1

Arrays

2. Common Operations1. Slicing2. Diminution3. Augmentation

1

ARRAY SLICINGUsed for:

Accessing more than one element of an arrayEliminating bad elements

22

Array Slicing

In general, when we “pull out” part of an array, we call that a “slice”. The range operator is frequently used when getting a slice.

% Pull out all elements in rows 1 and 2

% that are in columns 1 through 4

M1 = M(1:2, 1:4);

3

Real-life #1: Eliminating bad data In wind tunnels, the data is

obtained throughout the tunnel. However, data is usually

flawed around the walls, or far away form the object itself.

Given an array of pressure/temperature/or density obtained, only the ones far from the wall are kept for analysis!

4

ARRAY DIMINUTION

Making arrays smaller

Deleting an element

Deleting a row

Deleting a column

55

Pronounce:“Dim’ – min – yoo’ – shun”

Array Diminution

To eliminate the whole content, just re-define it as an empty-vector:

scores = []; %delete all scores

To eliminate a single value from a vector, you can either take a slice:

HighScores = [757, 65, -13, -89];HighScores = HighScores(1:3); %deletes last

%score

Or you can use the empty-vector:HighScores(4) = []; %removes 4th score

6

Example Diminution After analyzing data, you may wish to get rid of some data: in this

case, assign the empty brackets []

For example, get rid of the number 8 in b below:

7

This action changes the original vector and cannot be undone.

Array Diminution, cont. To eliminate an entire row/column:

1. Use the range operator, combined with2. the empty-vector

M = [1, 2, 3; 4, 5, 6];M(:, 1) = [] … Read it as:

QUESTION: Can we eliminate a single value from a matrix? M(2,2) = [] ?

No – because that would mean some rows or columns would have more values than others. 8

%”M , all-rows, 1stcolumn , delete!”

Real life#2 – similar example

9

Clearly, bad results on the walls…

10

Real life#2 – similar example

11

Real life#2 – similar example

12

Real life#2 – similar example

Suppose you want to delete the top now, since that is also a wall in the wind tunnel. What would be the command line?____________________________________

AUGMENTING AN ARRAY

1313

Array Augmentation, reviewAugmentation = “Adding to” = making an array bigger. For example:V = [1, 2, 3];

To augment more columns, it’s much like doing a running total or running product: to the current variable, perform an action:

V = [V, 4, 5, 6];

To augment with another row vector variable:V1 = [3, 4, 5];V2 = [6, 7, 8];V1 = [V1; V2];

To augment with a column vector variable:V1 = [6; 8; 9];V2 = [10; 20; 30];V1 = [V1, V2];

Makes a matrix!

Why did we use a comma? ________________

Result: _ [ _________________ ] _?

Result:

__ __ __. __ __ __.

Result:

__ __ .

__ __ __ __

14

Array Augmentation, reviewWorks for matrices, too:

M1 = [1, 2, 3; 4, 5, 6]; %original matrixM1 = [M1; 7, 8, 9]; % attach a row to M1M1 = [M1, [11, 2, 33; 44, 33, 22; 1, 0, 2]]

M1 =

1 2 3 11 2 33 4 5 6 44 33 22 7 8 9 1 0 2

Be sure that you augment with the correct number of rows / columns!

15

Extending an array

16

Array b does not have 4 columns… mmm… what will it do?

End of Presentation