# find(vector==value) not working

17 views (last 30 days)
Luke Stalder on 13 Nov 2019
Answered: Ruger28 on 13 Nov 2019
Why does find(vector==value) not work? I am trying to find the index of a value of 0.16 in a vector defined as vector=0:0.01:0.3. find(vector==0.16) should return 17 but returns an empty vector. Why?

Star Strider on 13 Nov 2019
You have encountered floating-point approximation error. See the documentation on Floating-Point Numbers for a full explanation.
Try this:
vector=0:0.01:0.3;
Result = find(vector<=0.16, 1, 'last');
producing:
Result =
17

Thomas Satterly on 13 Nov 2019
While it looks like the 17th value of your vector is 0.16, it's actually slightly off because of floating point precision. What you should do is find the closest value to your target value that's below an acceptable limit. For example:
vector = 0:0.01:0.3;
tolerance = eps * 2; % eps is the floating point number tolerance
target = 0.16;
match = find(abs(vector - target) <= tolerance);

Ruger28 on 13 Nov 2019
From the FIND documentation
To find a noninteger value, use a tolerance value based on your data. Otherwise, the result is sometimes an empty matrix due to floating-point roundoff error.
y = 0:0.1:1
y = 1×11
0 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 0.9000 1.0000
k = find(y==0.3)
k = 1x0 empty double row vector
k = find(abs(y-0.3) < 0.001)
k = 4