Need help with looping to populate matrix using if statement

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Kevin Hout
Kevin Hout 2016-3-7
回答： Kevin Hout 2016-3-7
clc;
clear all;
close all;
load('Stress_Strain.mat')
count = 0;
for i = 1:1000
if Stress_Strain_2(:,2) == count;
StressStrain(i,2) = Stress_Strain_2(i,2);
end
count = count + 0.001
end
% for i = 1:1000
%
% if Stress_Strain_2(i,2) = count && Stress_Strain_2(i,2) > 0.0999;
% Percent_Strain_2(1,2) = Stress_Strain_2(i,2);
% Percent_Strain_2(1,1) = Stress_Strain_2(i,1);
% elseif Stress_Strain_2(i,2) < 0.2001 && Stress_Strain_2(i,2) > 0.1999;
% Percent_Strain_2(2,2) = Stress_Strain_2(i,2);
% Percent_Strain_2(2,1) = Stress_Strain_2(i,1);

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回答（2 个）

Walter Roberson
Walter Roberson 2016-3-7
http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F
Two floating point numbers constructed different ways often do not exactly equal each other. You should be testing for being "close" rather than being "equal".
If you have R2015a or later you should consider using imembertol()
You also have the problem that
if Stress_Strain_2(:,2) == count
means the same thing as
if all(Stress_Strain_2(:,2) == count)
which is to say that all of the items in the column would have to be the same value, all exactly equal to what is in "count". This is unlikely to be what you wanted. You did not describe your desired match condition, so I cannot repair the code. Possibly you want
if any(Stress_Strain_2(:,2) == count)
except with correction for tolerances. Perhaps
if ismembertol(count, Stress_Strain_2(:,1))
Kevin Hout
Kevin Hout 2016-3-7
Thanks, Walter.
I was trying to take a value in the Stress column (at Strains equal to 0.001, 0.002, 0.003, etc) and populate it into a new table. I am trying to use an if statement, but it is not working.

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