picture.m

这里面有很多有用的东东

function picture(P)
%PICTURE
%Graphs lines of systems of linear equations in 2 variables.
%Gives the points of intersection of the lines.
%Calling format: picture(A), where A is aug matrix of system.

%Copyright Gareth Williams, Stetson University 
[e-mail], http://www.stetson.edu/~gwilliam
%Accompanies "Linear Algebra with Applications" by Gareth Williams

hold off %default graphics window mode
disp(' ')

clf; %clear graphics window
subplot(111) %full graphics window

X=[];Y=[]; %Initial values of X and Y, empty matrices.

S=size(P);
%construct all the 2x3 submatrices Q in P 
%each 2x3 matrix Q corresponds to 2 lines
%rref of Qs gives pts of int of lines
%store these points in X and Y

for i=1:S(1,1)-1
 for j=i+1:S(1,1)
  Q=[P(i,1) P(i,2) P(i,3);P(j,1) P(j,2) P(j,3)]; 
  R=rref(Q);
  if ((R(2,2)~=0) & (R(2,3)~=0)) 
   X=[X R(1,3)];
   Y=[Y R(2,3)];
  end
 end
end
  
H=[X;Y];
I=[H [2;2]];
  
if I(1,1)==2
  disp ('no points of intersection,')
  disp ('parallel lines')
 else
  Points_of_Intersection=[X;Y]
end
disp(' ')
  
%Use G and W to compute the axes
G=[X 0];
W=[Y 0];

%compute & plot axes
figure(gcf)
V=[min(G)-1 max(G)+1 min(W)-1 max(W)+1];
axis(V);
A1=[min(G)-1 max(G)+1];
B1=[0 0];
C1=[min(W)-1 max(W)+1];
plot(A1,B1,':')
hold on
plot(B1,C1,':')
hold on
%X=[X X(1)];Y=[Y Y(1)];
%plot the lines corresp to rows of P
for i=1:S(1,1)
 if P(i,2)~=0
  X1=[min(G)-1 max(G)+1];
  Y1=[(P(i,3)-P(i,1)*(min(G)-1))/P(i,2) (P(i,3)-P(i,1)*(max(G)+1))/P(i,2) ];
 end 
 if P(i,2)==0
  if P(i,1)~=0
   X1=[P(i,3)/P(i,1) P(i,3)/P(i,1)];
   Y1=[min(W)-1 max(W)+1];
  end
 end
 plot(X1,Y1)
end

hold off