机械振动加州大学University of CaliforniaMATLAB Commands要点

MATLAB Commands for Fourier Series

Relational and Logical Operations1

∙Relational Operators

∙Logical Operators

-Logical operators provide a way to combine or negate relational expressions.

-The logical operators make it easy to generate arrays representing signals with discontinuities or signals that are composed of segments of other signals. The

basic idea is to multiply those values in an array that you wish to keep with ones, and multiply all other values with zeros. For example,

» x = linspace(0,10,100); %create data

» y = sin(x); %compute sine

» z = (y >= 0).*y; %set negative values of sin(x) to zero

» z = z + 0.5*(y < 0); %where sin(x) is negative add 1/2

» z = (x <= 8) .*z; %set values past x = 8 to zero

» plot(x,z)

» xlabel('x'), ylabel('z = f(x)')

Control Flow1

FOR LOOPS

-It allows a group of commands to be repeated a fixed, predetermined number of times. The general form of a For Loop is

for x = array

(commands)

end

-For example,

» for n = 1:10

x(n) = sin(n*pi/10);

end

» x

x =

Columns 1 through 7

0.3090 0.5878 0.8090 0.9511 1.0000 0.9511 0.8090

Columns 8 through 10

0.5878 0.3090 0.0000

-Any valid MATLAB array is acceptable in the For Loop:

» data = [3 9 45 6;7 16 -1 5]

data =

3 9 45 6

7 16 -1 5

» for n = data

x = n(1) – n(2)

end

x =

-4

x =

-7

x =

46

x =

1

-For Loops should be avoided whenever there is an equivalent array approach to solving a given problem. For example, the first example can be rewritten as » n = 1:10;

» x = sin(n*pi/10)

x =

Columns 1 through 7

0.3090 0.5878 0.8090 0.9511 1.0000 0.9511 0.8090

Columns 8 through 10

0.5878 0.3090 0.0000

∙IF-ELSE-END CONSTRUCTION

-The simplest If-Else-End construction is

if expression

(commands)

end

-In case where there are two alternatives, the If-Else-End construction is

if expression

(commands evaluated if True)

else

(commands evaluated if False)

end

-When there are three or more alternatives, the If-Else-End construction takes the form

if expression1

(commands evaluated if expression1 is True)

elseif expression2

(commands evaluated if expression2 is True)

elseif expression3

(commands evaluated if expression3 is True)

elseif …

.

.

.

end

Function M-files1

∙M-File Construction rules:

-The function M-file name and the function name that appears in the first line of the file should be identical. In reality, MATLAB ignores the function name in the first line and executes function based on the file name stored on disk.

-Function M-file names can have up to 31 characters.

-Function names must begin with a letter

-The first line of a function M-file is called the function declaration line and must contain the word function followed by the calling syntax for the function in its

most general form. The input variables identified in the first line are variables

local to the function. The input variables contain data passed to the function and the output variables contain passed back from the function. It is not possible to

pass data back through the input variables.

-The first set of contiguous comment lines after the function declaration line are the help text for the function.

- A function M-file terminates after the last line in the file is executed or whenever

a return statement is encountered.

- A function can abort operation and return control to the command window by calling the function error. This function is useful for flagging improper function

usage:

if length(val) > 1

error(‘VAL must be a scalar.’)

end

∙Input and Output Arguments:

-Function M-files can have zero input and zero output arguments.