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

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.

Answer 1

[Image Analyst]

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';

[jungmin park]

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]

For some reason, you're ignoring the b that the fit gives you and instead using some b that you pulled out of a workbook (which you forgot to attach). Why?

Evidently that "ini" is a vector, and so log(ini) is a vector, which means that you can't stuff those multiple numbers into b(k) which is just a single element and can take only one number, not a bunch of numbers.

My code does what you said in your question. Yours does not. Your original question did not say anything about a workbook and using only the slopes and not the intercepts, and to use intercepts from a workbook instead of the computed ones.

[jungmin park]

First, the reason why b value is specified without using p(2) is to increase the accuracy of curve fitting using real time data. So how do I set the b value if I want to use it as a variable?

[jungmin park]

We have designated b(k) as p(2) apart from the previous question and applied it to the code.

As a result, both m(k) and b(k) are fitting in the same number.

Would you give me some advice on this?

file_name = '20200207_1.xlsx'
format short;
format compact;

numberOfTrials = 116;
for k = 1 : numberOfTrials;
        x = (3/60 : 3/60 : 117*3/60 );
        y = xlsread(file_name, 'sheet1','J4:J120' );
        y_t = y';
        Y = log(y_t - 430);
        
        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
hFig = figure;
plot(m, 'b.-', 'LineWidth', 2, 'MarkerSize', 1);
grid on;
hold on;
xlabel('Trial', 'FontSize', 1);
ylabel('m', 'FontSize', 1);
caption = sprintf('m for all %d points', numberOfTrials);
title(caption, 'FontSize', 1);

[Image Analyst]

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]

I forgot to attach the file. Please solve the problem.