I want to calculate m, b using the polyfit function over and over and over again.

Question

jungmin park 2020-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/505624-i-want-to-calculate-m-b-using-the-polyfit-function-over-and-over-and-over-again

评论： jungmin park 2020-2-19

x = randn(1, 100)
y= randn(1, 100)
P= polyfit (x,y,1)
m = P(1)
b = P(2)

In this way, m and b values, which are put into the primary function, will be displayed in the interval between 1 and 100.

But I'm one to five, five to ten, ... I would like to calculate m, b, and draw a scatterplot from 95 to 100.

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Answer 1

Image Analyst 2020-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/505624-i-want-to-calculate-m-b-using-the-polyfit-function-over-and-over-and-over-again#answer_415711

Try this, where I plot b and m for all 100 trials, and do a scatterplot for trials 95 through 100, as you asked:

% Initialization steps:
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clearvars;
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 24;
markerSize = 40;
numberOfTrials = 100;
for k = 1 : numberOfTrials
	x = randn(1, 100);
	y = randn(1, 100);
	% Fit a line.
	P = polyfit (x,y,1);
	m(k) = P(1);
	b(k) = P(2);
	fprintf('For trial #%3d of %3d, m = %.4f and b = %.4f.\n', ...
		k, numberOfTrials, m(k), b(k));
end
% DONE!  Now do the plotting.
% Plot m and b
hFig = figure;
subplot(2, 1, 1);
plot(m, 'b.-', 'LineWidth', 2, 'MarkerSize', markerSize);
grid on;
hold on;
plot(b, 'r.-', 'LineWidth', 2, 'MarkerSize', markerSize);
xlabel('Trial', 'FontSize', fontSize);
ylabel('m and b', 'FontSize', fontSize);
caption = sprintf('b and m for all %d points', numberOfTrials);
title(caption, 'FontSize', fontSize);
legend('m', 'b');
% Make a scatterplot of b vs. m but just for the indexes 95 to 100 (for some reason).
subplot(2, 1, 2);
scatter(m(95:100), b(95:100), markerSize, 'filled');
grid on;
xlabel('m', 'FontSize', fontSize);
ylabel('b', 'FontSize', fontSize);
title('b vs. m for the last 6 points (index 95 - 100)', 'FontSize', fontSize);
hFig.WindowState = 'maximized';

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jungmin park 2020-2-16

file_name = '20200207_1.xlsx'
y = xlsread(file_name, 'sheet1','J4:J120' );
format short;
format compact;
cini = xlsread(file_name, 'sheet1', 'K4:K120');
y_t = y';
cini_t = cini';
ini = cini_t - 430;
numberoftrials = 116;
for k = 1 : numberoftrials;
        x = linspace( 1*3/60, 117*3/60 , 117);
        Y = log(y_t - 430);
        
        P = polyfit (x, Y, 1);
        m(k) = P(1);
        b(k) = log(ini);
        fprintf('For trial #%3d of %3d, m = %.4f and b = %.4f.\n', ...
		k, numberOfTrials, m(k), b(k));
end
hFig = figure;
plot(m, 'b.-', 'LineWidth', 2, 'MarkerSize', markerSize);
grid on;
hold on;
xlabel('Trial', 'FontSize', fontSize);
ylabel('m', 'FontSize', fontSize);
caption = sprintf('m for all %d points', numberOfTrials);
title(caption, 'FontSize', fontSize);

The code was applied according to the method you told me, and an error occurred.

'The value cannot be substituted because the number of elements on the left and right sides is different.'

Error occurred: n1 (line 17)

b(k) = log(ini);

But I want to set the b(k) value as a variable.

Image Analyst 2020-2-18

Unfortunately you forgot to attach '20200207_1.xlsx', so now, when I had time to look at it, I can't because you didn't upload the workbook. I'll check back later for it.

jungmin park 2020-2-19