the range i need for my graph plot is from -10 to 10,but it seems like i cant use a negative number.help?

clc;clear all;
next = 1;
while (next > 0)
    x = input(' Enter selected starting value ');
    A = zeros(20,1);
    B = zeros(20,1);
    for count = (-10:10)
        main = (5*x^3+15*x^2+x+10)*sin(x);
        diff = (15*x^2+30*x+1)*sin(x)+(5*x^3+15*x^2+x+10)*cos(x);
        A(count) = x;
        B(count) = main;
        x = x - main/diff;
        if(abs(main)<1e-7)
            break; 
        end
    end
    disp([' The converged value of x is      '  num2str(x) ]);
    disp([' The associated equation error is '  num2str(main) ]);
    disp([' The number of iterations taken = '  int2str(count) ]);
    semilogy( (1:count)', abs(B(1:count)) );
    next = input(' Compute again ? (yes=1, no=0) : ');
end

[dpb]

...
semilogy( (1:count)', abs(B(1:count)) );

Hint--

What's log(0)???

Answer 1

[Geoff Hayes]

Ernest - you seem to be only using your interval of [-10,10] for the indexing variable count, which is used to update elements in an array. So you are probably getting an error message indicating that you can only use logical or positive integers to access an array element. What is the intent behind using this variable? If you are only using it to define how many iterations of the for loop to do before assuming that a solution has been found then this should be switched to

 for count=1:20

     % etc

 end

[ernest]

Geoff-I actually need to make a plot of some specific given points to that function.Will it help if I were to show you the problem statement?

Answer 2

[Image Analyst]

Why not just use count = 1 : 21? Here, run my code. It has a few other improvements in there also:

clc;
close all;
clear all;
fontSize = 20;
promptMessage = sprintf('Do you want to Continue and try another x,\nor Quit?');
titleBarCaption = 'Continue?';
next = 1;
while (next > 0)
    x = input(' Enter selected starting value ');
    A = zeros(20,1);
    B = zeros(20,1);
    for count = 1:21
        main = (5*x^3+15*x^2+x+10)*sin(x);
        diff = (15*x^2+30*x+1)*sin(x)+(5*x^3+15*x^2+x+10)*cos(x);
        A(count) = x;
        B(count) = main;
        x = x - main/diff;
        if(abs(main)<1e-7)
            break; 
        end
    end
    disp([' The converged value of x is      '  num2str(x) ]);
    disp([' The associated equation error is '  num2str(main) ]);
    disp([' The number of iterations taken = '  int2str(count) ]);
    hFig = semilogy( (1:count)', abs(B(1:count)), 'LineWidth', 2);
  grid on;
  xlabel('Iteration Number', 'FontSize', fontSize);
  % Enlarge figure to full screen.
  set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
  
  % Ask if they want to try again.
  button = questdlg(promptMessage, titleBarCaption, 'Continue', 'Cancel', 'Continue');
  delete(gcf);
  if strcmpi(button, 'Cancel')
    break;
  end
end

[ernest]

the problem statement did specifically state that i need to plot values in the range of -10 to 10.

[Image Analyst]

Yeah, probably for x. The "count" is the iteration number, not the x value. You need to change the x value. Since it's your homework and I can't just do the whole assignment for you outright, I'll let you do that. Hint A(1) = A at your starting x value.