Fondamenti di Informatica

Transcription

1 Fondamenti di Informatica Data abstraction: Vectors Prof. Emiliano Casalicchio

2 Objectives This lecture discusses the basic calculations involving rectangular collections of numbers in the form of vectors. For this collections, you will learn how to: Create them Manipulate them Access their elements Perform mathematical and logical operations on them 1-2

3 Concept: Using Built-in Functions n In this chapter we will see the use of some of the functions built into MATLAB. n At the end of each chapter that uses built-in functions, you will find a summary table listing the function specifications. n For help on a specific function, you can type the following: >> help <function name>! n For example: >> help sqrt! SQRT Square root.! SQRT(X) is the square root of the elements of X. Complex results are produced if X is not positive.!! 1-3

4 Concept: Data Collections This section considers two very common ways to group data: in arrays and in vectors. Data Abstraction allows us to refer to groups of data collectively: all the temperature readings for May or all the purchases from Wal-Mart. We can not only move these items around as a group, but also perform mathematical or logical operations on these groups, e.g.: compute the average, maximum, or minimum temperatures for a month A Homogeneous Collection is constrained to accept only items of the same data type in this case, they will all be numbers 1-4

5 MATLAB Vectors Individual items in a vector are usually referred to as its elements. Vector elements have two separate and distinct attributes that make them unique in a specific vector: their numerical value and their position in that vector. For example, the individual number 66 is the third element in this vector. Its value is 66 and its index is 3. There may be other items in the vector with the value of 66, but no other item will be located in this vector at position

6 Vector Manipulation We consider the following basic operations on vectors: Creating a Vector Determining the size of a Vector Extracting data from a vector by indexing Shortening a Vector Mathematical and logical operations on Vectors 1-6

7 Creating a Vector Constant Values n Entering the values directly, e.g.!!a = [2, 5, 7, 1, 3] n Automatically entering values e.g. as a range of numbers as equally distributed in a linear space as random, zeros, ones 1-7

8 Creating a Vector Entering the values as a range B = [v 1, v 2,, vn]={vi v min <=v i <=v max,v i =v i-1 +K, v 1 =v min, i=2,..}! dove k è un intero costante. Ad esempio K=3, vmin=1 e vmax=20 B=[1,4,7,10,13,16, 19]! come fare? Proviamo a definire un algoritmo: 1. leggi v min, v max e K 2. i=1, v i =v min 3. i=i+1 4. se v i-1 +K > v max allora termina 5. altrimenti v i =v i-1 +K 6. vai a passo 3. In the following lectures you will learn how to implement in Matlab this algorithm and more 1-8

9 Creating a Vector Entering the values as a range (2) n In Matlab B = {v i v min <=v i <=v max,v i =v i-1 +K, v 1 =v min, i=2,..} =! = v min :K:v max! ad esempio B = 1:3:20! = [1,4,7,10,13,16, 19]! 1. leggi v min, v max e K 2. i=1, v i =v min 3. i=i+1 4. se v i-1 +K > v max allora termina 5. altrimenti v i =v i-1 +K 6. vai a passo

10 Creating a Vector as equally distributed in a linear space B = [v 1, v 2,, v N ]={N equally spaced points between X min and X max }! ad esempio N=11, X min =0 e X max =20 B=[ ]! n Matlab offers a built in function to do that C = linspace (Xmin, Xmax, N)! Homework: To design an algorithm that implements linspace 1-10

11 Creating a Vector - filled with 0, 1, random values B = [v 1, v 2,, v N ];!!v i =0 for i=1,..,n!!v i =1 for i=1,..,n!!v i =x k where x k is a pseudorandom value uniformly or normally distributed! All the cases can use the same algorithm! 1. i=0; 2. i=i+1; 3. v i =x; 4. if i=n then exit; 5. go to 2; x = 0 or 1 or Normal() or Uniform() 1-11

12 Creating a Vector - filled with 0, 1, random values Matlab offers built in function implementing the previous algorithm! %B = [v 1, v 2,, v N ];! B=zeros(1,n) %v i =0 for i=1,..,n! B=ones(1,n) %v i =1 for i=1,..,n! B=rand(1,n) %v i =x k where x k is a pseudorandom value!! %uniformly distributed! B=randn(1,n) %v i =x k where x k is a pseudorandom value!! %normally distributed! 1-12

13 Size of Vectors and Arrays Problem: we have a vector and we want to know the number of its elements MATLAB provides two functions to determine the size of arrays in general (a vector is an array with one row): the function size(a) when applied to the array A returns vector containing two quantities: the number of rows and the number of columns The function length(a) returns the maximum value in the size of an array; for a vector, this is its length. 1-13

14 Indexing a Vector n The process of extracting values from a vector, or inserting values into a vector n Syntax: v(index) returns the element(s) at the location(s) specified by the vector index. v(index) = value replaces the elements at the location(s) specified by the vector index. n The indexing vector may contain either numerical or logical values >> A=1:3:12! A =! ! >> A(1)! ans =! 1! >> B=[2,4]! indexing vector B =! 2 4! >> A(B)! ans =! 4 10! 1-14

15 Numerical Indexing n The indexing vector may be of any length n It should contain integer (non-fractional) numbers n The values in the indexing vector are constrained by the following rules: For reading elements, all index values must be 1 <= element <= length(vector) For replacing elements, all index values must be 1 <= element 1-15

