells Each cell can have a different content Note how content 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] }

3. Different way of accessing cells via 'content indexing' B = cell(2,3) creates a 2 by matrix 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)

4. 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, not just 2, 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

5. 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)

6. 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] ;

7. Difficulties with cells Cells of array hold 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

8. Structures Structures are similar to cells, except the cells can be referenced symbolically by a meaningful name (instead of an index) student.name='John Smith' student.SSN='123-45-6789' student.email='smithj@email.uc.edu' student.tests=[23,45,34] when entering Matlab will repeat entry unless a semicolon is used

9. Array of structures student(2).name='Mary Poppins' student(2).SSN='456-12-2345' student(2).email='poppins@email.uc.edu' student(2).tests=[12,0,13] when entering Matlab will respond each time with student= 1 * 2 struct array with fields: name SSN email tests in this case better to use semicolon

10. Preallocation of structure students=struct('name','Karl',tests,[12,34,67]) students(2).name='Cindy' ; students(2).tests=[13,24,34]; Additional fields can be added later, that is dynamically students(2).id = '123-23-3333'; All existing array elements will now have the same entry available; initially set to empty. All entries can be change by using the same notation With the name notation each field can be retrieved

11. Copying structure arrays studentcopy = students all entries of structure array are copied operation can be time consuming for very large arrays and also will require a lot of memory

12. Modifying a structure array To remove a field in a structure array: stdnt=rmfield( student,'email' ) function returns a new structure array after copying all fields except the one for 'email' Both structure arrays student and stdnt are accessible unless same name is used student = rmfield( student, 'email' ) To add a field can also use stdnts=setfield( students, {2}, 'phone','985-5566') entire structure array has to be copied with new field inserted. array element 2, will have given phone number all other entries are empty if structure array is two dimensional {2} needs to be replaced by reference to correct array element {i,j}

13. Accessing structures getfield( student,{2},'name' ) Matlab allows access to fields of a structure with a variable, i.e. dynamic access xyz = 'name' student(2).(xyz) other programming languages typically do not have this feature and require that the field is known at compilation time, i.e. static access Benefits of dynamic versus static access probably slower in complied languages probably faster in interpreted languages (i.e. interactive systems) like Matlab