How do I let the user enter an equation that is then able to be used in abs() calcualtions

Question

Colten 2023-1-23

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https://ww2.mathworks.cn/matlabcentral/answers/1899210-how-do-i-let-the-user-enter-an-equation-that-is-then-able-to-be-used-in-abs-calcualtions

评论： Walter Roberson 2023-1-23

I want the user to be able to enter a function that is then stored in y3 that I am able to use. I currently get an error with er=max(abs(y4-y3)); when i try and I and run it. I am able to make it work when I type the equation direcly into the code but not when getting from user input

x = 0:0.001:1; 
fprintf(1,'For example: cos(x)\n');
s = input(' ', 's');
F = str2func(['@(x) ', s]);
y3 =F;
y4=x;
er=max(abs(y4-y3)); %maximum error
plot(x,y3);
title(['f(x) = ' s ' and Hermite interpolant, max. error is ' , num2str(er)]);

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Walter Roberson 2023-1-23

The code works for me when I use your input() instead of assigning a constant to s .

What problem do you observe?

It would be a problem if the user typed in a @() anonymous function definition instead of an expression. Or if the user expression does not include x

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Answer 1

Sulaymon Eshkabilov 2023-1-23

1
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https://ww2.mathworks.cn/matlabcentral/answers/1899210-how-do-i-let-the-user-enter-an-equation-that-is-then-able-to-be-used-in-abs-calcualtions#answer_1154725

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Use this small change by specifying the input argument (x) in your code:

x = 0:0.001:1; 
fprintf(1,'For example: cos(x)\n');
s = input(' ', 's');
F = str2func(['@(x) ', s]);
y3 =F(x);                     % This is necessary
y4=x;
er=max(abs(y4-y3)); %maximum error
plot(x,y3);
title(['f(x) = ' s ' and Hermite interpolant, max. error is ' , num2str(er)]);