16 Replacement Rules 1. Either: All dimensions of the blocks on either side of the replacement instruction must be equal, or There must be a single element on the RHS of the replacement 2. If you replace beyond the end of the existing vector, the vector length is automatically increased. Any element not specifically replaced remains unchanged. Elements beyond the existing length not replaced are set to 0. >> A=1:3:12! A =! ! >> B=[2,4];! >> A(B)=[0,0]! A =! ! >> A(4)=99! A =! ! >> A(7)=99! A =! ! 1-16

17 Logical Indexing n The indexing vector length must be less than or equal to the original vector length n It must contain logical values (true or false) n Access to the vector elements is by their relative position in the logical vector When reading elements, only the elements corresponding to true index values are returned When replacing elements, the elements corresponding to true index values are replaced n Beware logical vectors in Matlab echo in the Command window as 1 or 0, but they are not the same thing. >> a=true! a =! 1! >> a=false! a =! 0! >> 13>10! ans =! 1! >> mask=[true,false,true,true]! mask =! ! >> A(mask)! ans =! ! >> A(mask)=[66,66,66];! >> A! A =! ! ! 1-17

18 Shortening an Array n Never actually necessary. It is advisable to extract what you want by indexing rather than removing what you don t want. Can lead to logic problems when changing the length of a vector n Accomplished by assigning the empty vector ([]) to elements of a vector, or to complete rows or columns of an array. 1-18

19 Operating on Vectors Three techniques extend directly from operations on scalar values: Arithmetic operations Logical operations Applying library functions Two techniques are unique to arrays in general, and to vectors in particular: Concatenation Slicing (generalized indexing) 1-19

20 Operators precedence n Operator description and precedences 20

21 Arithmetic operations In the Command window, enter the following: >> A = [ ];! >> A + 5! ans =! ! >> A.* 2! ans =! ! >> B = -1:1:3! B =! ! 1-21

22 Arithmetic operations (continued)! >> A.* B % element-by-element multiplication! ans =! ! >> A * B % matrix multiplication!!!??? Error using ==> mtimes! Inner matrix dimensions must agree.! >> C = [1 2 3]! C =! 1 2 3! >> A.* C % A and C must have the same length!??? Error using ==> times! Matrix dimensions must agree.! 1-22

23 Logical operations >> A = [ ];! >> B = [ ];! >> A >= 5! ans =! ! >> A >= B! ans =! ! >> C = [1 2 3]! >> A > C!??? Error using ==> gt! Matrix dimensions must agree. 1-23

24 Logical operators: NOT, AND, OR NOT, - Truth tables AND, & OR, 24

25 Logical operations (continued) >> A = [true true false false];! >> B = [true false true false];! >> A & B! ans =! ! >> A B! ans =! ! >> C = [1 0 0]; % NOT a logical vector! >> A(C) % yes, you can index logical vectors, but...!??? Subscript indices must either be real positive integers or logicals.! 1-25

26 A Footnote: the find function n Continuing the code above: n >> C = find(b)! ans =! [1 3]! The find(...) function consumes a logical vector and returns the numerical indices of the elements of that vector that are true. 1-26

27 Applying library functions All MATLAB functions accept vectors of numbers rather than single values and return a vector of the same length. Special Functions: sum(v) and mean(v) consume a vector and return a number min(v) and max(v) return two quantities: the minimum or maximum value in a vector, plus the position in that vector where that value occurred. round(v), ceil(v), floor(v), and fix(v) remove the fractional part of the numbers in a vector by conventional rounding, rounding up, rounding down, and rounding toward zero, respectively. 1-27

28 Concatenation n MATLAB lets you construct a new vector by concatenating other vectors: A = [B C D... X Y Z]! where the individual items in the brackets may be any vector defined as a constant or variable, and the length of A will be the sum of the lengths of the individual vectors. A = [ ]! is a special case where all the component elements are scalar quantities. 1-28

29 Slicing (generalized indexing) n A(4) actually creates an anonymous 1 1 index vector, 4, and then using it to extract the specified element from the array A. n In general, B(<rangeB>) = A(<rangeA>)! where <rangea> and <rangeb> are both index vectors, A is an existing array, and B can be an existing array, a new array, or absent altogether (giving B the name ans). The values in B at the indices in <rangeb> are assigned the values of A from <rangea>. 1-29

30 Rules for Slicing Either the dimensions of <rangeb> must be equal to the dimensions of <rangea> or <rangea> must be of size 1 If B did not exist before this statement was implemented, it is zero filled where assignments were not explicitly made If B did exist before this statement, the values not directly assigned in <rangeb> remain unchanged 1-30

31 Representing Mathematical Vectors n An unfortunate clash of names n Vectors in mathematics can be represented by Matlab vectors The first, second and third values being the x, y and z components Matlab vector addition and subtraction work as expected. Matlab magnitude and scaling works as expected. Dot product is just the sum of A.* B Cross product has a Matlab function 1-31

32 Engineering Example Forces and Moments n So given a pair of forces A and B acting at a point P, find: The resultant force, and The moment of that resultant about the origin 1-32

33 Vector Solution clear! clc! PA = [0 1 1]! PB = [1 1 0]! P = [2 1 1]! M = [4 0 1]! % find the resultant of PA and PB! PC = PA + PB! % find the unit vector in the direction of PC! mag = sqrt(sum(pc.^2))! unit_vector = PC/mag! % find the moment of the force PC about M! % this is the cross product of MP and PC! MP = P - M! moment = cross( MP, PC )!! 1-33

34 n Homework exercise of the book. 34