I'm trying to use a while loop to delete the final row in an array every time it loops, however, it's not working

Question

Mehrab 2023-9-14

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https://ww2.mathworks.cn/matlabcentral/answers/2021051-i-m-trying-to-use-a-while-loop-to-delete-the-final-row-in-an-array-every-time-it-loops-however-it

评论： Dyuman Joshi 2023-9-15

TensileData.xlsx

Basically, using the file TensileData.xlsx that I have attached to the question, I have to use a loop, work backwards through the entire data set reducing the number of points to find the equation of the elastic region. This can be done by calculating the r^2 value until the r^2 value is larger than 0.9997. However, when I try to create write the code for this I am unable to do it. I'm not completely sure where to begin, however, I have made a start. When I run the code I receive the error: Index in position 1 is invalid. Array indices must be positive integers or logical values.

Error in lab8t5 (line 26)

strain(numRows_strain, :) = [];

The code that my question relates to is part b onwards, however, I have put all the code here in case it is helpful.

clear all; close all; clc
% Part a.
data = importdata('TensileData.xlsx');
strain = data.data(:, 1);
stress = data.data(:, 2);
figure(1)
plot(strain, stress, 'r*')
% Part b.
numRows_strain = length(strain);
numRows_stress = size(stress, 1);
[a0, a1, r2] = linreg(strain, stress);
slope = a1;
constant = a0;
error = r2;
while error <= 0.9997
    [a0, a1, r2] = linreg(strain, stress);
    numRows_strain = numRows_strain - 1;
    numRows_stress = numRows_stress - 1;
    strain(numRows_strain, :) = [];
    stress(numRows_stress, :) = [];    
end

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Answer 1

Dyuman Joshi 2023-9-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2021051-i-m-trying-to-use-a-while-loop-to-delete-the-final-row-in-an-array-every-time-it-loops-however-it#answer_1310431

在 MATLAB Online 中打开

Firstly, use readmatrix or readtable instead of importdata.

Secondly, you can define variables from function calls, instead of reassinging them. And as mentioned before, do not use error as a variable name, because it is a built-in MATLAB function.

%Directly use this
[slope, constant, err] = linreg(strain, stress);

Thirdly, you are not updating the variable error in the while loop, so depending upon its value, the while loop will not run or run forever.

% Part a.
data = readmatrix('TensileData.xlsx');
strain = data(:, 1);
stress = data(:, 2);
figure(1)
plot(strain, stress, 'r*')
% Part b.
[slope, constant, err] = linreg(strain, stress);
while err <= 0.9997
    %Removing the last row
    strain(end, :) = [];
    stress(end, :) = [];  
    %Computing the r_square value again
    [~, ~, err] = linreg(strain, stress);
end

Since I do not have the linreg() function, I can not run the above code, but it should do what you asked for - delete the last row till the condition is met.

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Image Analyst 2023-9-14

在 MATLAB Online 中打开

You can make it more robust like this

while err <= 0.9997 && ~isempty(strain) && ~isempty(stress)
    % Remove the last row from each matrix:
    strain(end, :) = [];
    stress(end, :) = [];  
    
    % Bail out if either matrix is empty.
    if isempty(strain) || isempty(stress)
        break;
    end
    
    % Compute the r_square value again.
    [~, ~, err] = linreg(strain, stress);
end