Empty plot is showing because of my er

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Panda05 2023-11-30

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评论： Panda05 2023-11-30

Hello guys , is there a way i can store my er as a vector that i can later on use to plot with N ? thank you for your suggestions. The code is shown below. I just want to be able to use er in the for loop to plot with N , how can i retrieve the er values into a vector

N=[10,20,40,80];

for i = 1:length(N)

x = (0:N(i))./N(i);

xsamples = 0:0.005:1;

y = f(xsamples);

fxn = Lagpoly(xsamples,x,y,N);

er= max(abs(fxn-y));

end

plot(N, er, '*-', 'DisplayName', 'Data Points');

%plot(N,N);

%set(gca, 'XScale', 'log')

%set(gca, 'YScale', 'log')

function y = f(xsamples)

y = 1 ./ (1+ (xsamples.*xsamples));

end

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Dyuman Joshi 2023-11-30

编辑：Dyuman Joshi 2023-11-30

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Even if the er is a scalar, the code should plot something, see below.

If you get an empty plot, your data could have NaN values.

Please attach the defition of the parameter Lagpoly, so that we can run the code and reproduce the error.

plot(1:10, 10, '*-')

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Answer 1

Image Analyst 2023-11-30

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在 MATLAB Online 中打开

Index er. This works:

% Initialization steps.

clc; % Clear the command window.

close all; % Close all figures (except those of imtool.)

clear; % Erase all existing variables. Or clearvars if you want.

workspace; % Make sure the workspace panel is showing.

format long g;

format compact;

fontSize = 18;

N = [10, 20, 40, 80];

for k = 1:length(N)

x = (0:N(k))./N(k);

xsamples = 0:0.005:1;

y = f(xsamples);

% fxn = Lagpoly(xsamples,x,y,N);

fxn = rand;

er(k) = max(abs(fxn-y));

end

% Plot on a log-log scale. Or use plot() for a linear scale.

loglog(N, er, '*-', 'DisplayName', 'Data Points',...

'LineWidth', 2, 'MarkerSize', 30);

grid on;

xlabel('N', 'FontSize', fontSize);

ylabel('er', 'FontSize', fontSize);

title('er vs. N', 'FontSize', fontSize);

%=======================================================================

function y = f(xsamples)

y = 1 ./ (1+ (xsamples.*xsamples));

end

Since you didn't provide Lagpoly I had to just use a random number to test it. Replace the rand line with your Lagpoly line of code.

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Panda05 2023-11-30

Thank you so much !!!!

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Answer 2

Torsten 2023-11-30

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https://ww2.mathworks.cn/matlabcentral/answers/2054514-empty-plot-is-showing-because-of-my-er#answer_1363329

编辑：Torsten 2023-11-30

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What x-y data are given to build the Lagrange Polynomial and in which x-data do you want to interpolate ? I assume that x-y are given arrays to build the Lagrange Interpolation upon and that you want to interpolate in xsamples. But your code takes xsamples and y to build the Lagrange polynomial and x for evaluation. I don't think this is what you want.

Further, you have to save "er" for each value of i.

Use

er(i)= max(abs(fxn-y));

instead of

er= max(abs(fxn-y));