08. Use of octave language - control statements, drawing lines and other commands

One 、 Control statement

1.1 if … else sentence

• Basic grammar ：

  if expression
  statements
  elseif expression
  statements
  else
  statements
  end
  

• Octave There is no need to use parentheses in the condition () Cover up ( Of course, using parentheses will make the program more explicit

  了 ), and statments The program block does not need curly braces *{ }* Cover up .

• Octave We need to use end Command to identify if Of the statement end .. stay Octave in ,if…end and if…endif It's all legal . Use end It can make Octave Documents and Matlab file compatible

• image if Such control statements , Usually used in Octave In the script , It can also be used on the command line

• Octave Indentation has no effect

  >> a=1;b=2;
  >> if a>b
  c=5
  else
  c=6
  end
  c = 6
  >>
  

1.2 switch sentence

• If multiple if/elseif Statement to select from multiple options , Use switch Better statement

• The sentence format is ：

  switch x
  case x1
  statements
  case x2
  statements
  otherwise
  statements
  end
  

  • stay switch In the sentence ,x The value of is in sequence with case Comparison of values in , If you find an equal value , Corresponding statements Will be carried out .
  • Notice in Octave in switch The statement is not like C++ That needs to be break sentence ——Octave Found matching options in , perform statements And exit swith sentence . If there is no matching item ,otherwise The statements in are executed .

  >> a=1;
  >> switch a
  case 0
    disp('a is zero')
  case 1
    disp('a is one')
  otherwise
    disp('a is not a binary digit')
  end
  a is one
  >>
  

1.3 for sentence

• stay Octave You should use more Use the calculation of vectors instead of for loop , Because usually for The cycle will be much slower . But sometimes for Circulation is inevitable .

• for The syntax of circular statements ：

  for variable=vector
  statements
  end
  

  • among vector Contains traversal values . Usually , The vector is expressed with a colon Write

  >> for n=1:5
  sum=0;
  sum=sum+n;
  end
  >> disp(sum)
  5
  >>
  

  • Be careful ： To at for Adding a semicolon to the loop can avoid octave Output each time the loop body is executed sum Value

1.4 while sentence

• If you don't know how many cycles you need to execute , Instead, it knows to end the cycle when a condition is met .Octave Medium while Statement can be realized Now this function

• while The basic format of the statement

  while expression 
  statements 
  end
  

Two 、 Simple drawing

• Octave By calling another open source software GNUPLOT To achieve very rich drawing functions

• The most basic drawing command is plot(x,y), among x,y They are horizontal and vertical axis data respectively

• Use plot() After that, the program will open a new window to display the picture

  

2.1 plot() Function details

• stay Octave You can be in plot Add more options to the command to specify the style of graphic lines

  • for example , Use red and circles to draw the previous picture , Input plot(angles,y,'ro');

    The last parameter is the string that specifies the line style

• adopt title,xlabel and ylabel Name the picture and add a title and xy Axis name ：

  title('Graph of y=sin(x)') ;

  xlabel('Angle');

  ylabel('Value')

• grid on Command to add gridlines to the picture ：

  

• plot Color and style options in the command （ Can also pass help plot see ）

  

2.2 Draw multiple curves

• Multiple curves can be drawn on a graph . By means of plot The... Of each curve is given in order in the command x and y vector

• legend () The command adds the corresponding legend to the picture

• By using hold Command to implement . Use this command , We can realize the presentation on the same picture by two （ Multiple ） plot Command line drawn

  • hold on, Continue to draw lines on the graph
  • hold off, This function will be turned off .

  

2.3 Draw multiple images

• Multiple pictures can be passed figure Command to control , Enter... On the command line figure So next plot The command will be drawn in the newly created window ,

• If you want to return to the previous picture , Input figure(1)

  

2.4 Save and print pictures

• Octave There is no mouse or shortcut key operation to realize the printing operation of pictures .

• Octave Provides print command To print pictures to the default printer .help print The command gives all print The options provided by the command

• print('graph1.eps','-deps') To save the current image to a eps Documents or

• print('graph1.png','-dpng') Save the current image as a png file .

3、 ... and 、 Advanced drawing

• Octave Its drawing function is not only to draw a simple curve in a two-dimensional rectangular coordinate system . By calling GNUPLOT, It can draw Bar chart ,3D The surface of the , Contour map and polar coordinate map, etc

3.1 Draw a subgraph

• To create multiple pictures in the same window, go through subplot Command implementation . This command can separate the window into a series of sub windows

• Its basic grammar is subplot(row,columns,select)

  • among subplot Parameter specifies the sequence of the current drawing in the sub window
  • The sequence number of the sub window is from top to bottom , The order increases from left to right

  

3.2 3D drawing

• Octave It also provides a variety of 3D Data visualization tool , The simplest 3D The drawing implementation is

  plot3 command

• This command accepts input by x,y and z Axis of the data , Draw points or lines

• stay 3D In drawing xlabel and ylabel Use as usual , And new zlabel The order can be z Axis naming . And drawing 3D Picture time , The option of style of table line can also be used .

  

• Adjust the perspective ：

  • If you want to change 3D Perspective of drawing results , The simplest way is to use the mouse （ Click and drag with the mouse to rotate the image , See table for specific operation options ）.

  • and Octave Of view Commands can quantitatively determine a specific perspective ( Use help view Get more Help information )

    Mouse action	operation
    LMB+ motion +LMB + motion	Shift perspective Transfer axis
    Shortcut key	operation
    →	Right hand
    ↑	Topspin
    ←	left-handed
    ↓	Downward rotation

3.3 3D drawing —— Draw surfaces

• Define a binary function f(x, y), The surface of this function can be represented by a series of Octave Draw . First , To initialize a grid point , use meshgird Command implementation ：

• Such as drawing ：f(x, y) = (x− 3)^2 − (y− 2)^2

  

Four 、 Other commands

4.1 Load and save data

• When you quit Octave When , You will lose the variables you created . If you need to quit halfway through your work Octave, So you can To save the data of the current session and reload it later

• save anyname Store the variables in the entire workspace in the current directory with a name of anyname.mat In the file of ( Be careful .mat yes octave Automatically added ). So you can quit Octave, Then restart Octave Program , By input load anyname

  The previously saved namespace will be reloaded , And start working again from where you left off

• Octave You can also import data from files

• You can load or save specific changes The amount . The format is ：

  save filename var1 var2 ...

  • for example , If you want to store deg Variable , Then you can enter save degconv deg This will deg Variables are stored in a file named degconv.mat In the file of , You can reload variables with the following command load degconv

4.2 Repeat the previous order

• Octave It will record the commands you have entered in a session , And you can pass In the direction key ↓ and ↑ To view the previously entered command ( The most recent commands are at the top ). If you want to repeat one of these commands , You just need to use the arrow keys to find the command and press enter .
• Once an executed command is recalled , You can edit it before executing it again . You can use it. ← and → Move the cursor Go in parallel Line corresponding Edit operation

4.4 Get help

• If you don't know someone Octave Command function or you need to find a specific function , have access to help Order to see help

  help commandname

• If you don't know the specific function name , From the command line Input help -i,Octave A list of topics for their help will be given

• Details about a help topic can be obtained by moving the cursor to the topic and entering . You can go through Up,Next, Previous Wait for shortcut keys to browse the corresponding topic . Input ‘q’ To exit the help system and return to octave Command line

4.5 Cancel a command

• Ctrl-C ： Abort the program and return to the command prompt interface

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