### Powerpoint

```Array Accessing and Strings
ENGR 1181
MATLAB 3
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.
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.
 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
Define a vector with 9 elements:
>> v = [ 12 15 18 21 24 27 30 33 36];
We can access the elements individually:
>> v(4)
ans =
21
We can retrieve any
element by indexing:
elements to variables:
>> v(7)
ans =
30
>> B= v(7)
B=
30
>> v(9)
ans =
36
>> C=v(9)
C=
36
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
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 6th element of v to 90:
v= [12 15 18 21 24 27 30 33 36]
>> v(6) = 90;
>> v
v=
12 15 18 21 24 90 30 33 36
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
Vector Functions
 MATLAB has many, many built-in functions we
can use with vectors
max()
min()
sum()
length()
…etc.
Vector Functions
length() gives us the number of elements in a vector
>> fun = [4 6 8 10 12];
>> length(fun)
ans =
5
Vector Functions
Zeros() gives us a vector or matrix of zeros
>> nothing = zeros (1 , 7)
nothing =
0 0 0 0 0 0 0
Vector Functions
ones() gives us a vector/matrix of all ones
>> single = ones(1, 12)
single =
1 1 1 1 1 1 1 1 1 1 1 1
 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
Define a vector:
>> vec = [ 1 3 5 7 9 11 13 15 ];
Select elements 3 through 7 in 'vec':
>> vec(3:7)
vec =
5 7 9 11 13
We can access a range of elements in any vector
and assign them to a new variable. Recall that
vec = [ 1 3 5 7 9 11 13 15 ]
>> t= vec(2:5)
t=
3 5 7 9
Vector Modifications
We can add elements to any existing vector.
Recall that 'vec' has 8 elements:
vec = [ 1 3 5 7 9 11 13 15 ]
>> vec(9: 12)= [ 2 4 6 8]
vec =
1 3 5 7 9 11 13 15 2 4 6 8
Vector Modifications
We can create new vectors made up of elements
from previously defined vectors:
>> E = [ 3 6 9 12 ];
>> G = [ 2 4 8 5];
>> K = [ E(1:3) G(3:4)]
K=
3 6 9 8 5
 Matrix addressing works very similarly to
 Individual elements are addressed by their row
number and column number: (m, n)
Let's define a matrix, then access some elements:
>> data = [ 2 3 4 5 ; 1 6 8 9]
data =
2 3 4 5
1 6 8 9
>> data (2,3)
ans =
8
We can perform mathematical operation with matrix elements.
Let's add two values from our matrix called 'data':
data =
2 3 4 5
1 6 8 9
>> data_sum= data(1,2) + data(2,4)
data_sum =
12
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)
Elements in columns 2 to 5 in all rows
 A(2:4 , :)
Elements in rows 2 to 4 in all columns
 A(1:3 , 2:4)
Elements in rows 1 to 3 and in
columns 2 to 4
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
Example: Extracting Elements
>> A = [ 1 3 5 7
2468
3 6 9 12
4 8 12 16]
A=
1
2
3
4
3 5
4 6
6 9
8 12
7
8
12
16
>> B = A(1:3, 2:4)
B=
3 5 7
4 6 8
6 9 12
Example: Extracting Elements
>> C = A(1:3 , : )
>> D = A( : , 2:4)
C=
1 3 5 7
2 4 6 8
3 6 9 12
D=
3
4
6
8
5
6
9
12
7
8
12
16
Remember
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
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
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.