Why is my plot printing dots instead of a curve???

Question

coy 2014-4-20

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/126401-why-is-my-plot-printing-dots-instead-of-a-curve

评论： Star Strider 2014-4-20

My code is supposed to print a nice curve instead it is printing dots. I have not specified any dots in my loglog line command and I have not set any default to be dots.

sprintf('      x            numerical               actual                       error')
for i =1:numsteps
    x = a:h:b;
    f = -h^2 * rfun(x);
    f(1) = 2*h*lambda;
    f(end) = 2*h*rho;
    p = -1 -0.5*h.* pfun(x(2:end));
    P = -1 + 0.5*h.*pfun(x(1:end-1));
    q =  2 + h^2* qfun(x);
    A = gallery('tridiag',p,q,P);
    A(1,1) =-3;A(1,2)=4; A(1,3) =-1;
    A(end,end)= 1; A(end,end-1) = -4; A(end,end-2) = 3;
    y = A\f';
    hold all
    actual = exp(x') - (1/3)*x'.^2.*exp(x');
    error =abs(y - actual);
%     loglog(h,error);
     set(gca, 'XScale', 'log', 'YScale', 'log');
   while i ==1
        n = 1/h;
        disp(sprintf('h = %.4f',h'))
        for j =3:2:n
            fprintf('%3.6f       %3.6f             %3.6f                %3.6f\n',x(j),y(j),actual(j),error(j))
        end
          loglog(h,error,'k-');
          set(gca, 'XScale', 'log', 'YScale', 'log');
       break
      end
      while i ==2
          n =1/h;
          disp(sprintf('h = %.4f',h))
          for j = 5:4:n-2
              fprintf('%3.6f       %3.6f             %3.6f                %3.6f\n',x(j),y(j),actual(j),error(j))
              end
          loglog(h,error,'m-')
      break
      end
      while i ==3
          n =1/h;
          disp(sprintf('h = %.4f',h))
          for j = 9:8:n-2
                   fprintf('%3.6f       %3.6f             %3.6f                %3.6f\n',x(j),y(j),actual(j),error(j))
          end
         loglog(h,error,'g-')
      break
      end
      while i ==4
           n =1/h;
          disp(sprintf('h = %.4f',h))
          for j = 17:16:n-2
                   fprintf('%3.6f       %3.6f             %3.6f                %3.6f\n',x(j),y(j),actual(j),error(j))
          end
          loglog(h,error,'b-')
       break
      end
       h=h/2;
end

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Walter Roberson 2014-4-20

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/126401-why-is-my-plot-printing-dots-instead-of-a-curve#answer_133928

Your "h" must be a scalar or else you would have received error messages or warning messages that you did not mention.

You have calls of the form loglog(h,error,'b-') where the color changes, and there is evidence that error is a vector. With h being a scalar, all of those error values are going to be drawn at the same x position. That is going to produce anywhere from a dot to a vertical line -- with the vertical line probably retracing itself several times (the error values are not sorted.) If any of the error values are negative or 0, they will not appear on a loglog() plot.

Are you sure the above is what you desire??

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

Answer 2

Star Strider 2014-4-20

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/126401-why-is-my-plot-printing-dots-instead-of-a-curve#answer_133932

在 MATLAB Online 中打开

I did not run your code, but I believe your problem is due to your plotting your ‘error’ variable in your loop.

You only calculate error once:

error =abs(y - actual);

and you don’t assign it as an array.

I suggest you change it to:

error(i) =abs(y - actual);

and then plot it once at the end of your ‘i’ loop. (That means taking the loglog statements out of your loop.)

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

Answer 3

Image Analyst 2014-4-20

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/126401-why-is-my-plot-printing-dots-instead-of-a-curve#answer_133936

DO NOT USE error AS THE NAME OF A VARIABLE! It is a built in function and you just wiped it out by redefining it as one of your variables. Call it "theError" or "residuals" or something else, but never "error".

Also, we can't run your code because you didn't give all the information, like values for numsteps, a, h, b, rfun, etc.