Array Accessing and Strings ENGR 1181 MATLAB 3

Transcription

1 Array Accessing and Strings ENGR 1181 MATLAB 3

2 Array Accessing In The Real World Recall from the previously class that seismic data is important in structural design for civil engineers. Accessing data from an array at a certain location in California allows engineers to design their structures according to vibrational data in a specific region. This allows the building to be designed to this standard but not overdesigned to more extreme data in other regions.

3 Today's Learning Objectives After today s class, students will be able to: Demonstrate proper notation for accessing elements from previously assigned onedimensional arrays (e.g., single elements, list of elements) and two-dimensional arrays (e.g., those with rows and columns). Explain that a string is a one dimensional array and can be used the same way as numeric arrays.

4 What is Addressing? Each element in a vector has an address, also called an index MATLAB indexing starts at 1 (not at 0!) We can access/retrieve/extract the individual elements by referring to their addresses Useful for transforming data or doing calculations with only part of a vector

5 Vector Addressing Example Define a vector with 9 elements: >> v = [ ]; We can access the elements individually: >> v(4) ans = 21

6 Vector Addressing Example We can retrieve any element by indexing: >> v(7) ans = 30 >> v(9) ans = 36 We can assign individual vector elements to variables: >> B= v(7) B = 30 >> C=v(9) C = 36

7 Vector Addressing Examples We can add elements together. Recall: B = v(7), C = v(9) >> D= B + C D = 66 We can also add elements directly: >> v(4) + v(7) ans = 51

8 Changing Element Values We can change an element in a vector by directly assigning a new value to a specific address. Let s change the 6 th element of v to 90: v= [ ] >> v(6) = 90; >> v v =

9 Addressing Column Vectors Addressing an element in a column vector works the same way as with a row vector: >> col = [25; 30; 35; 40; 45; 50] >> t = col(4) t = 40

10 Vector Functions MATLAB has many, many built-in functions we can use with vectors max() min() sum() length() etc.

11 Vector Functions length() gives us the number of elements in a vector >> fun = [ ]; >> length(fun) ans = 5

12 Vector Functions Zeros() gives us a vector or matrix of zeros >> nothing = zeros (1, 7) nothing =

13 Vector Functions ones() gives us a vector/matrix of all ones >> single = ones(1, 12) single =

14 Addressing a Range of Elements The colon operator allows us to access a range of elements in a vector This is useful if we want to extract or alter only a portion of an existing vector

15 Example: Addressing a Range Define a vector: >> vec = [ ]; Select elements 3 through 7 in 'vec': >> vec(3:7) vec =

16 Example: Addressing a Range We can access a range of elements in any vector and assign them to a new variable. Recall that vec = [ ] >> t= vec(2:5) t =

17 Vector Modifications We can add elements to any existing vector. Recall that 'vec' has 8 elements: vec = [ ] >> vec(9: 12)= [ ] vec =

18 Vector Modifications We can create new vectors made up of elements from previously defined vectors: >> E = [ ]; >> G = [ ]; >> K = [ E(1:3) G(3:4)] K =

19 Matrix Addressing Matrix addressing works very similarly to vector addressing Individual elements are addressed by their row number and column number: (m, n)

20 Matrix Addressing Example Let's define a matrix, then access some elements: >> data = [ ; ] data = >> data (2,3) ans = 8

21 Matrix Addressing Example We can perform mathematical operation with matrix elements. Let's add two values from our matrix called 'data': data = >> data_sum= data(1,2) + data(2,4) data_sum = 12

22 Colon Operator With Matrices A(:, 3) Elements in all rows of column 3 A(2, : ) Elements in all columns of row 2 A(:, 2:5) A(2:4, :) A(1:3, 2:4) Elements in columns 2 to 5 in all rows Elements in rows 2 to 4 in all columns Elements in rows 1 to 3 and in columns 2 to 4

23 Extracting Matrix Elements We can extract a portion of a matrix and assign it to a new variable new_matrix =matrix( r1 : r2, c1 : c2) r1 is the starting row r2 is the ending row c1 is the starting column c2 is the ending column

24 Example: Extracting Elements >> A = [ ] A = >> B = A(1:3, 2:4) B =

25 Example: Extracting Elements >> C = A(1:3, : ) C = >> D = A( :, 2:4) D = Remember

26 Important Takeaways An element in a defined vector can be accessed with v(x) - an element in a vector can be defined, or redefined with v(x)=z An element in a defined matrix can be accessed with v(x:y)- an element in a matrix can be defined, or redefined with v(x:y)=z Strings are lines of text and can be used instead of numerical values - they are defined inside single apostrophes, e.g. Your text here.

27 Preview of Next Class Array Operations Scalar vector operations Vector vector operations Dot operator, when to use it Built-in vector functions Ex: max, min, mean etc. Examples

28 What s Next? Review today s Quiz #03 Open the in-class activity from the EEIC website and we will go through it together. Then, start working on MAT-03 homework. Before next class, you will read about array operations, this is an introduction of mathematical operations in MATLAB and basics of linear algebra.