gjelim.m

这里面有很多有用的东东

function gjelim(P)
%GJELIM
%Gauss-Jordan elimination.
%Options: rational number format 
%         count of operations 
%         all steps
%Calling format: gjelim(A)

%Can use given tolerance of 1e-20
%or change to own tolerance
%MATLAB computes to about 16 decimal digits

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

%Initial values
adds=0;totadds=0;mults=0;totmults=0;swaps=0;totswaps=0;
ops=[0];

%hold off 
%default graphics window mode
tol=1e-20;
[n,m]=size(P);
format compact

disp(' ')
h=input('Rational numbers? y/n: ','s');
q=input('Count of operations? y/n: ','s');
g=input('All steps? y/n: ','s');

disp(' ')
disp('initial matrix')
if h=='y' 
  disp(rats(P))
 else 
  disp(P)
end

if g=='y';
 disp('[press Enter at each step to continue]')
 disp(' ')
 pause
end

%find a pivot
j=1;
for i=1:n,
 if j <= m
  found=0;
  if abs(P(i, j)) <= tol %end is in line 101 
    while (found == 0) %over 2 spaces since 1st part of
                       % if-else-end, lines 99,101 
%search for a leading one and interchange rows if necessary
     for s=i:n,
      if (abs(P(s, j)) > tol) 
	    if  (found == 0)
        found=1;
        if s~=i
         for r=j:m,
          temp=P(i, r);
          P(i, r)=P(s, r);
          P(s, r) = temp;
         end
         swaps = m-j+1;
         totswaps = totswaps + swaps;
         if g=='y'; %allsteps
          disp('swap rows')
          if h=='y' 
            disp(rats(P))
           else 
            disp(P)
          end
          if q=='y'
           disp('element swaps:')
           disp(swaps)
          end
          disp('--------------------')
          pause
         end %allsteps
        end
       end
      end
     end

     if (found==0) 
      if (j <= m)
       j = j + 1;
      end
     end

     if j>m  
      found=1;
     end 
    end  %of while loop, line 52 
    if  j > m
     found = 0;
    end
   else
    found = 1;
  end  %starts line 51

%normalize leading element in row changing the rest of the row accordingly
  if found == 1 
   k=i;
   if (P(k, j) ~= 1) 
    if (abs(P(k, j)) > tol)
     y = P(i, j);
     for l=j:m,
      P(k, l) = P(k, l)/y ;
     end
     mults = m-j;
     totmults = totmults + mults;
     if g=='y'; %allsteps
      disp('normalize')
      if h=='y' 
        disp(rats(P))
       else 
        disp(P)
      end
      if q=='y'
       disp('multiplications:')
       disp(mults)
      end
      disp('--------------------')
      pause
     end %allsteps
    end
   end
   for r=1:n,
    if (abs(P(r, j)) >tol) 
     if (r ~= i)
      z=P(r, j);
      for c=j:m, 
       P(r, c)=P(r, c) - z * P(i, c);
      end
      adds = m-j;
      mults = m-j;
      totadds = totadds + adds;
      totmults = totmults + mults;
      if g=='y'; %allsteps
       disp('create zero')
       if h=='y' 
         disp(rats(P))
        else 
         disp(P)
       end
       if q=='y'
        disp('additions, multiplications:')
        ops=[adds mults];
        disp(ops)
       end
       disp('--------------------')
       pause
      end %allsteps
     end
    end
   end
  end

  j = j + 1;

 end
end %end i loop

%print out final matrix
disp('-reduced echelon form-')

if h=='y' 
   disp(rats(P))
else 
   disp(P)
end

if q=='y'
 disp('Total additions, multiplications, element-swaps:')
 ops=[totadds totmults totswaps];
 disp(ops)
 disp('--------------------')
end

disp(' ')
format loose