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

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

Answer 1

[KALYAN ACHARJYA]

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

[FRANCESCA ROSSI]

Actually, I don't want to set it to 0, but I want to save in the new array the corresponding current value from A_Zn1

[KALYAN ACHARJYA]

I have added another answer, please check.

[FRANCESCA ROSSI]

Ok I think we are on the right way, but when I try to run it an error occurs:

Operands to the || and && operators must be convertible to logical scalar values.

Error in pot_selection (line 19)

idx=(find(V_Zn1==-1.7007 || V_Zn1==-1.7001));

[KALYAN ACHARJYA]

idx=(find(V_Zn1==-1.7007 | V_Zn1==-1.7001));

[FRANCESCA ROSSI]

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]

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?

Answer 2

[David Hill]

AA_Zn1=Zn1_data(:,1).*(Zn1_data(:,1)==-1.7001|Zn1_data(:,1)==-1.70007);

[FRANCESCA ROSSI]

I am actually looking for the current data (A_Zn1) corresponding to that value of voltage (V_Zn1=-1.0007)

[David Hill]

A_Zn1(V_Zn1==-1.7001|V_Zn1==-1.7007);

Answer 3

[Steven Lord]

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