Strange Problem when i write same code in function

Question

moonman 2011-11-5

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/20432-strange-problem-when-i-write-same-code-in-function

This is my original code for Bisection Methods of Numerical Analysis

% A program in matlab  
% bisection method to find the roots of x^3+x^3+5=0
      xleft = -2.5 ;  xright = -1.5 ; n = 20   
      format longe
% Input:  xleft,xright = left and right brackets of the root
% n =  (optional) number of iterations; default: n =15
% Output:     x = estimate of the root
 if nargin<3, n=15; end   % Default number of iterations
 a = xleft;   b =xright;  % Copy orig bracket to local variables
fa  = a^3 + a^2 + 5;   % Initial values    
fb  = b^3 + b^2 +5;   % Initial values
fprintf('  k         a            xmid            b     f(xmid)\n');  
for k=1:n
     xm  = a + 0.5*(b-a);    % Minimize roundoff in the midpoint
     fm  = xm^3 + xm^2 +5;  % f(x) at midpoint
 fprintf('%3d  %12.8f   %12.8f  %12.8f   %12.3e\n',k,a,xm,b,fm);
    if sign(fm) == sign(fa)
         a  = xm;   fa = fm;
    else
         b  = xm;   fb = fm;
    end
end

**************************************** Now i am writing the same function by manually giving input

my main file is this one

input('Type value of a \n');
input('Type value of b\n');
bisec(a,b)

and my function file is this one

function bisec(a,b)   
      format longe
      n=20;
% Output:     x = estimate of the root
*fa  = a^3 + a^2 + 5;   % Initial values    
fb  = b^3 + b^2 +5;   % Initial values* _ITALIC TEXT_
fprintf('  k         a            xmid            b     f(xmid)\n');  
for k=1:n
     xm  = a + 0.5*(b-a);    % Minimize roundoff in the midpoint
     fm  = xm^3 + xm^2 +5;  % f(x) at midpoint
 fprintf('%3d  %12.8f   %12.8f  %12.8f   %12.3e\n',k,a,xm,b,fm);
    if sign(fm) == sign(fa)
         a  = xm;   fa = fm;
    else
         b  = xm;   fb = fm;
    end
end
end

But my results are comming different plz guide me

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Answer 1

Fangjun Jiang 2011-11-5

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https://ww2.mathworks.cn/matlabcentral/answers/20432-strange-problem-when-i-write-same-code-in-function#answer_26988

在 MATLAB Online 中打开

try

a=input('Type value of a \n');
b=input('Type value of b\n');

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moonman 2011-11-5

Thanks a lot Fangjun

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Answer 2

moonman 2011-11-5

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https://ww2.mathworks.cn/matlabcentral/answers/20432-strange-problem-when-i-write-same-code-in-function#answer_26990

Thanks a lot Fangjun u saved my time and problem is resolved Can u explain what is wrong in my input commands

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Walter Roberson 2011-11-5

You are not assigning the results of the input() to any variable.

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Answer 3

moonman 2011-11-5

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/20432-strange-problem-when-i-write-same-code-in-function#answer_26992

I also need one more help. I have to follow this instruction

write a separate function file called f.m that stores the definition of the function: for part (i) this will be function y=f(x) y=x.^3+x.^2+5; You can then use f(x) in your bisection M-file

Right now i am writing this function manually and it is shown in italic