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How can I select y data given a certain x value?

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Hi,
The attached files contains two columns: the first contains voltage data, while the second current data. I would like to select only those current data whose potential is -1.70007 or -1.7001, depending on the rounding of the value. I am trying like this, but it never enters in the first condition loop, even if I know there are some values with V=-1.7001.
Thank you fo your time
Francesca
clear all
load Zn1.txt
Zn1_data=Zn1;
V_Zn1=Zn1_data(:,1);
A_Zn1=Zn1_data(:,2);
for i=1,length(V_Zn1)
if V_Zn1(i)==-1.7001
AA_Zn1(i)=A_Zn1(i);
else
AA_Zn1(i)=0;
end
end

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Accepted Answer

KALYAN ACHARJYA
KALYAN ACHARJYA on 19 Nov 2019
Edited: KALYAN ACHARJYA on 19 Nov 2019
load Zn1.txt
Zn1_data=Zn1;
V_Zn1=Zn1_data(:,1);
A_Zn1=Zn1_data(:,2);
AA_Zn1(find(V_Zn1~=-1.7001))=0;
Is This?
"I would like to select only those current data whose potential is -1.70007 or -1.7001"
Edited:
load Zn1.txt
Zn1_data=Zn1;
V_Zn1=Zn1_data(:,1);
A_Zn1=Zn1_data(:,2);
idx=(find(V_Zn1==-1.7007 | V_Zn1==-1.7001));
A_new_data=A_Zn1(idx);
A_new_data

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KALYAN ACHARJYA
KALYAN ACHARJYA on 19 Nov 2019
idx=(find(V_Zn1==-1.7007 | V_Zn1==-1.7001));
FRANCESCA ROSSI
FRANCESCA ROSSI on 19 Nov 2019
Ok the script runs but I got :
A_new_data =
0×1 empty double column vector
I think that could be a problem for the value I am using. Actually I know that in my data there is -1.70007 but matlab converts it to -1.7001, so that why I tried with this value. But maybe it is not the one anyway.
Thank you for you answer
KALYAN ACHARJYA
KALYAN ACHARJYA on 19 Nov 2019
Please note on floating points number and here Read the @Stevan's Answer also (Just below). Please check with other known non decimal numbers, any issue let me know?

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More Answers (2)

David Hill
David Hill on 19 Nov 2019
AA_Zn1=Zn1_data(:,1).*(Zn1_data(:,1)==-1.7001|Zn1_data(:,1)==-1.70007);

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FRANCESCA ROSSI
FRANCESCA ROSSI on 19 Nov 2019
I am actually looking for the current data (A_Zn1) corresponding to that value of voltage (V_Zn1=-1.0007)

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Steven Lord
Steven Lord on 19 Nov 2019
What you're seeing is the first example in the "Avoiding Common Problems with Floating-Point Arithmetic" section on this documentation page. Be very, very careful when using == (which performs exact, down-to-the-last-bit comparison) on floating point numbers. Use a tolerance or use ismembertol.
a = 0:0.1:1;
% ==
a == 0.3 % all elements are false
a(4) - 0.3 % small but not EXACTLY zero
% Tolerance -- eps is "close enough" in this case
abs(a-0.3) < eps % one element is true, the rest are false
% Ismembertol
ismembertol(a, 0.3) % one element is true, the rest are false

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