How can it be that a value shown in a plot cannot be found in the matrix whose values are used to generate the plot?

Consider a plot of a frequency spectrum:

plot(f, X_dBm);

This plot has a peak at a frequency f_peak = 11 GHz. The amplitude in dBm = -6 dBm. You can click on the peak of the plot and it shows exactly these values for x and y. X_dBm is a matrix of t rows and n columns. I plot column n=30. When I search for the entry -6 the result is 0. How can that be? I am using:

find(X_dBm(:,30) = -6)

Answer 1

[Image Analyst]

Because it's not exactly -6. See the FAQ: http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

To fix:

oneColumn = X_dBm(:,30);
targetValue = -6;
tolerance = .01; % or whatever closeness you want.
rows = find(abs(oneColumn - targetValue) <= tolerance);