Matlab | those matlab tips you have to know (I)

1： Preset symbolic variable types

Suppose we write the following code ：

syms x
f=x^3+x^2+9*x+9;
solve(f==0)
 At this time, the solution result is ：

ans =
-1
-3i
3i

But if you preset x It's a real number ：

syms x
assume(x,'real')
f=x^3+x^2+9*x+9;
solve(f==0)

The result is ：

ans =
-1

There are many other presets waiting for you to study ：

Assume ‘x’ is	Syntax
real	assume(x,‘real’)
rational	assume(x,‘rational’)
positive	assume(x,‘positive’)
positive integer	assume(x,{‘positive’,‘integer’})
less than -1 or greater than 1	assume(x<-1)
an integer from 2 through 10	assume(in(x,‘integer’) & x>2 & x<10)
not an integer	assume(~in(z,‘integer’))
not equal to 0	assume(x ~= 0)
even	assume(x/2,‘integer’)
odd	assume((x-1)/2,‘integer’)
from 0 through 2π	assume(x>0 & x<2*pi)
a multiple of π	assume(x/pi,‘integer’)

2：if turn switch ( Logical value as input variable )

Suppose you write the following code ：

score=68
if 90<=score&&score<=100
    disp(' good ');
elseif 70<=score&&score<90
    disp(' good ');
elseif 60<=score&&score<70
    disp(' commonly ');
elseif score<60
    disp(' fail, ');
end

There is a way to avoid a lot of if else：

Is to use logical value as input switch:

score=68
switch true
    case 90<=score&&score<=100
        disp(' good ');
    case 70<=score&&score<90
        disp(' good ');
    case 60<=score&&score<70
        disp(' commonly ');
    case score<60
        disp(' fail, ');
end

3：matlab After fitting the curve, it is converted into an anonymous function

x=[2 2.5 3 3.5 4 4.5 5 5.5 6];
y=[41 38 34 32 29 28 25 22 20];
p=polyfit(x,y,3);
f=matlabFunction(poly2sym(p))

xx=2:.1:6;
plot(xx,f(xx),'LineWidth',2)
grid on

f =
With the following values function_handle:
@(x)x.(-8.57e+2./5.4e+1)+x.^{2.*(2.56e+2./9.9e+1)-x.}3.(5.8e+1./2.97e+2)+7.05e+2./1.1e+1

4： View the functions contained in each module faster

If you haven't read Miss Liu's article , I may never click on this little triangle , This triangle is too detailed, Ba hiahiahia picture , After clicking on it, an electronic dictionary will appear help A collection of documents :

5：sub2ind And ind2sub

Suppose we want to put all zero matrix number (2,3) Elements and (3,2) The first element becomes 1, If you write the following code ：

A=zeros(4,4);
A([2,3],[3,2])=1

A =
0 0 0 0
0 1 1 0
0 1 1 0
0 0 0 0

We will find that not only two elements have been changed , But we can pass sub2ind The function changes the bivariate index to the univariate index , for example ([4,4] Is the matrix size )：

A=zeros(4,4);
A(sub2ind([4,4],[2,3],[3,2]))=1

A =
0 0 0 0
0 0 1 0
0 1 0 0
0 0 0 0

ind2sub Function can be used to reduce the number of nested loops , A simple example , Suppose you write the following code ：

for i1=1:5
    for i2=1:3
        for i3=1:2
            disp([i1,i2,i3])
        end
    end
end

It can be rewritten as the following code ( Not much use and need to pay attention to the order , Far less important than the previous one )：

sz=[5,3,2];
for i=1:prod(sz)
    [i3,i2,i1]=ind2sub(sz(end:-1:1),i);
    disp([i1,i2,i3])
end

6： Brush the data

No one has not ordered this small brush yet ：

Click to highlight some data , Then right click on the highlighted data , You can export the data , Color change , Or delete the data directly , Is there any interactive graphics killer ：

7： The size is 0 Matrix

For example, I want to create a [x1,y1;x2,y2;x3,y3;… …] Such a matrix , Add new points at the end of each round , But the number of points added each time is unknown , The number of times to add is unknown , We might as well create a 0xn The size of the matrix continues to add new elements to the back ：

%  establish 0 That's ok 2 Column matrices 
P=zeros(0,2);
%  Cyclic random 5-10 Between random wheels 
for i=1:randi([5,10],[1,1]) 
    %  Increase randomly each time 2-5 Row random number 
    P=[P;rand(randi([2,5],[1,1]),2)]
end

8： Any base to base conversion

It is to convert to decimal system first , Then convert to other base numbers ：

% 12 Turn into the system 16 Base number 
bs1=12;
bs2=16;
%  Original character 
baseStr='1B';

%  transformation 
decStr=base2dec(baseStr,bs1);
result=dec2base(decStr,bs2)
% result='17'

So there is a simpler RGB Color 、16 Binary code mutual conversion function :

RGB It's worth turning 16 Binary code function ：

function HEX=RGB2HEX(RGB)
    hexVec=dec2base(RGB,16)';
    HEX=['#',hexVec(:)'];
end

call ：

hex=RGB2HEX([251,255,250])
% hex = '#FBFFFA' 

16 Binary code conversion RGB Value function ：

function RGB=HEX2RGB(HEX)
    hexVec=reshape(HEX(2:end),2,[])';
    RGB=base2dec(hexVec,16)';
end

call ：

rgb=HEX2RGB('#FBFFFA')
% rgb = [251,255,250]

9： Virtual plane drawing

MATLAB Support virtual plane drawing , Is the real part correspondence X Axis coordinates , The imaginary part corresponds to Y Axis coordinates , Using virtual plane drawing, you can draw implicit functions with only one array , for instance , Draw a circle with a virtual plane drawing ：

t=0:pi/100:2*pi;
plot(exp(t.*1i))

10：MATLAB High precision calculation

commonly MATLAB If the difference between the two numbers is less than eps, It is considered that the value is the same , For example, the following two judgments MATLAB Will think it is right ：

1==1+eps/2
1==1+1e-20

ans =
logical
1
ans =
logical
1

eps The value of is 2.2204e-16, As long as we set the accuracy higher, the above problems will not occur , for example ：

digits(50)
logical(vpa(1)==vpa(1)+vpa(1e-20))

ans =
logical
0

Another example , I want to show the front of PI 100 Decimal place ：

digits(100)
pi100=vpa(pi)

pi100=3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117068