# how to make something between quotation automatic!

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Ham Man on 16 Aug 2022
Commented: Ham Man on 16 Aug 2022
I want to make 'poly2' automatic by changing n? How can I do this using for example eval function?
n=2;% 3,4,....
f1 = fit(x,y,'poly2')

Steven Lord on 16 Aug 2022
If you're just fitting polynomials you could use polyfit and polyval, specifying n as the third input argument.
n = 2;
[p, s] = polyfit(cdate, pop, n);
plot(cdate, pop, 'o', cdate, polyval(p, cdate, s), '-') But you can also use fit if you want. There's no need to use eval here, just use string + or char manipulation.
theFit = fit(cdate, pop, "poly" + n, 'Normalize', 'on') % or
theFit =
Linear model Poly2: theFit(x) = p1*x^2 + p2*x + p3 where x is normalized by mean 1890 and std 62.05 Coefficients (with 95% confidence bounds): p1 = 25.18 (23.58, 26.79) p2 = 75.43 (74.04, 76.83) p3 = 61.74 (59.7, 63.79)
theFit2 = fit(cdate, pop, ['poly', num2str(n)], 'Normalize', 'on') % or
theFit2 =
Linear model Poly2: theFit2(x) = p1*x^2 + p2*x + p3 where x is normalized by mean 1890 and std 62.05 Coefficients (with 95% confidence bounds): p1 = 25.18 (23.58, 26.79) p2 = 75.43 (74.04, 76.83) p3 = 61.74 (59.7, 63.79)
theFit2 = fit(cdate, pop, sprintf('poly%d', n), 'Normalize', 'on')
theFit2 =
Linear model Poly2: theFit2(x) = p1*x^2 + p2*x + p3 where x is normalized by mean 1890 and std 62.05 Coefficients (with 95% confidence bounds): p1 = 25.18 (23.58, 26.79) p2 = 75.43 (74.04, 76.83) p3 = 61.74 (59.7, 63.79)
figure
plot(cdate, pop, 'o')
hold on
plot(theFit, '-') ##### 1 CommentShowHide None
Ham Man on 16 Aug 2022
Steven thank you so much for nice explanation with many choices. Much appreciated!