How do I create a function and run it.

Question

Jay Gourley 2017-2-3

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/323155-how-do-i-create-a-function-and-run-it

编辑： Stephen23 2017-2-3

采纳的回答： Richard Zappulla

在 MATLAB Online 中打开

I don't know anything about MATLAB beyond how to create a new script file and type stuff into it.

I copied the following code from a course handout. So I assume it is correct.

    %Program 1.1 Bisection Method
    %Computes approximate solution of f(x)=0
    %Input: function handle f; a,b such that f(a)*f(b)<0,and tolerance tol
    %Output: Approximate solution xc
    xc=bisect(f,a,b,tol)
    if sign(f(a))*sign(f(b)) >= 0
        error('f(a)f(b)<0 not satisfied!') %ceases execution
    end
    fa=f(a)
    fb=f(b)
    while (b-a)/2>tol
        c=(a+b)/2;
        fc=f(c);
        if fc == 0  %c is a solution, done        
            break
        end
    end
    if sign(fc)*sign(fa)<0  %a and c make the new interval
        b=c;fb=fc;
    else                    %c and b make the new interval
        a=c;fa=fc;
    end
    xc=(a+b)/2;

It is supposed to solve a single-variable equation by bisection. I understand the bisection logic and the function flow logic seems obvious to me.

I want to give it a function, say x^5+x=1, and have it bracket in on x and get something close to 0.754877666. What do I type into the MATLAB command window to do this?

Question 2. Where do I find answers to simple questions like this? Is there a list of simple examples somewhere? I have gone through the "onramp" stuff and read the MATLAB Primer, which took hours, without learning how to just pass some variables to a pre-defined function.

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Stephen23 2017-2-3

编辑：Stephen23 2017-2-3

"Question 2. Where do I find answers to simple questions like this?"

The MATLAB documentation is the most readable, browsable documentation of any language that I have seen or used. It is organized by topic, has links everywhere to related topics and functions, and gives working examples. The introductory tutorials are a good place to start learning fundamental MATLAB concepts:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

Ten minutes practicing using the contents (LHS of the page) will be time well spent: click up a level, see the topics, get used to navigating around!

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

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Answer 1

Richard Zappulla 2017-2-3

3
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/323155-how-do-i-create-a-function-and-run-it#answer_253156

在 MATLAB Online 中打开

Every function in MATLAB uses the following structure:

 function [ouput_vars] = foo(input,vars)
 %
 % Code goes here....
 % 
 end

In your case, you want the first line to read:

function xc = bisect(funcHandle, a, b, tol)

where funcHandle is a handle for your function. The syntax of a function handle is:

funcHandle = @(variables)(equation)

To use the function, you simply call the variable name as a function, i.e.

f_of_x = funcHandle(pi);

Since your bisection method expects a function of the form f(x) = 0, your function handle would be,

your_fun = @(x)(x^5 + x -1);

Hope this helps!!

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Jay Gourley 2017-2-3

Yes, that helps. Thanks for anticipating that I needed more than just the command to type. Your explanation of the breakdown helps a lot.

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Answer 2

John BG 2017-2-3

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/323155-how-do-i-create-a-function-and-run-it#answer_253155

在 MATLAB Online 中打开

function   xc=bisect(f,a,b,tol)
    if sign(f(a))*sign(f(b)) >= 0
        error('f(a)f(b)<0 not satisfied!') %ceases execution
    end
    fa=f(a)
    fb=f(b)
    while (b-a)/2>tol
        c=(a+b)/2;
        fc=f(c);
        if fc == 0  %c is a solution, done        
            break
        end
    end
    if sign(fc)*sign(fa)<0  %a and c make the new interval
        b=c;fb=fc;
    else                    %c and b make the new interval
        a=c;fa=fc;
    end
    xc=(a+b)/2;
end