how can I set the decimal digits of all coefficients at one time when outputting a polynomial f?

Question

marvellous 2023-10-27

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评论： marvellous 2023-10-27

In Windows 10, MATLAB R2018a, I try to use fprintf to output a polynomial f, for example, 5.327416*x^2+3.660092*x+1.5799301. How can I set the decimal digits(e.g. 3) of all coefficients at one time instead of setting them one by one? That is, I want the result to be "5.327*x^2+3.660*x+1.580". Anyone can help me? Thanks!

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Answer 1

Walter Roberson 2023-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/2039336-how-can-i-set-the-decimal-digits-of-all-coefficients-at-one-time-when-outputting-a-polynomial-f#answer_1341651

在 MATLAB Online 中打开

P = [5.327416, -3.660092, 1.5799301]

P = 1×3

5.3274 -3.6601 1.5799

%method 1

vpa(poly2sym(P, sym('x')),4)

ans =

%method 2

poly2sym(round(sym(P),3))

ans =

%method 3

fmt = [repmat('%.3f*x^%d + ', 1, length(P)-1), '%.3f\n'];

temp = reshape([P; length(P)-1:-1:0], 1, []);

fprintf(fmt, temp(1:end-1));

5.327*x^2 + -3.660*x^1 + 1.580

If you want the + -3.660 to instead show up as - 3.660 then it takes more work.

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Dyuman Joshi 2023-10-27

编辑：Dyuman Joshi 2023-10-27

在 MATLAB Online 中打开

You can use disp on the symbolic expression or convert it to char and use fprintf as well -

P = [5.327416, -3.660092, 1.5799301]

P = 1×3

5.3274 -3.6601 1.5799

y = vpa(poly2sym(P, sym('x')),4);

disp(y)

z = char(y);

fprintf('%s', z)

5.327*x^2 - 3.66*x + 1.58

marvellous 2023-10-27

Bravo! Many thanks!

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Answer 2

Sam Chak 2023-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/2039336-how-can-i-set-the-decimal-digits-of-all-coefficients-at-one-time-when-outputting-a-polynomial-f#answer_1341581

在 MATLAB Online 中打开

a = 5.327416;
b = 3.660092;
c = 1.5799301;
fprintf('f(x) = %.3f*x^2 + %.3f*x + %.3f', a, b, c)
f(x) = 5.327*x^2 + 3.660*x + 1.580

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marvellous 2023-10-27

Thank you for your help, but this is not what I want. Since I may not know the degree of f, I can not write out the 'x' items in advance. Thus, I do not intend to set the decimal digit for each coefficient separately. I prefer to write like ' fprintf (' ? ' , f) ', but I can not figure out what to fill in at the question mark to make it work? Can you help me?

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