Index not working - very simple problem!

I want to find the indices of a vector of shortwave radiation values which = 3 and replace them with 0. Should be simple.

My code is:

idx=INSW==3;
INSW(idx)=0;

I know there are 3s in the data, but sum(idx)=0. What on earth is going on?

I can even do

idx=INSW(122)==3 %I know there is a 3 here

but still idx=0.

If I look for 0s or other numbers instead the code works and finds them. I don't know if its something to do with the data format? It should be just double. Here's some screenshots of the data and problem. I have tried also using 3.0000 and 3.000000000000000.

[Stephen23]

"I don't know if its something to do with the data format?"

It has nothing to do with the display format. The values are different, that is all.

Read about the behaviors of binary floating point numbers:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

http://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug

http://www.mathworks.com/matlabcentral/answers/140656-about-numerical-difficulty-of-equality-comparison-operation-between-two-double-real-numbers-in-matlab

This is worth reading as well:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

http://faculty.tarleton.edu/agapie/documents/cs_343_arch/papers/1991_Goldberg_FloatingPoint.pdf

Answer 1

[madhan ravi]

 idx = abs(INSW - 3) < 1e-2; set tolerance

[Catriona Fyffe]

Nice that's brilliant and works, thank you!! I thought I was going mad.

Answer 2

[Bruno Luong]

Your 3 in array is actually not 3 but something like 3.0000000001 or other variant.

When you see MATLAB dispay with .0000 if often means that.

[Catriona Fyffe]

Ok thanks also for this info, I'll look out for that in future!