Unknown glitch when finding an exact array value

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Samuel Gaspar
Samuel Gaspar 2020-11-12
评论： Star Strider 2020-11-12
All,
I am experiencing a strange glitch within the following code. I understand this was probably not the best way to execute what i was looking to do, and have since re-worked the code to avoid this error.
In the following code, Matlab is failing to recognize t=0.3 within the t array. That is, T=find() skips over 0.3000 when the step size h is 0.05 or 0.025 in the t array. T=find() DOES correctly find 0.3000 in the t array when the step size h is <=0.01. I can also call for an indexing array:
a=t==0.3;
that fails to find the index of 0.3 within t, when the step size h is 0.05 or 0.025. Again, this indexing array works with step size h=0.01.
This has been verrified to happen on two machines running two different releases of Matlab. Mine is R2020a
clc
clear
h=0.05;%when h <=0.01, the error does not occur
t=0:h:2;
y=zeros(size(t));
y(1)=1;
n=numel(y);
%syms x real
%Y(x)=0.25*x-(3/16)+(19/16)*exp(4*x); %actual solution
for i=1:n-1
f=1-t(i)+4.*y(i);
y(i+1) = y(i)+h*f;
end
T=find(t==0.1 | t==0.2 | t==0.3 | t==0.4 | t==0.5 | t==1 | t==1.5 | t==2);
for j=1:numel(T)
fprintf('y=%.6f\n',y(T(j)));
end
%plot(t,y,t,Y(t)); grid on
The code following the first loop was re-written and will properly find t=0.3 with step size h=0.05:
inds = [0.1,0.2,0.3,0.4,0.5,1,1.5,2];
T=zeros(size(inds));
for i=1:length(inds)
T(i)=find(t>=inds(i),1);
end
for j=1:length(T)
fprintf('y=%.5f\n',y(T(j)));
end

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采纳的回答

Star Strider
Star Strider 2020-11-12
That reflects the way the colon operator works to calculate the vector elements. See the documentation section on Floating-Point Numbers for an illustrative discussion.
Include a tolerance with find, as you did in the version that worked.
  2 个评论
Samuel Gaspar
Samuel Gaspar 2020-11-12
Thanks! Defining t using linspace also appears to fix it.
Star Strider
Star Strider 2020-11-12
As always, my pleasure!
The linspace function produces more exact elements, however it requires a bit of forethought to produce exact integers with it.

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