How do I save a SINGLE value from the workspace?

Hello, I am using the diff function to find the value of 0 of my function in the first derivative. Turns out I got the results shown in the image.

Now I need to know how I can save ONLY the value that is marked in a box of the image. Since later I need to perform a for and save the marked value for each iteration. Thank you.

[Luca Ferro]

could you share the data? if not, could you say what data type (array 24x2, struct,...) is the source?

[Camilo Mahnert Cataldo]

Hi, is a data type 2x99. I need save the value x(:,1) when x(:,2) is near to 0. How I save this date.

Answer 1

[Fangjun Jiang]

You will have to assign that value to a temparary variable and then save it.

a=rand(10,2);
%save test.mat a(8,:) % this will cause error
temp=a(8,:);
save test.mat temp;

Answer 2

[Star Strider]

‘Hi, is a data type 2x99. I need save the value x(:,1) when x(:,2) is near to 0. How I save this date.’

It depends what ‘near to 0’ means. One option is to look for the indices of the zero crossings —

x = [0:98; sin(2*pi*(0:98)/25)+randn(1,99)*0.05].'              % Create Data

x = 99×2

0 -0.0154 1.0000 0.2117 2.0000 0.4525 3.0000 0.7344 4.0000 0.8145 5.0000 0.8883 6.0000 1.0496 7.0000 0.8734 8.0000 0.8215 9.0000 0.7359

Near0Idx = find(diff(sign(x(:,2))))                             % Indices Of Zero Crossings

Near0Idx = 8×1

1 13 25 38 51 63 75 88

figure
plot(x(:,1), x(:,2), 'DisplayName','Data')
hold on
plot(x(Near0Idx,1), x(Near0Idx,2), 'rs', 'DisplayName','Near to 0')
hold off
grid
axis('padded')
legend('Location','best')

If there are no zero-crossings, then equivalently something like this would work:

Offset = 1;

Near0Idx = find(diff(sign(x(:,2)-Offset)))

The ‘Offset’ value can be whatever works in your application, either positive or negative, and it could be either added or subtracted from ‘x(:,2)’.

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